diff options
Diffstat (limited to 'open_issues')
27 files changed, 2594 insertions, 185 deletions
diff --git a/open_issues/binutils_gold.mdwn b/open_issues/binutils_gold.mdwn index aa6843a3..9eeebf2d 100644 --- a/open_issues/binutils_gold.mdwn +++ b/open_issues/binutils_gold.mdwn @@ -1,4 +1,5 @@ -[[!meta copyright="Copyright © 2010, 2011 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2010, 2011, 2012 Free Software Foundation, +Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this @@ -8,180 +9,8 @@ Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled [[GNU Free Documentation License|/fdl]]."]]"""]] -[[!tag open_issue_binutils]] +[[!tag open_issue_binutils open_issue_porting]] -Have a look at GOLD / port as needed. +Have a look at gold / port as needed. - -# teythoon's try / `mremap` issue - -IRC, #hurd, 2011-01-12 - - <teythoon> I've been looking into building gold on hurd and it built fine - with one minor tweak - <teythoon> and it's working fine according to its test suite - <teythoon> the only problem is that the build system is failing to detect - the hurdish mremap which lives in libmemusage - <teythoon> on linux it is in the libc so the check succeeds - <teythoon> any hints on how to fix this properly? - <antrik> hm... it's strange that it's a different library on the Hurd - <antrik> are the implementations compatible? - <teythoon> antrik: it seems so, though the declarations differ slightly - <antrik> I guess the best thing is to ask on the appropriate list(s) why - they are different... - <teythoon> teythoon@ganymede:~/build/gold/binutils-2.21/gold$ grep -A1 - mremap /usr/include/sys/mman.h - <teythoon> extern void *mremap (void *__addr, size_t __old_len, size_t - __new_len, int __flags, ...) __THROW; - <teythoon> vs - <antrik> of course it would be possible to modify the configure script to - check for the Hurd variant too; but first we should establish whether - here is actually any reason for being different, or it's just some - historical artefact that should be fixed... - <teythoon> teythoon@ganymede:~/build/gold/binutils-2.21/gold$ fgrep 'extern - void *mremap' mremap.c - <teythoon> extern void *mremap (void *, size_t, size_t, int, ...); - <teythoon> the problem is that the test fails to link due to the fact that - mremap isn't in the libc on hurd - <antrik> yeah, it would be possible for the configure script to check - whether it works when the hurdish extra library is added explicitely - <antrik> but again, I don't see any good reason for being different here in - the first place... - <teythoon> so should I create a patch to move mremap? - <antrik> if it's not too complicated, that would be nice... it's always - easier to discuss when you already have code :-) - <antrik> OTOH, asking first might spare you some useless work if it turns - out there *is* some reason for being different after all... - so where is the right place to discuss this? - <antrik> bug-hurd mailing list and/or glibc mailing list. not sure which - one is better -- I guess it doesn't hurt to crosspost... - -[[mailing_lists/libc-alpha]] is the correct list, and cross-posting to -[[mailing_lists/bug-hurd]] would be fine, too. - - <teythoon> antrik: some further digging revealed that mremap belongs to - /lib/libmemusage.so on both hurd and linux - <teythoon> the only difference is that on linux there is a weak reference - to that function in /lib/libc-2.11.2.so - <teythoon> $ objdump -T /lib/libc-2.11.2.so | fgrep mremap - <teythoon> 00000000000cf7e0 w DF .text 0000000000000028 GLIBC_2.2.5 - mremap - <antrik> ah, it's probably simply a bug that we don't have this weak - reference too - <antrik> IIRC we had similar bugs before - <antrik> teythoon: can you provide a patch for that? - <teythoon> antrik: unfortunately I have no idea how that weak ref ended up - there - - <guillem> teythoon: also the libmemusage.s seems to be just a debugging - library to be used by LD_PRELOAD or similar - <guillem> which override those memory functions - <guillem> the libc should provide actual code for those functions, even if - the symbol is declared weak (so overridable) - <guillem> teythoon: are you sure that's the actual problem? can you paste - somewhere the build logs with the error? - <teythoon> guillem: sure - <teythoon> http://paste.debian.net/104437/ - <teythoon> that's the part of config.log that shows the detection (or the - failure to detect it) of mremap - <teythoon> this results in HAVE_MREMAP not being defined - <teythoon> as a consequence it is declared in gold.h and this declaration - conflicts with the one from sys/mman.h http://paste.debian.net/104438/ - <teythoon> on linux the test for mremap succeeds - <guillem> teythoon: hmm oh I guess it's just what that, mremap is linux - specific so it's not available on the hurd - <guillem> teythoon: I just checked glibc and seems to confirm that - <braunr> CONFORMING TO This call is Linux-specific, and should not be used - in programs intended to be portable. - <teythoon> ah okay - <teythoon> so I guess we shouldn't ship an header with that declaration... - <guillem> teythoon: yeah :/ good luck telling that to drepper :) - <guillem> teythoon: I guess he'll suggest that everyone else needs to get - our own copy of sys/mman.h - <guillem> s/our/their/ - <teythoon> hm, so how should I proceed? - <braunr> what's your goal ? - <braunr> detecting mremap ? - <teythoon> making binutils/gold compile ootb on hurd - <teythoon> I picked it from the open issues page ;) - <braunr> well, if there is no mremap, you need a replacement - <teythoon> gold has a replacement - <braunr> ok - <braunr> so your problem is fixing the detection of mremap right ? - <teythoon> yes - <braunr> ok, that's a build system question then :/ - <braunr> you need to ask an autotools guy - <teythoon> well, actually the build system correctly detects the absence of - mremap - <braunr> (gold does use the autotools right ?) - <teythoon> yes - <braunr> oh, i'm lost now (i admit i didn't read the whole issue :/) - <teythoon> it is just that the declaration in sys/mman.h conflicts with - their own declaration - <braunr> ah - <braunr> so in the absence of mremap, they use their own builtin function - <teythoon> yes - <teythoon> and according to the test suite it is working perfectly - <teythoon> gold that is - <teythoon> the declaration in mman.h has an extra __THROW - <guillem> a workaround would be to rename gold's mremap to something else, - gold_mremap for example - <braunr> that's really the kind of annoying issue - <braunr> you either have to change glibc, or gold - <guillem> yeah - <braunr> you'll face difficulty changing glibc, as guillem told you - <guillem> the correct solution though IMO is to fix glibc - <braunr> but this may be true for gold too - <braunr> guillem: i agree - <antrik> maybe it would be easiest actually to implement mremap()?... - <braunr> but as this is something quite linux specific, it makes sense to - use another internal name, and wrap that to the linux mremap if it's - detected - <braunr> antrik: i'm nto sure - <antrik> braunr: I don't think using such workarounds is a good - idea. clearly there would be no issue if the header file wouldn't be - incorrect on Hurd - <braunr> antrik: that's why i said i agree with guillem when he says "the - correct solution though IMO is to fix glibc" - <teythoon> what exactly is the problem with getting a patch into glibc? - <braunr> the people involved - <guillem> teythoon: and touching a generic header file - <braunr> but feel free to try, you could be lucky - <teythoon> but glibc is not an linux specific piece of software, right? - <braunr> teythoon: no, it's not - <guillem> erm... - <braunr> teythoon: but in practice, it is - <guillem> supposedly not :) - <antrik> braunr: BTW, by "easiest" I don't mean coding alone, but - coding+pushing upstream :-) - <guillem> so the problem is, misc/sys/mman.h should be a generic header and - as such not include linux specific parts, which are not present on hurd, - kfreebsd, etc etc - <braunr> antrik: yes, that's why guillem and i suggested the workaround - thing in gold - <antrik> that also requires pushing upstream. and quite frankly, if I were - the gold maintainer, I wouldn't accept it. - <guillem> but the easiest (and wrong) solution in glibc to avoid maintainer - conflict will probably be copying that file under hurd's glibc tree and - install that instead - <braunr> antrik: implementing mremap could be relatively easy to do - actually - <braunr> antrik: IIRC, vm_map() supports overlapping - <antrik> well, actually the easiest solution would be to create a patch - that never goes upstream but is included in Debian, like many - others... but that's obviously not a good long-term plan - <antrik> braunr: yes, I think so too - <antrik> braunr: haven't checked, but I have a vague recollection that the - fundamentals are pretty much there - <antrik> teythoon: so, apart from an ugly workaround in gold, there are - essentially three options: 1. implement mremap; 2. make parts of mman.h - conditional; 3. use our own copy of mman.h - <antrik> 1. would be ideal, but might be non-trivial; 2. would might be - tricky to get right, and even more tricky to get upstream; 3. would be - simple, but a maintenance burden in the long term - <teythoon> looking at golds replacement code (mmap & memcpy) 1 sounds like - the best option performance wise - -[[!taglink open_issue_glibc]]: check if it is possible to implement `mremap`. -[[I|tschwinge]] remember some discussion about this, but have not yet worked on -locating it. [[Talk to me|tschwinge]] if you'd like to have a look at this. +Apparently it needs [[glibc/mremap]]. diff --git a/open_issues/code_analysis.mdwn b/open_issues/code_analysis.mdwn index d776d81a..00915651 100644 --- a/open_issues/code_analysis.mdwn +++ b/open_issues/code_analysis.mdwn @@ -1,4 +1,5 @@ -[[!meta copyright="Copyright © 2010, 2011 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2010, 2011, 2012 Free Software Foundation, +Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this @@ -110,6 +111,20 @@ There is a [[!FF_project 276]][[!tag bounty]] on some of these tasks. glibc's heap structure. its kinda handy, might help? <vsrinivas> MALLOC_CHECK_ was the envvar you want, sorry. + * In context of [[!message-id + "1341350006-2499-1-git-send-email-rbraun@sceen.net"]]/the `alloca` issue + mentioned in [[gnumach_page_cache_policy]]: + + IRC, freenode, #hurd, 2012-07-08: + + <youpi> braunr: there's actually already an ifdef REDZONE in libthreads + + It's `RED_ZONE`. + + <youpi> except it seems clumsy :) + <youpi> ah, no, the libthreads code properly sets the guard, just for + grow-up stacks + * Input fuzzing Not a new topic; has been used (and a paper published) for early UNIX diff --git a/open_issues/dde.mdwn b/open_issues/dde.mdwn index 725af646..aff988d5 100644 --- a/open_issues/dde.mdwn +++ b/open_issues/dde.mdwn @@ -451,3 +451,13 @@ At the microkernel davroom at [[community/meetings/FOSDEM_2012]]: any movement in that regard :-( <braunr> wasn't it needed for dde ? <antrik> hm... good point + + +# virtio + + +## IRC, freenode, #hurd, 2012-07-01 + + <braunr> hm, i haven't looked but, does someone know if virtio is included + in netdde ? + <youpi> braunr: nope, there's an underlying virtio layer needed before diff --git a/open_issues/fcntl_locking_dev_null.mdwn b/open_issues/fcntl_locking_dev_null.mdwn new file mode 100644 index 00000000..4c65a5c4 --- /dev/null +++ b/open_issues/fcntl_locking_dev_null.mdwn @@ -0,0 +1,38 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!meta title="fcntl locking /dev/null"]] + +[[!tag open_issue_hurd]] + + +# IRC, OFTC, #debian-hurd, 2012-07-06 + + <pinotree> regarding the libwibble failure (which holds libbuffy → + libbuffy-bindings), the failing test happens because it logs to /dev/null + as test file, + <pinotree> and while doing that, it wants to lock it first, having a + ENOTSUP in return + <youpi> oh + <youpi> locking null, how interesting + <youpi> what is that supposed to do ? :o) + <pinotree> from what i was reading posix, it would seem that such object is + considered a "File" + <youpi> is it our unimplemented record lock, or just the lock operation + that /dev/null doesn't support ? + <youpi> what size is null supposed to be? zero, right? + <pinotree> the latter + <youpi> ah + <youpi> so we can simply make lock return 0 + <youpi> since there's no byte to lock? + <youpi> I don't remember whether you can lock unexistant bytes + <pinotree> indeed, if i change the libwibble unit test to use eg /tmp/foo, + they pas + <pinotree> s diff --git a/open_issues/gcc.mdwn b/open_issues/gcc.mdwn index 04d399f0..9019939d 100644 --- a/open_issues/gcc.mdwn +++ b/open_issues/gcc.mdwn @@ -237,6 +237,60 @@ Last reviewed up to the [[Git mirror's 9aa4b6a8046270a9dbdf47827f1ea873217d7aa5 to find out why some stuff wasn't compiling even after kfreebsd porting patches adding preprocessors checks for __GLIBC__ + IRC, freenode, #hurd, 2012-05-25: + + <gnu_srs> Hi, looks like __GLIBC__ is not defined by default for GNU? + <gnu_srs> touch foo.h; cpp -dM foo.h|grep LIBC: empty + <braunr> gnu_srs: well, this only tells your the compiler defaults + <tschwinge> gnu_srs: See the email I just sent. + + [[!message-id "87396od3ej.fsf@schwinge.name"]] + + <braunr> __GLIBC__ would probably be introduced by a glibc header + <gnu_srs> tschwinge: I saw your email. I wonder if features.h is + included in the kFreeBSD build of webkit. + <gnu_srs> It is defined in their build, but not in the Hurd build. + <pinotree> gcc on kfreebsd unconditionally defines __GLIBC__ + <pinotree> (a bit stupid choice imho, but hardly something that could + be changed now...) + <braunr> :/ + <braunr> personally i don't consider this only "a bit" stupid, as + kfreebsd is one of the various efforts pushing towards portability + <braunr> and using such hacks actually hinders portability ... + <pinotree> yeah don't tell me, i can remember at least half dozen of + occasions when a code wouldn't have been compiling at all on other + glibc platforms otherwise + <pinotree> sure, i have nothing against kfreebsd's efforts, but making + gcc define something which is proper of the libc used is stupid + <braunr> it is + <pinotree> i spotted changes like: + <pinotree> -#ifdef __linux + <pinotree> +#if defined(__linux__) || defined(__GLIBC__) + <pinotree> and wondered why they wouldn't work at all for us... and + then realized there were no #include in that file before that + preprocessor check + <tschwinge> This is even in upstream GCC gcc/config/kfreebsd-gnu.h: + <tschwinge> #define GNU_USER_TARGET_OS_CPP_BUILTINS() \ + <tschwinge> do \ + <tschwinge> { \ + <tschwinge> builtin_define ("__FreeBSD_kernel__"); \ + <tschwinge> builtin_define ("__GLIBC__"); \ + <tschwinge> builtin_define_std ("unix"); \ + <tschwinge> builtin_assert ("system=unix"); \ + <tschwinge> builtin_assert ("system=posix"); \ + <tschwinge> } \ + <tschwinge> while (0) + <tschwinge> I might raise this upstream at some point. + <pinotree> tschwinge: i could guess the change was proposed by the + kfreebsd people, so asking them before at d-bsd@d.o would be a start + <tschwinge> pinotree: Ack. + <pinotree> especially that they would need to fix stuff afterwards + <pinotree> imho we could propose them the change, and if they agree put + that as local patch to debian's gcc4.6/.7 after wheezy, so there is + plenty of time for them to fix stuff + <pinotree> what should be done first is, however, find out why that + define has been added to gcc + * [low] Does `-mcpu=native` etc. work? (For example, 2ae1f0cc764e998bfc684d662aba0497e8723e52.) diff --git a/open_issues/gdb.mdwn b/open_issues/gdb.mdwn index 2ae3518c..dae18227 100644 --- a/open_issues/gdb.mdwn +++ b/open_issues/gdb.mdwn @@ -69,7 +69,7 @@ harmonized. There are several occurences of *error: dereferencing type-punned pointer will break strict-aliasing rules* in the MIG-generated stub files; thus no `-Werror` -until that is resolved. +until that is resolved ([[strict_aliasing]]). This takes up around 140 MiB and needs roughly 6 min on kepler.SCHWINGE and 30 min on coulomb.SCHWINGE. diff --git a/open_issues/gdb_attach.mdwn b/open_issues/gdb_attach.mdwn new file mode 100644 index 00000000..4e4f2ea0 --- /dev/null +++ b/open_issues/gdb_attach.mdwn @@ -0,0 +1,41 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!meta title="GDB: attach"]] + +[[!tag open_issue_gdb]] + + +# [[gdb_thread_ids]] + + +# IRC, freenode, #hurd, 2012-06-30 + + <braunr> hm, gdb isn't able to determine which thread is running when + attaching to a process + + +# IRC, freenode, #hurd, 2012-07-02 + + <braunr> woah, now that's a weird message ! + <braunr> when using gdb on a hanged ext2fs : + <braunr> Pid 938 has an additional task suspend count of 1; clear it? (y or + n) + <braunr> when hanged, gdb thinks the target task is already being debugged + :/ + <braunr> no wonder why it's completely stuck + <braunr> hm, the task_suspend might actually be the crash-dump-core server + attempting to create the core :/ + <braunr> hm interesting, looks like a problem with the + diskfs_catch_exception macro + <pinotree> braunr: what's up with it? + <braunr> pinotree: it uses setjmp + <braunr> hm random corruptions :/ + <braunr> definitely looks like a concurrency problem diff --git a/open_issues/glibc.mdwn b/open_issues/glibc.mdwn index 1ce47560..2dea816a 100644 --- a/open_issues/glibc.mdwn +++ b/open_issues/glibc.mdwn @@ -267,6 +267,8 @@ Last reviewed up to the [[Git mirror's d40c5d54cb551acba4ef1617464760c5b3d41a14 initialization <tschwinge> OK, that at least matches my understanding. + * [[`mremap`|mremap]] + * `syncfs` We should be easily able to implement that one. diff --git a/open_issues/glibc/mremap.mdwn b/open_issues/glibc/mremap.mdwn new file mode 100644 index 00000000..a293eea0 --- /dev/null +++ b/open_issues/glibc/mremap.mdwn @@ -0,0 +1,221 @@ +[[!meta copyright="Copyright © 2011, 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_glibc]] + +[[!toc]] + + +# binutils gold + +## IRC, freenode, #hurd, 2011-01-12 + + <teythoon> I've been looking into building gold on hurd and it built fine + with one minor tweak + <teythoon> and it's working fine according to its test suite + <teythoon> the only problem is that the build system is failing to detect + the hurdish mremap which lives in libmemusage + <teythoon> on linux it is in the libc so the check succeeds + <teythoon> any hints on how to fix this properly? + <antrik> hm... it's strange that it's a different library on the Hurd + <antrik> are the implementations compatible? + <teythoon> antrik: it seems so, though the declarations differ slightly + <antrik> I guess the best thing is to ask on the appropriate list(s) why + they are different... + <teythoon> teythoon@ganymede:~/build/gold/binutils-2.21/gold$ grep -A1 + mremap /usr/include/sys/mman.h + <teythoon> extern void *mremap (void *__addr, size_t __old_len, size_t + __new_len, int __flags, ...) __THROW; + <teythoon> vs + <antrik> of course it would be possible to modify the configure script to + check for the Hurd variant too; but first we should establish whether + here is actually any reason for being different, or it's just some + historical artefact that should be fixed... + <teythoon> teythoon@ganymede:~/build/gold/binutils-2.21/gold$ fgrep 'extern + void *mremap' mremap.c + <teythoon> extern void *mremap (void *, size_t, size_t, int, ...); + <teythoon> the problem is that the test fails to link due to the fact that + mremap isn't in the libc on hurd + <antrik> yeah, it would be possible for the configure script to check + whether it works when the hurdish extra library is added explicitely + <antrik> but again, I don't see any good reason for being different here in + the first place... + <teythoon> so should I create a patch to move mremap? + <antrik> if it's not too complicated, that would be nice... it's always + easier to discuss when you already have code :-) + <antrik> OTOH, asking first might spare you some useless work if it turns + out there *is* some reason for being different after all... + so where is the right place to discuss this? + <antrik> bug-hurd mailing list and/or glibc mailing list. not sure which + one is better -- I guess it doesn't hurt to crosspost... + +[[mailing_lists/libc-alpha]] is the correct list, and cross-posting to +[[mailing_lists/bug-hurd]] would be fine, too. + + <teythoon> antrik: some further digging revealed that mremap belongs to + /lib/libmemusage.so on both hurd and linux + <teythoon> the only difference is that on linux there is a weak reference + to that function in /lib/libc-2.11.2.so + <teythoon> $ objdump -T /lib/libc-2.11.2.so | fgrep mremap + <teythoon> 00000000000cf7e0 w DF .text 0000000000000028 GLIBC_2.2.5 + mremap + <antrik> ah, it's probably simply a bug that we don't have this weak + reference too + <antrik> IIRC we had similar bugs before + <antrik> teythoon: can you provide a patch for that? + <teythoon> antrik: unfortunately I have no idea how that weak ref ended up + there + + <guillem> teythoon: also the libmemusage.s seems to be just a debugging + library to be used by LD_PRELOAD or similar + <guillem> which override those memory functions + <guillem> the libc should provide actual code for those functions, even if + the symbol is declared weak (so overridable) + <guillem> teythoon: are you sure that's the actual problem? can you paste + somewhere the build logs with the error? + <teythoon> guillem: sure + <teythoon> http://paste.debian.net/104437/ + <teythoon> that's the part of config.log that shows the detection (or the + failure to detect it) of mremap + <teythoon> this results in HAVE_MREMAP not being defined + <teythoon> as a consequence it is declared in gold.h and this declaration + conflicts with the one from sys/mman.h http://paste.debian.net/104438/ + <teythoon> on linux the test for mremap succeeds + <guillem> teythoon: hmm oh I guess it's just what that, mremap is linux + specific so it's not available on the hurd + <guillem> teythoon: I just checked glibc and seems to confirm that + <braunr> CONFORMING TO This call is Linux-specific, and should not be used + in programs intended to be portable. + <teythoon> ah okay + <teythoon> so I guess we shouldn't ship an header with that declaration... + <guillem> teythoon: yeah :/ good luck telling that to drepper :) + <guillem> teythoon: I guess he'll suggest that everyone else needs to get + our own copy of sys/mman.h + <guillem> s/our/their/ + <teythoon> hm, so how should I proceed? + <braunr> what's your goal ? + <braunr> detecting mremap ? + <teythoon> making binutils/gold compile ootb on hurd + <teythoon> I picked it from the open issues page ;) + <braunr> well, if there is no mremap, you need a replacement + <teythoon> gold has a replacement + <braunr> ok + <braunr> so your problem is fixing the detection of mremap right ? + <teythoon> yes + <braunr> ok, that's a build system question then :/ + <braunr> you need to ask an autotools guy + <teythoon> well, actually the build system correctly detects the absence of + mremap + <braunr> (gold does use the autotools right ?) + <teythoon> yes + <braunr> oh, i'm lost now (i admit i didn't read the whole issue :/) + <teythoon> it is just that the declaration in sys/mman.h conflicts with + their own declaration + <braunr> ah + <braunr> so in the absence of mremap, they use their own builtin function + <teythoon> yes + <teythoon> and according to the test suite it is working perfectly + <teythoon> gold that is + <teythoon> the declaration in mman.h has an extra __THROW + <guillem> a workaround would be to rename gold's mremap to something else, + gold_mremap for example + <braunr> that's really the kind of annoying issue + <braunr> you either have to change glibc, or gold + <guillem> yeah + <braunr> you'll face difficulty changing glibc, as guillem told you + <guillem> the correct solution though IMO is to fix glibc + <braunr> but this may be true for gold too + <braunr> guillem: i agree + <antrik> maybe it would be easiest actually to implement mremap()?... + <braunr> but as this is something quite linux specific, it makes sense to + use another internal name, and wrap that to the linux mremap if it's + detected + <braunr> antrik: i'm nto sure + <antrik> braunr: I don't think using such workarounds is a good + idea. clearly there would be no issue if the header file wouldn't be + incorrect on Hurd + <braunr> antrik: that's why i said i agree with guillem when he says "the + correct solution though IMO is to fix glibc" + <teythoon> what exactly is the problem with getting a patch into glibc? + <braunr> the people involved + <guillem> teythoon: and touching a generic header file + <braunr> but feel free to try, you could be lucky + <teythoon> but glibc is not an linux specific piece of software, right? + <braunr> teythoon: no, it's not + <guillem> erm... + <braunr> teythoon: but in practice, it is + <guillem> supposedly not :) + <antrik> braunr: BTW, by "easiest" I don't mean coding alone, but + coding+pushing upstream :-) + <guillem> so the problem is, misc/sys/mman.h should be a generic header and + as such not include linux specific parts, which are not present on hurd, + kfreebsd, etc etc + <braunr> antrik: yes, that's why guillem and i suggested the workaround + thing in gold + <antrik> that also requires pushing upstream. and quite frankly, if I were + the gold maintainer, I wouldn't accept it. + <guillem> but the easiest (and wrong) solution in glibc to avoid maintainer + conflict will probably be copying that file under hurd's glibc tree and + install that instead + <braunr> antrik: implementing mremap could be relatively easy to do + actually + <braunr> antrik: IIRC, vm_map() supports overlapping + <antrik> well, actually the easiest solution would be to create a patch + that never goes upstream but is included in Debian, like many + others... but that's obviously not a good long-term plan + <antrik> braunr: yes, I think so too + <antrik> braunr: haven't checked, but I have a vague recollection that the + fundamentals are pretty much there + <antrik> teythoon: so, apart from an ugly workaround in gold, there are + essentially three options: 1. implement mremap; 2. make parts of mman.h + conditional; 3. use our own copy of mman.h + <antrik> 1. would be ideal, but might be non-trivial; 2. would might be + tricky to get right, and even more tricky to get upstream; 3. would be + simple, but a maintenance burden in the long term + <teythoon> looking at golds replacement code (mmap & memcpy) 1 sounds like + the best option performance wise + +[[!taglink open_issue_glibc]]: check if it is possible to implement `mremap`. +[[I|tschwinge]] remember some discussion about this, but have not yet worked on +locating it. [[Talk to me|tschwinge]] if you'd like to have a look at this. + + +# IRC, OFTC, #debian-hurd, 2012-06-19 + + <bdefreese> OK, how the heck do you get an undefined reference to mremap? + <youpi> simply because we don't have it + <pinotree> mremap exists only on linux + <bdefreese> It's in sys/mman.h + <pinotree> on linux? + <bdefreese> No, on GNU/Hurd + <bdefreese> /usr/include/i386-gnu/sys/mman.h + <youpi> that's just the common file with linux + <youpi> containing just the prototype + <youpi> that doesn't mean there's an implementation behind + <pinotree> youpi: hm no, linux has an own version + <youpi> uh + <bdefreese> Ah, aye, I didn't look at the implementation.. :( + <youpi> it's then odd that it was added to the generic sys/mman.h :) + <bdefreese> Just another stub? + <pinotree> ah, only few linux archs have own versions + <youpi> for the macro values I guess + <pinotree> http://paste.debian.net/175173/ on glibc/master + <bdefreese> Hmm, so where is MREMAP_MAYMOVE coming in from? + <youpi> rgrep on a linux box ;) + <youpi> <bits/mman.h> + <youpi> but that's again linuxish + <bdefreese> Aye but with us having that in the header it is causing some + code to be run which utilizes mremap. If that wasn't defined we wouldn't + be calling it. + <youpi> ah + <youpi> we could try to remove it indeed + <bdefreese> Should I change the code to #ifdef MREMAP_MAYMOVE & !defined + __GNU__? + <youpi> no, I said we could remove the definition of MREMAP_MAYMOVE itself diff --git a/open_issues/gnumach_i686.mdwn b/open_issues/gnumach_i686.mdwn new file mode 100644 index 00000000..b34df73b --- /dev/null +++ b/open_issues/gnumach_i686.mdwn @@ -0,0 +1,26 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach]] + + +# IRC, freenode, #hurd, 2012-07-05 + + <braunr> we could use a gnumach-i686 too + <pinotree> how would you compile gnumach as i686 variant btw? add + -march=.. or something like that in CFLAGS? + <braunr> yes + <braunr> at least we'll get some cmovs :) + + +## IRC, freenode, #hurd, 2012-07-07 + + <braunr> it was rejected in the past because we didn't think it would bring + real performance benefit, but it actually may diff --git a/open_issues/gnumach_integer_overflow.mdwn b/open_issues/gnumach_integer_overflow.mdwn new file mode 100644 index 00000000..2166e591 --- /dev/null +++ b/open_issues/gnumach_integer_overflow.mdwn @@ -0,0 +1,17 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach]] + + +# IRC, freenode, #hurd, 2012-07-04 + + <braunr> yes, we have integer overflows on resident_page_count, but + luckily, the member is rarely used diff --git a/open_issues/gnumach_page_cache_policy.mdwn b/open_issues/gnumach_page_cache_policy.mdwn index 75fcdd88..6f51d713 100644 --- a/open_issues/gnumach_page_cache_policy.mdwn +++ b/open_issues/gnumach_page_cache_policy.mdwn @@ -10,6 +10,11 @@ License|/fdl]]."]]"""]] [[!tag open_issue_gnumach]] +[[!toc]] + + +# [[page_cache]] + # IRC, freenode, #hurd, 2012-04-26 @@ -33,3 +38,587 @@ License|/fdl]]."]]"""]] have either lots of free pages because tha max limit is reached, or lots of pressure and system freezes :/ <youpi> yes + + +## IRC, freenode, #hurd, 2012-06-17 + + <braunr> youpi: i don't understand your patch :/ + <youpi> arf + <youpi> which part don't you understand? + <braunr> the global idea :/ + <youpi> first, drop the limit on number of objects + <braunr> you added a new collect call at pageout time + <youpi> (i.e. here, hack overflow into 0) + <braunr> yes + <braunr> obviously + <youpi> but then the cache keeps filling up with objects + <youpi> which sooner or later become empty + <youpi> thus the collect, which is supposed to look for empty objects, and + just drop them + <braunr> but not at the right time + <braunr> objects should be collected as soon as their ref count drops to 0 + <braunr> err + <youpi> now, the code of the collect is just a crude attempt without + knowing much about the vm + <braunr> when their resident page count drops to 0 + <youpi> so don't necessarily read it :) + <braunr> ok + <braunr> i've begin playing with the vm recently + <braunr> the limits (arbitrary, and very old obviously) seem far too low + for current resources + <braunr> (e.g. the threshold on free pages is 50 iirc ...) + <youpi> yes + <braunr> i'll probably use a different approach + <braunr> the one i mentioned (collecting one object at a time - or pushing + them on a list for bursts - when they become empty) + <braunr> this should relax the kernel allocator more + <braunr> (since there will be less empty vm_objects remaining until the + next global collecttion) + + +## IRC, freenode, #hurd, 2012-06-30 + + <braunr> the threshold values of the page cache seem quite enough actually + <youpi> braunr: ah + <braunr> youpi: yes, it seems the problems are in ext2, not in the VM + <youpi> k + <youpi> the page cache limitation still doesn't help :) + <braunr> the problem in the VM is the recycling of vm_objects, which aren't + freed once empty + <braunr> but it only wastes some of the slab memory, it doesn't prevent + correct processing + <youpi> braunr: thus the limitation, right? + <braunr> no + <braunr> well + <braunr> that's the policy they chose at the time + <braunr> for what reason .. i can't tell + <youpi> ok, but I mean + <youpi> we can't remove the policy because of the non-free of empty objects + <braunr> we must remove vm_objects at some point + <braunr> but even without it, it makes no sense to disable the limit while + ext2 is still unstable + <braunr> also, i noticed that the page count in vm_objects never actually + drop to 0 ... + <youpi> you mean the limit permits to avoid going into the buggy scenarii + too often? + <braunr> yes + <youpi> k + <braunr> at least, that's my impression + <braunr> my test case is tar xf files.tar.gz, which contains 50000 files of + 12k random data + <braunr> i'll try with other values + <braunr> i get crashes, deadlocks, livelocks, and it's not pretty :) + <braunr> and always in ext2, mach doesn't seem affected by the issue, other + than the obvious + <braunr> (well i get the usual "deallocating an invalid port", but as + mentioned, it's "most probably a bug", which is the case here :) + <youpi> braunr: looks coherent with the hangs I get on the buildds + <braunr> youpi: so that's the nasty bug i have to track now + <youpi> though I'm also still getting some out of memory from gnumach + sometimes + <braunr> the good thing is i can reproduce it very quickly + <youpi> a dump from the allocator to know which zone took all the room + might help + <braunr> youpi: yes i promised that too + <youpi> although that's probably related with ext2 issues :) + <braunr> youpi: can you send me the panic message so i can point the code + which must output the allocator state please ? + <youpi> next time I get it, sure :) + <pinotree> braunr: you could implement a /proc/slabinfo :) + <braunr> pinotree: yes but when a panic happens, it's too late + <braunr> http://git.sceen.net/rbraun/slabinfo.git/ btw + <braunr> although it's not part of procfs + <braunr> and the mach_debug interface isn't provided :( + + +## IRC, freenode, #hurd, 2012-07-03 + + <braunr> it looks like pagers create a thread per memory object ... + <antrik> braunr: oh. so if I open a lot of files, ext2fs will *inevitably* + have lots of threads?... + <braunr> antrik: i'm not sure + <braunr> it may only be required to flush them + <braunr> but when there are lots of them, the threads could run slowly, + giving the impression there is one per object + <braunr> in sync mode i don't see many threads + <braunr> and i don't get the bug either for now + <braunr> while i can see physical memory actually being used + <braunr> (and the bug happens before there is any memory pressure in the + kernel) + <braunr> so it definitely looks like a corruption in ext2fs + <braunr> and i have an idea .... :> + <braunr> hm no, i thought an alloca with a big size parameter could erase + memory outside the stack, but it's something else + <braunr> (although alloca should really be avoided) + <braunr> arg, the problem seems to be in diskfs_sync_everything -> + ports_bucket_iterate (pager_bucket, sync_one); :/ + <braunr> :( + <braunr> looks like the ext2 problem is triggered by calling pager_sync + from diskfs_sync_everything + <braunr> and is possibly related to + http://lists.gnu.org/archive/html/bug-hurd/2010-03/msg00127.html + <braunr> (and for reference, the rest of the discussion + http://lists.gnu.org/archive/html/bug-hurd/2010-04/msg00012.html) + <braunr> multithreading in libpager is scary :/ + <antrik> braunr: s/in libpager/ ;-) + <braunr> antrik: right + <braunr> omg the ugliness :/ + <braunr> ok i found a bug + <braunr> a real one :) + <braunr> (but not sure it's the only one since i tried that before) + <braunr> 01:38 < braunr> hm no, i thought an alloca with a big size + parameter could erase memory outside the stack, but it's something else + <braunr> turns out alloca is sometimes used for 64k+ allocations + <braunr> which explains the stack corruptions + <pinotree> ouch + <braunr> as it's used to duplicate the node table before traversing it, it + also explains why the cache limit affects the frequency of the bug + <braunr> now the fun part, write the patch following GNU protocol .. :) + +[[!message-id "1341350006-2499-1-git-send-email-rbraun@sceen.net"]] + + <braunr> if someone feels like it, there are a bunch of alloca calls in the + hurd (like around 30 if i'm right) + <braunr> most of them look safe, but some could trigger that same problem + in other servers + <braunr> ok so far, no problem with the upstream ext2fs code :) + <braunr> 20 loops of tar xf / rm -rf consuming all free memory as cache :) + <braunr> the hurd uses far too much cpu time for no valid reason in many + places :/ + * braunr happy + <braunr> my hurd is completely using its ram :) + <gnu_srs> Meaning, the bug is solved? Congrats if so :) + <braunr> well, ext2fs looks way more stable now + <braunr> i haven't had a single issue since the change, so i guess i messed + something with my previous test + <braunr> and the Mach VM cache implementation looks good enough + <braunr> now the only thing left is to detect unused objects and release + them + <braunr> which is actually the core of my work :) + <braunr> but i'm glad i could polish ext2fs + <braunr> with luck, this is the issue that was striking during "thread + storms" in the past + * pinotree hugs braunr + <braunr> i'm also very happy to see the slab allocator reacting well upon + memory pressure :> + <mcsim> braunr: Why alloca corrupted memory diskfs_node_iterate? Was + temporary node to big to keep it in stack? + <braunr> mcsim: yes + <braunr> 17:54 < braunr> turns out alloca is sometimes used for 64k+ + allocations + <braunr> and i wouldn't be surprised if our thread stacks are + simplecontiguous 64k mappings of zero-filled memory + <braunr> (as Mach only provides bottom-up allocation) + <braunr> our thread implementation should leave unmapped areas between + thread stacks, to easily catch such overflows + <pinotree> braunr: wouldn't also fatfs/inode.c and tmpfs/node.c need the + same fix? + <braunr> pinotree: possibly + <braunr> i haven't looked + <braunr> more than 300 loops of tar xf / rm -rf on an archive of 20000 + files of 12 KiB each, without any issue, still going on :) + <youpi> braunr: yay + + +## [[!message-id "20120703121820.GA30902@mail.sceen.net"]], 2012-07-03 + + +## IRC, freenode, #hurd, 2012-07-04 + + <braunr> mach is so good it caches objects which *no* page in physical + memory + <braunr> hm i think i have a working and not too dirty vm cache :> + <kilobug> braunr: congrats :) + <braunr> kilobug: hey :) + <braunr> the dangerous side effect is the increased swappiness + <braunr> we'll have to monitor that on the buildds + <braunr> otherwise the cache is effectively used, and the slab allocator + reports reasonable amounts of objects, not increasing once the ram is + full + <braunr> let's see what happens with 1.8 GiB of RAM now + <braunr> damn glibc is really long to build :) + <braunr> and i fear my vm cache patch makes non scalable algorithms negate + some of its benefits :/ + <braunr> 72 tasks, 2090 threads + <braunr> we need the ability to monitor threads somewhere + + +## IRC, freenode, #hurd, 2012-07-05 + + <braunr> hm i get kernel panics when not using the host cache :/ + <braunr> no virtual memory for stack allocations + <braunr> that's scary + <antrik> ? + <braunr> i guess the lack of host cache makes I/O slow enough to create a + big thread storm + <braunr> that completely exhausts the kernel space + <braunr> my patch challenges scalability :) + <antrik> and not having a zalloc zone anymore, instead of getting a nice + panic when trying to allocate yet another thread, you get an address + space exhaustion on an unrelated event instead. I see ;-) + <braunr> thread stacks are not allocated from a zone/cache + <braunr> also, the panic concerned aligned memory, but i don't think that + matters + <braunr> the kernel panic clearly mentions it's about thread stack + allocation + <antrik> oh, by "stack allocations" you actually mean allocating a stack + for a new thread... + <braunr> yes + <antrik> that's not what I normally understand when reading "stack + allocations" :-) + <braunr> user stacks are simple zero filled memory objects + <braunr> so we usually get a deadlock on them :> + <braunr> i wonder if making ports_manage_port_operations_multithread limit + the number of threads would be a good thing to do + <antrik> braunr: last time slpz did that, it turned out that it causes + deadlocks in at least one (very specific) situation + <braunr> ok + <antrik> I think you were actually active at the time slpz proposed the + patch (and it was added to Debian) -- though probably not at the time + where youpi tracked it down as the cause of certain lockups, so it was + dropped again... + <braunr> what seems very weird though is that we're normally using + continuations + <antrik> braunr: you mean in the kernel? how is that relevant to the topic + at hand?... + <braunr> antrik: continuations have been designed to reduce the number of + stacks to one per cpu :/ + <braunr> but they're not used everywhere + <antrik> they are not used *anywhere* in the Hurd... + <braunr> antrik: continuations are supposed to be used by kernel code + <antrik> braunr: not sure what you are getting at. of course we should use + some kind of continuations in the Hurd instead of having an active thread + for every single request in flight -- but that's not something that could + be done easily... + <braunr> antrik: oh no, i don't want to use continuations at all + <braunr> i just want to use less threads :) + <braunr> my panic definitely looks like a thread storm + <braunr> i guess increasing the kmem_map will help for the time bein + <braunr> g + <braunr> (it's not the whole kernel space that gets filled up actually) + <braunr> also, stacks are kept on a local cache until there is memory + pressure oO + <braunr> their slab cache can fill the backing map before there is any + pressure + <braunr> and it makes a two level cache, i'll have to remove that + <antrik> well, how do you reduce the number of threads? apart from + optimising scheduling (so requests are more likely to be completed before + new ones are handled), the only way to reduce the number of threads is to + avoid having a thread per request + <braunr> exactly + <antrik> so instead the state of each request being handled has to be + explicitly stored... + <antrik> i.e. continuations + <braunr> hm actually, no + <braunr> you use thread migration :) + <braunr> i don't want to artificially use the number of kernel threads + <braunr> the hurd should be revamped not to use that many threads + <braunr> but it looks like a hard task + <antrik> well, thread migration would reduce the global number of threads + in the system... it wouldn't prevent a server from having thousands of + threads + <braunr> threads would allready be allocated before getting in the server + <antrik> again, the only way not to use a thread for each outstanding + request is having some explicit request state management, + i.e. continuations + <braunr> hm right + <braunr> but we can nonetheless reduce the number of threads + <braunr> i wonder if the sync threads are created on behalf of the pagers + or the kernel + <braunr> one good thing is that i can already feel better performance + without using the host cache until the panic happens + <antrik> the tricky bit about that is that I/O can basically happen at any + point during handling a request, by hitting a page fault. so we need to + be able to continue with some other request at any point... + <braunr> yes + <antrik> actually, readahead should help a lot in reducing the number of + request and thus threads... still will be quite a lot though + <braunr> we should have a bunch of pageout threads handling requests + asynchronously + <braunr> it depends on the implementation + <braunr> consider readahead detects that, in the next 10 pages, 3 are not + resident, then 1 is, then 3 aren't, then 1 is again, and the last two + aren't + <braunr> how is this solved ? :) + <braunr> about the stack allocation issue, i actually think it's very + simple to solv + <braunr> the code is a remnant of the old BSD days, when processes were + heavily swapped + <braunr> so when a thread is created, its stack isn't allocated + <braunr> the allocation happens when the thread is dispatched, and the + scheduler finds it's swapped (which is the initial state) + <braunr> the stack is allocated, and the operation is assumed to succeed, + which is why failure produces a panic + <antrik> well, actually, not just readahead... clustered paging in + general. the thread storms happen mostly on write not read AIUI + <braunr> changing that to allocate at thread creation time will allow a + cleaner error handling + <braunr> antrik: yes, at writeback + <braunr> antrik: so i guess even when some physical pages are already + present, we should aim at larger sizes for fewer I/O requests + <antrik> not sure that would be worthwhile... probably doesn't happen all + that often. and if some of the pages are dirty, we would have to make + sure that they are ignored although they were part of the request... + <braunr> yes + <braunr> so one request per missing area ? + <antrik> the opposite might be a good idea though -- if every other page is + dirty, it *might* indeed be preferable to do a single request rewriting + even the clean ones in between... + <braunr> yes + <braunr> i personally think one request, then replace only what was + missing, is simpler and preferable + <antrik> OTOH, rewriting clean pages might considerably increase write time + (and wear) on SSDs + <braunr> why ? + <antrik> I doubt the controller is smart enough to recognies if a page + doesn't really need rewriting + <antrik> so it will actually allocate and write a new cluster + <braunr> no but it won't spread writes on different internal sectors, will + it ? + <braunr> sectors are usually really big + <antrik> "sectors" is not a term used in SSDs :-) + <braunr> they'll be erased completely whatever the amount of data at some + point if i'm right + <braunr> ah + <braunr> need to learn more about that + <braunr> i thought their internal hardware was much like nand flash + <antrik> admittedly I don't remember the correct terminology either... + <antrik> they *are* NAND flash + <antrik> writing is actually not the problem -- it can happen in small + chunks. the problem is erasing, which is only possible in large blocks + <braunr> yes + <braunr> so having larger requests doesn't seem like a problem to me + <braunr> because of that + <antrik> thus smart controllers (which pretty much all SSD nowadays have, + and apparently even SD cards) do not actually overwrite. instead, writes + always happen to clean portions, and erasing only happens when a block is + mostly clean + <antrik> (after relocating the remaining used parts to other clean areas) + <antrik> braunr: the problem is not having larger requests. the problem is + rewriting clusters that don't really need rewriting. it means the dist + performs unnecessary writing actions. + <antrik> it doesn't hurt for magnetic disks, as the head has to pass over + the unchanged sectors anyways; and rewriting the unnecessarily doesn't + increase wear + <antrik> but it's different for SSDs + <antrik> each write has a penalty there + <braunr> i thought only erases were the real penalty + <antrik> well, erase happens in the background with modern controllers; so + it has no direct penalty. the write has a direct performance penalty when + saturating the bandwith, and always has a direct wear penalty + <braunr> can't controllers handle 32k requests ? like everything does ? :/ + <antrik> sure they can. but that's beside the point... + <braunr> if they do, they won't mind the clean data inside such large + blocks + <antrik> apparently we are talking past each other + <braunr> i must be missing something important about SSD + <antrik> braunr: the point is, the controller doesn't *know* it's clean + data; so it will actually write it just like the really unclean data + <braunr> yes + <braunr> and it will choose an already clean sector for that (previously + erased), so writing larger blocks shouldn't hurt + <braunr> there will be a slight increase in bandwidth usage, but that's + pretty much all of it + <braunr> isn't it ? + <antrik> well, writing always happens to clean blocks. but writing more + blocks obviously needs more time, and causes more wear... + <braunr> aiui, blocks are always far larger than the amount of pages we + want to writeback in one request + <braunr> the only way to use more than one is crossing a boundary + <antrik> no. again, the blocks that can be *written* are actually quite + small. IIRC most SSDs use 4k nowadays + <braunr> ok + <antrik> only erasing operates on much larger blocks + <braunr> so writing is a problem too + <braunr> i didn't think it would cause wear leveling to happen + <antrik> well, I'm not sure whether the wear actually happens on write or + on erase... but that doesn't matter, as the number of blocks that need to + be erased is equivalent to the number of blocks written... + <braunr> sorry, i'm really not sure + <braunr> if you erase one sector, then write the first and third block, + it's clearly not equivalent + <braunr> i mean + <braunr> let's consider two kinds of pageout requests + <braunr> 1/ a big one including clean pages + <braunr> 2/ several ones for dirty pages only + <braunr> let's assume they both need an erase when they happen + <braunr> what's the actual difference between them ? + <braunr> wear will increase only if the controller handle it on writes, if + i'm right + <braunr> but other than that, it's just bandwidth + <antrik> strictly speaking erase is only *necessary* when there are no + clean blocks anymore. but modern controllers will try to perform erase of + unused blocks in the background, so it doesn't delay actual writes + <braunr> i agree on that + <antrik> but the point is that for each 16 pages (or so) written, we need + to erase one block so we get 16 clean pages to write... + <braunr> yes + <braunr> which is about the size of a request for the sequential policy + <braunr> so it fits + <antrik> just to be clear: it doesn't matter at all how the pages + "fit". the controller will reallocate them anyways + <antrik> what matters is how many pages you write + <braunr> ah + <braunr> i thought it would just put the whole request in a single sector + (or two) + <antrik> I'm not sure what you mean by "sector". as I said, it's not a term + used in SSD technology + <braunr> so do you imply that writes can actually get spread over different + sectors ? + <braunr> the sector is the unit at the nand flash level, its size is the + erase size + <antrik> actually, I used the right terminology... the erase unit is the + block; the write unit is the page + <braunr> sector is a synonym of block + <antrik> never seen it. and it's very confusing, as it isn't in any way + similar to sectors in magnetic disks... + <braunr> http://en.wikipedia.org/wiki/Flash_memory#NAND_flash + <braunr> it's actually in the NOR part right before, paragraph "Erasing" + <braunr> "Modern NOR flash memory chips are divided into erase segments + (often called blocks or sectors)." + <antrik> ah. I skipped the NOR part :-) + <braunr> i've only heard sector where i worked, but i don't consider french + computer engineers to be authorities on the matter :) + <antrik> hehe + <braunr> let's call them block + <braunr> so, thread stacks are allocated out of the kernel map + <braunr> this is already a bad thing (which is probably why there is a + local cache btw) + <antrik> anyways, yes. modern controllers might split a contiguous write + request onto several blocks, as well as put writes to completely + different logical pages into one block. the association between addresses + and actual blocks is completely free + <braunr> now i wonder why the kernel map is so slow, as the panic happens + at about 3k threads, so about 11M of thread stacks + <braunr> antrik: ok + <braunr> antrik: well then it makes sense to send only dirty pages + <braunr> s/slow/low/ + <antrik> it's different for raw flash (using MTD subsystem in Linux) -- but + I don't think this is something we should consider any time soon :-) + <antrik> (also, raw flash is only really usable with specialised + filesystems anyways) + <braunr> yes + <antrik> are the thread stacks really only 4k? I would expect them to be + larger in many cases... + <braunr> youpi reduced them some time ago, yes + <braunr> they're 4k on xen + <braunr> uh, 16k + <braunr> damn, i'm wondering why i created separate submaps for the slab + allocator :/ + <braunr> probably because that's how it was done by the zone allocator + before + <braunr> but that's stupid :/ + <braunr> hm the stack issue is actually more complicated than i thought + because of interrupt priority levels + <braunr> i increased the kernel map size to avoid the panic instead + <braunr> now libc0.3 seems to build fine + <braunr> and there seems to be a clear decrease of I/O :) + + +### IRC, freenode, #hurd, 2012-07-06 + + <antrik> braunr: there is a submap for the slab allocator? that's strange + indeed. I know we talked about this; and I am pretty sure we agreed + removing the submap would actually be among the major benefits of a new + allocator... + <braunr> antrik: a submap is a good idea anyway + <braunr> antrik: it avoids fragmenting the kernel space too much + <braunr> it also breaks down locking + <braunr> but we could consider it + <braunr> as a first step, i'll merge the kmem and kalloc submaps (the ones + used for the slab caches and the malloc-like allocations respectively) + <braunr> then i'll change the allocation of thread stacks to use a slab + cache + <braunr> and i'll also remove the thread swapping stuff + <braunr> it will take some time, but by the end we should be able to + allocate tens of thousands of threads, and suffer no panic when the limit + is reached + <antrik> braunr: I'm not sure "no panic" is really a worthwhile goal in + such a situation... + <braunr> antrik: uh ?N + <braunr> antrik: it only means the system won't allow the creation of + threads until there is memory available + <braunr> from my pov, the microkernel should never fail up to a point it + can't continue its job + <antrik> braunr: the system won't be able to recover from such a situation + anyways. without actual resource management/priorisation, not having a + panic is not really helpful. it only makes it harder to guess what + happened I fear... + <braunr> i don't see why it couldn't recover :/ + + +## IRC, freenode, #hurd, 2012-07-07 + + <braunr> grmbl, there are a lot of issues with making the page cache larger + :( + <braunr> it actually makes the system slower in half of my tests + <braunr> we have to test that on real hardware + <braunr> unfortunately my current results seem to indicate there is no + clear benefit from my patch + <braunr> the current limit of 4000 objects creates a good balance between + I/O and cpu time + <braunr> with the previous limit of 200, I/O is often extreme + <braunr> with my patch, either the working set is less than 4k objects, so + nothing is gained, or the lack of scalability of various parts of the + system add overhead that affect processing speed + <braunr> also, our file systems are cached, but our block layer isn't + <braunr> which means even when accessing data from the cache, accesses + still cause some I/O for metadata + + +## IRC, freenode, #hurd, 2012-07-08 + + <braunr> youpi: basically, it works fine, but exposes scalability issues, + and increases swapiness + <youpi> so it doens't help with stability? + <braunr> hum, that was never the goal :) + <braunr> the goal was to reduce I/O, and increase performance + <youpi> sure + <youpi> but does it at least not lower stability too much? + <braunr> not too much, no + <youpi> k + <braunr> most of the issues i found could be reproduced without the patch + <youpi> ah + <youpi> then fine :) + <braunr> random deadlocks on heavy loads + <braunr> youpi: but i'm not sure it helps with performance + <braunr> youpi: at least not when emulated, and the host cache is used + <youpi> that's not very surprising + <braunr> it does help a lot when there is no host cache and the working set + is greater (or far less) than 4k objects + <youpi> ok + <braunr> the amount of vm_object and ipc_port is gracefully adjusted + <youpi> that'd help us with not having to tell people to use the complex + -drive option :) + <braunr> so you can easily run a hurd with 128 MiB with decent performance + and no leak in ext2fs + <braunr> yes + <braunr> for example + <youpi> braunr: I'd say we should just try it on buildds + <braunr> (it's not finished yet, i'd like to work more on reducing + swapping) + <youpi> (though they're really not busy atm, so the stability change can't + really be measured) + <braunr> when building the hurd, which takes about 10 minutes in my kvm + instances, there is only a 30 seconds difference between using the host + cache and not using it + <braunr> this is already the case with the current kernel, since the + working set is less than 4k objects + <braunr> while with the previous limit of 200 objects, it took 50 minutes + without host cache, and 15 with it + <braunr> so it's a clear benefit for most uses, except my virtual machines + :) + <youpi> heh + <braunr> because there, the amount of ram means a lot of objects can be + cached, and i can measure an increase in cpu usage + <braunr> slight, but present + <braunr> youpi: isn't it a good thing that buildds are resting a bit ? :) + <youpi> on one hand, yes + <youpi> but on the other hand, that doesn't permit to continue + stress-testing the Hurd :) + <braunr> we're not in a hurry for this patch + <braunr> because using it really means you're tickling the pageout daemon a + lot :) + + +## [[metadata_caching]] diff --git a/open_issues/gnumach_tick.mdwn b/open_issues/gnumach_tick.mdwn new file mode 100644 index 00000000..eed447f6 --- /dev/null +++ b/open_issues/gnumach_tick.mdwn @@ -0,0 +1,35 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach]] + + +# IRC, freenode, #hurd, 2012-07-05 + + <pinotree> braunr: wrt to mach: it seems to me it ticks every 10ms or so, + it is true? + <braunr> yes + <braunr> and it's not preemptible + <pinotree> braunr: that means a gnumach kernel currently has a maximum + uptime of almost 500 days + <braunr> pinotree: what do you mean ? + <pinotree> there's an int (or uint, i don't remember) variable that keeps + the tick count + <braunr> yes the tick variable should probably be a 64-bits type + <braunr> or a struct + <braunr> but the tick count should only be used for computation on "short" + delays + <braunr> and it should be safe to use it even when it overflows + <braunr> it's not the wall clock + <pinotree> i found that when investigating why the maximum timeout for a + mach_msg is like INT_MAX >> 2 (or 4) or something like that, also due to + the tick count + <braunr> iirc, in linux, they mostly use the lower 32-bits on 32-bits + architecture, updating the 32 upper only when necessary diff --git a/open_issues/gnumach_vm_map_red-black_trees.mdwn b/open_issues/gnumach_vm_map_red-black_trees.mdwn index 17263099..d7407bfe 100644 --- a/open_issues/gnumach_vm_map_red-black_trees.mdwn +++ b/open_issues/gnumach_vm_map_red-black_trees.mdwn @@ -152,3 +152,23 @@ License|/fdl]]."]]"""]] entries) [[glibc/fork]]. + + +## IRC, freenode, #hurdfr, 2012-06-02 + + <youpi> braunr: oh, un bug de rbtree + <youpi> Assertion `diff != 0' failed in file "vm/vm_map.c", line 1002 + <youpi> c'est dans rbtree_insert() + <youpi> vm_map_enter (vm/vm_map.c:1002). + <youpi> vm_map (vm/vm_user.c:373). + <youpi> syscall_vm_map (kern/ipc_mig.c:657). + <youpi> erf j'ai tué mon débuggueur, je ne peux pas inspecter plus + <youpi> le peu qui me reste c'est qu'apparemment target_map == 1, size == + 0, mask == 0 + <youpi> anywhere = 1 + <braunr> youpi: ça signifie sûrement que des adresses overlappent + <braunr> je rejetterai un coup d'oeil sur le code demain + <braunr> (si ça se trouve c'est un bug rare de la vm, le genre qui fait + crasher le noyau) + <braunr> (enfin jveux dire, qui faisait crasher le noyau de façon très + obscure avant le patch rbtree) diff --git a/open_issues/gnumach_vm_object_resident_page_count.mdwn b/open_issues/gnumach_vm_object_resident_page_count.mdwn new file mode 100644 index 00000000..cc1b8897 --- /dev/null +++ b/open_issues/gnumach_vm_object_resident_page_count.mdwn @@ -0,0 +1,22 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach]] + + +# IRC, freenode, #hurd, 2012-07-03 + + <braunr> omg the ugliness + <braunr> the number of pages in physical memory for on object is a short + ... which limits the amount to .. 128 MiB + * braunr cries + <braunr> luckily, this should be easy to solve + +`vm/vm_object.h:vm_object:resident_page_count`. diff --git a/open_issues/libpthread_CLOCK_MONOTONIC.mdwn b/open_issues/libpthread_CLOCK_MONOTONIC.mdwn index f9195540..2c8f10f8 100644 --- a/open_issues/libpthread_CLOCK_MONOTONIC.mdwn +++ b/open_issues/libpthread_CLOCK_MONOTONIC.mdwn @@ -15,7 +15,7 @@ License|/fdl]]."]]"""]] [[!message-id "201204220058.37328.toscano.pino@tiscali.it"]] -# IRC, freenode, #hurd- 2012-04-22 +# IRC, freenode, #hurd, 2012-04-22 <pinotree> youpi: what i thought would be creating a glib/hurd/hurdtime.{c,h}, adding _hurd_gettimeofday and @@ -34,7 +34,7 @@ License|/fdl]]."]]"""]] <youpi> (and others) -## IRC, freenode, #hurd- 2012-04-23 +## IRC, freenode, #hurd, 2012-04-23 <youpi> pinotree: about librt vs libpthread, don't worry too much for now <youpi> libpthread can lib against the already-installed librt @@ -56,3 +56,23 @@ License|/fdl]]."]]"""]] at all <youpi> pinotree: yes, things work even with -lrt <pinotree> wow + + +## IRC, OFTC, #debian-hurd, 2012-06-04 + + <youpi> pinotree: -lrt in libpthread is what is breaking glib2.0 + <youpi> during ./configure it makes clock_gettime linked in, while at + library link it doesn't + <youpi> probably for obscure reasons + <youpi> I'll have to disable it in debian + + +### IRC, OFTC, #debian-hurd, 2012-06-05 + + <pinotree> youpi: i saw your message about the linking issues with + pthread/rt; do you want me to provide a patch to switch clock_gettime → + gettimeofday in libpthread? + <youpi> you mean switch it back as it was previously? + <pinotree> kind of, yes + <youpi> I have reverted the change in libc for now + <pinotree> ok diff --git a/open_issues/low_memory.mdwn b/open_issues/low_memory.mdwn new file mode 100644 index 00000000..22470c65 --- /dev/null +++ b/open_issues/low_memory.mdwn @@ -0,0 +1,113 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach open_issue_glibc open_issue_hurd]] + +Issues relating to system behavior under memory pressure. + +[[!toc]] + + +# [[gnumach_page_cache_policy]] + + +# IRC, freenode, #hurd, 2012-07-08 + + <braunr> am i mistaken or is the default pager simply not vm privileged ? + <braunr> (which would explain the hangs when memory is very low) + <youpi> no idea + <youpi> but that's very possible + <youpi> we start it by hand from the init scripts + <braunr> actually, i see no way provided by mach to set that + <braunr> i'd assume it would set the property when a thread would register + itself as the default pager, but it doesn't + <braunr> i'll check at runtime and see if fixing helps + <youpi> thread_wire(host, thread, 1) ? + <youpi> ./hurd/mach-defpager/wiring.c: kr = + thread_wire(priv_host_port, + <braunr> no + <braunr> look in cprocs.c + <braunr> iir + <braunr> iirc + <braunr> iiuc, it sets a 1:1 kernel/user mapping + <youpi> ?? + <youpi> thread_wire, not cthread_wire + <braunr> ah + <braunr> right, i'm getting tired + <braunr> youpi: do you understand the comment in default_pager_thread() ? + <youpi> well, I'm not sure to know what external vs internal is + <braunr> i'm almost sure the default pager is blocked because of a relation + with an unprivlege thread + <braunr> +d + <braunr> when hangs happen, the pageout daemon is still running, waiting + for an event so he can continue + <braunr> it* + + <braunr> all right, our pageout stuff completely sucks + <braunr> when you think the system is hanged, it's actually not + <pinotree> and what's happening instead? + <braunr> instead, it seems it's in a very complex resursive state which + ends in the slab allocator not being able to allocate kernel map entries + <braunr> recursive* + <braunr> the pageout daemon, unable to continue, progressively slows + <braunr> in hope the default pager is able to service the pageout requests, + but it's not + <braunr> probably the most complicated deadlock i've seen :) + <braunr> luckily ! + <braunr> i've been playing with some tunables involved in waking up the + pageout daemon + <braunr> and got good results so far + <braunr> (although it's clearly not a proper solution) + <braunr> one thing the kernel lacks is a way to separate clean from dirty + pages + <braunr> this stupid kernel doesn't try to free clean pages first .. :) + <braunr> hm + <braunr> now i can see the system recover, but some applications are still + stuck :( + <braunr> (but don't worry, my tests are rather aggressive) + <braunr> what i mean by aggressive is several builds and various dd of a + few hundred MiB in parallel, on various file systems + <braunr> so far the file systems have been very resilient + <braunr> ok, let's try running the hurd with 64 MiB of RAM + <braunr> after some initial swapping, it runs smoothly :) + <braunr> uh ? + <braunr> ah no, i'm still doing my parallel builds + <braunr> although less + <braunr> gcc: internal compiler error: Resource lost (program as) + <braunr> arg + <braunr> lol + <braunr> the file system crashed under the compiler + <pinotree> too much memory required during linking? or ram+swap should have + been enough? + <braunr> there is a lot of swap, i doubt it + <braunr> the hurd is such a dumb and impressive system at the same time + <braunr> pinotree: what does this tell you ? + <braunr> git: hurdsig.c:948: post_signal: Unexpected error: (os/kern) + failure. + <pinotree> something samuel spots often during the builds of haskell + packages + +Probably also the *sigpost* case mentioned in [[!message-id +"87bol6aixd.fsf@schwinge.name"]]. + + <braunr> actually i should be asking jkoenig + <braunr> it seems the lack of memory has a strong impact on signal delivery + <braunr> which is bad + <antrik> braunr: I have a vague recollection of slpz also saying something + about missing dirty page tracking a while back... I might be confusing + stuff though + <braunr> pinotree: yes it happens often during links + <braunr> which makes sense + <pinotree> braunr: "happens often" == "hurdsig.c:948: post_signal: ..."? + <braunr> yes + <pinotree> if you can reproduce it often, what about debugging it? :P + <braunr> i mean, the few times i got it, it was often during a link :p + <braunr> i'd rather debug the pageout deadlock :( + <braunr> but it's hard diff --git a/open_issues/mach-defpager_swap.mdwn b/open_issues/mach-defpager_swap.mdwn new file mode 100644 index 00000000..7d3b001c --- /dev/null +++ b/open_issues/mach-defpager_swap.mdwn @@ -0,0 +1,20 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach]] + +[[!toc]] + + +# IRC, OFTC, #debian-hurd, 2012-06-16 + + <lifeng> I allocated a 5GB partition as swap, but hurd only found 1GB + <youpi> use 2GiB swaps only, >2Gib are not supported + <youpi> (and apparently it just truncates the size, to be investigated) diff --git a/open_issues/metadata_caching.mdwn b/open_issues/metadata_caching.mdwn new file mode 100644 index 00000000..f7f4cb53 --- /dev/null +++ b/open_issues/metadata_caching.mdwn @@ -0,0 +1,31 @@ +[[!meta copyright="Copyright © 2011, 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_gnumach open_issue_hurd]] + +[[!toc]] + + +# IRC, freenode, #hurd, 2012-07-08 + + <braunr> youpi: there is still quite a lot of I/O even for cached objects + <braunr> youpi: i strongly suspect these are for the metadata + <braunr> i.e. we don't have a "buffer cache", only a file cache + <braunr> (gnu is really not unix lol) + <youpi> doesn't ext2fs cache these? + <youpi> (as long as the corresponding object is cached + <youpi> ) + <braunr> i didn't look too much, but if it does, it does a bad job + <braunr> i would guess it does, but possibly only writethrough + <youpi> iirc it does writeback + <youpi> there's a sorta "node needs written" flag somewhere iirc + <braunr> but that's for the files, not the metadata + <youpi> I mean the metadata of the node + <braunr> then i have no idea what happens diff --git a/open_issues/multithreading.mdwn b/open_issues/multithreading.mdwn index 0f6b9f19..5924d3f9 100644 --- a/open_issues/multithreading.mdwn +++ b/open_issues/multithreading.mdwn @@ -1,4 +1,5 @@ -[[!meta copyright="Copyright © 2010, 2011 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2010, 2011, 2012 Free Software Foundation, +Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this @@ -36,6 +37,18 @@ Control*](http://soft.vub.ac.be/~tvcutsem/talks/presentations/T37_nobackground.p Tom Van Cutsem, 2009. +## IRC, freenode, #hurd, 2012-07-08 + + <youpi> braunr: about limiting number of threads, IIRC the problem is that + for some threads, completing their work means triggering some action in + the server itself, and waiting for it (with, unfortunately, some lock + held), which never terminates when we can't create new threads any more + <braunr> youpi: the number of threads should be limited, but not globally + by libports + <braunr> pagers should throttle their writeback requests + <youpi> right + + # Alternative approaches: * <http://www.concurrencykit.org/> diff --git a/open_issues/nfs_trailing_slash.mdwn b/open_issues/nfs_trailing_slash.mdwn new file mode 100644 index 00000000..90f138e3 --- /dev/null +++ b/open_issues/nfs_trailing_slash.mdwn @@ -0,0 +1,36 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_glibc open_issue_hurd]] + + +# IRC, freenode, #hurd, 2012-05-27 + + <gg0> ok, on nfs "mkdir dir0" succeeds, "mkdir dir0/" fails. RPC struct is bad + + +## IRC, freenode, #hurd, 2012-05-27 + + <gg0> 150->dir_mkdir ("foo1/" 493) = 0x40000048 (RPC struct is bad) + <gg0> task2876->mach_port_deallocate (pn{ 18}) = 0 + <gg0> mkdir: 136->io_write_request ("mkdir: " -1) = 0 7 + <gg0> cannot create directory `/nfsroot/foo1/' 136->io_write_request + ("cannot create directory `/nfsroot/foo1/'" -1) = 0 40 + <gg0> : RPC struct is bad 136->io_write_request (": RPC struct is bad" -1) + = 0 19 + <gg0> 136->io_write_request (" + <gg0> " -1) = 0 1 + <tschwinge> gg0: Yes, I think we knew about this before. Nobody felt like + working on it yet. Might be a nfs, libnetfs, glibc issue. + <tschwinge> gg0: If you want to work on it, please ask here or on bug-hurd + if you need some guidance. + <gg0> yeah found this thread + http://lists.gnu.org/archive/html/bug-hurd/2008-04/msg00069.html I don't + think I'll work on it diff --git a/open_issues/page_cache.mdwn b/open_issues/page_cache.mdwn index 062fb8d6..fd503fdc 100644 --- a/open_issues/page_cache.mdwn +++ b/open_issues/page_cache.mdwn @@ -1,4 +1,4 @@ -[[!meta copyright="Copyright © 2011 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2011, 2012 Free Software Foundation, Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this @@ -10,7 +10,10 @@ License|/fdl]]."]]"""]] [[!tag open_issue_gnumach]] -IRC, freenode, #hurd, 2011-11-28: +[[!toc]] + + +# IRC, freenode, #hurd, 2011-11-28 <braunr> youpi: would you find it reasonable to completely disable the page cache in gnumach ? @@ -71,3 +74,6 @@ IRC, freenode, #hurd, 2011-11-28: <youpi> restarting them every few days is ok <youpi> so I'd rather keep the performance :) <braunr> ok + + +# [[gnumach_page_cache_policy]] diff --git a/open_issues/performance.mdwn b/open_issues/performance.mdwn index 2fd34621..8dbe1160 100644 --- a/open_issues/performance.mdwn +++ b/open_issues/performance.mdwn @@ -1,4 +1,5 @@ -[[!meta copyright="Copyright © 2010, 2011 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2010, 2011, 2012 Free Software Foundation, +Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this @@ -38,3 +39,16 @@ call|/glibc/fork]]'s case. * [[microbenchmarks]] * [[microkernel_multi-server]] + + * [[gnumach_page_cache_policy]] + + * [[metadata_caching]] + +--- + + +# IRC, freenode, #hurd, 2012-07-05 + + <braunr> the more i study the code, the more i think a lot of time is + wasted on cpu, unlike the common belief of the lack of performance being + only due to I/O diff --git a/open_issues/performance/io_system/read-ahead.mdwn b/open_issues/performance/io_system/read-ahead.mdwn index d6a98070..710c746b 100644 --- a/open_issues/performance/io_system/read-ahead.mdwn +++ b/open_issues/performance/io_system/read-ahead.mdwn @@ -16,6 +16,9 @@ License|/fdl]]."]]"""]] # [[community/gsoc/project_ideas/disk_io_performance]] +# [[gnumach_page_cache_policy]] + + # 2011-02 [[Etenil]] has been working in this area. @@ -389,3 +392,1176 @@ License|/fdl]]."]]"""]] with appropriate frame size. Is that right? <youpi> question of taste, better ask on the list <mcsim> ok + + +## IRC, freenode, #hurd, 2012-06-09 + + <mcsim> hello. What fictitious pages in gnumach are needed for? + <mcsim> I mean why real page couldn't be grabbed straight, but in sometimes + fictitious page is grabbed first and than converted to real? + <braunr> mcsim: iirc, fictitious pages are needed by device pagers which + must comply with the vm pager interface + <braunr> mcsim: specifically, they must return a vm_page structure, but + this vm_page describes device memory + <braunr> mcsim: and then, it must not be treated like normal vm_page, which + can be added to page queues (e.g. page cache) + + +## IRC, freenode, #hurd, 2012-06-22 + + <mcsim> braunr: Ah. Patch for large storages introduced new callback + pager_notify_evict. User had to define this callback on his own as + pager_dropweak, for instance. But neal's patch change this. Now all + callbacks could have any name, but user defines structure with pager ops + and supplies it in pager_create. + <mcsim> So, I just changed notify_evict to confirm it to new style. + <mcsim> braunr: I want to changed interface of mo_change_attributes and + test my changes with real partitions. For both these I have to update + ext2fs translator, but both partitions I have are bigger than 2Gb, that's + why I need apply this patch.z + <mcsim> But what to do with mo_change_attributes? I need somehow inform + kernel about page fault policy. + <mcsim> When I change mo_ interface in kernel I have to update all programs + that use this interface and ext2fs is one of them. + + <mcsim> braunr: Who do you think better to inform kernel about fault + policy? At the moment I've added fault_strategy parameter that accepts + following strategies: randow, sequential with single page cluster, + sequential with double page cluster and sequential with quad page + cluster. OSF/mach has completely another interface of + mo_change_attributes. In OSF/mach mo_change_attributes accepts structure + of parameter. This structure could have different formats depending o + <mcsim> This rpc could be useful because it is not very handy to update + mo_change_attributes for kernel, for hurd libs and for glibc. Instead of + this kernel will accept just one more structure format. + <braunr> well, like i wrote on the mailing list several weeks ago, i don't + think the policy selection is of concern currently + <braunr> you should focus on the implementation of page clustering and + readahead + <braunr> concerning the interface, i don't think it's very important + <braunr> also, i really don't like the fact that the policy is per object + <braunr> it should be per map entry + <braunr> i think it mentioned that in my mail too + <braunr> i really think you're wasting time on this + <braunr> http://lists.gnu.org/archive/html/bug-hurd/2012-04/msg00064.html + <braunr> http://lists.gnu.org/archive/html/bug-hurd/2012-04/msg00029.html + <braunr> mcsim: any reason you completely ignored those ? + <mcsim> braunr: Ok. I'll do clustering for map entries. + <braunr> no it's not about that either :/ + <braunr> clustering is grouping several pages in the same transfer between + kernel and pager + <braunr> the *policy* is held in map entries + <antrik> mcsim: I'm not sure I properly understand your question about the + policy interface... but if I do, it's IMHO usually better to expose + individual parameters as RPC arguments explicitly, rather than hiding + them in an opaque structure... + <antrik> (there was quite some discussion about that with libburn guy) + <mcsim> antrik: Following will be ok? kern_return_t vm_advice(map, address, + length, advice, cluster_size) + <mcsim> Where advice will be either random or sequential + <antrik> looks fine to me... but then, I'm not an expert on this stuff :-) + <antrik> perhaps "policy" would be clearer than "advice"? + <mcsim> madvise has following prototype: int madvise(void *addr, size_t + len, int advice); + <mcsim> hmm... looks like I made a typo. Or advi_c_e is ok too? + <antrik> advise is a verb; advice a noun... there is a reason why both + forms show up in the madvise prototype :-) + <mcsim> so final variant should be kern_return_t vm_advise(map, address, + length, policy, cluster_size)? + <antrik> mcsim: nah, you are probably right that its better to keep + consistency with madvise, even if the name of the "advice" parameter + there might not be ideal... + <antrik> BTW, where does cluster_size come from? from the filesystem? + <antrik> I see merits both to naming the parameter "policy" (clearer) or + "advice" (more consistent) -- you decide :-) + <mcsim> antrik: also there is variant strategy, like with inheritance :) + I'll choose advice for now. + <mcsim> What do you mean under "where does cluster_size come from"? + <antrik> well, madvise doesn't have this parameter; so the value must come + from a different source? + <mcsim> in madvise implementation it could fixed value or somehow + calculated basing on size of memory range. In OSF/mach cluster size is + supplied too (via mo_change_attributes). + <antrik> ah, so you don't really know either :-) + <antrik> well, my guess is that it is derived from the cluster size used by + the filesystem in question + <antrik> so for us it would always be 4k for now + <antrik> (and thus you can probably leave it out alltogether...) + <antrik> well, fatfs can use larger clusters + <antrik> I would say, implement it only if it's very easy to do... if it's + extra effort, it's probably not worth it + <mcsim> There is sense to make cluster size bigger for ext2 too, since most + likely consecutive clusters will be within same group. + <mcsim> But anyway I'll handle this later. + <antrik> well, I don't know what cluster_size does exactly; but by the + sound of it, I'd guess it makes an assumption that it's *always* better + to read in this cluster size, even for random access -- which would be + simply wrong for 4k filesystem clusters... + <antrik> BTW, I agree with braunr that madvice() is optional -- it is way + way more important to get readahead working as a default policy first + + +## IRC, freenode, #hurd, 2012-07-01 + + <mcsim> youpi: Do you think you could review my code? + <youpi> sure, just post it to the list + <youpi> make sure to break it down into logical pieces + <mcsim> youpi: I pushed it my branch at gnumach repository + <mcsim> youpi: or it is still better to post changes to list? + <youpi> posting to the list would permit feedback from other people too + <youpi> mcsim: posix distinguishes normal, sequential and random + <youpi> we should probably too + <youpi> the system call should probably be named "vm_advise", to be a verb + like allocate etc. + <mcsim> youpi: ok. A have a talk with antrik regarding naming, I'll change + this later because compiling of glibc take a lot of time. + <youpi> mcsim: I find it odd that vm_for_every_page allocates non-existing + pages + <youpi> there should probably be at least a flag to request it or not + <mcsim> youpi: normal policy is synonym to default. And this could be + treated as either random or sequential, isn't it? + <braunr> mcsim: normally, no + <youpi> yes, the normal policy would be the default + <youpi> it doesn't mean random or sequential + <youpi> it's just to be a compromise between both + <youpi> random is meant to make no read-ahead, since that'd be spurious + anyway + <youpi> while by default we should make readahead + <braunr> and sequential makes even more aggressive readahead, which usually + implies a greater number of pages to fetch + <braunr> that's all + <youpi> yes + <youpi> well, that part is handled by the cluster_size parameter actually + <braunr> what about reading pages preceding the faulted paged ? + <mcsim> Shouldn't sequential clean some pages (if they, for example, are + not precious) that are placed before fault page? + <braunr> ? + <youpi> that could make sense, yes + <braunr> you lost me + <youpi> and something that you wouldn't to with the normal policy + <youpi> braunr: clear what has been read previously + <braunr> ? + <youpi> since the access is supposed to be sequential + <braunr> oh + <youpi> the application will proabably not re-read what was already read + <braunr> you mean to avoid caching it ? + <youpi> yes + <braunr> inactive memory is there for that + <youpi> while with the normal policy you'd assume that the application + might want to go back etc. + <youpi> yes, but you can help it + <braunr> yes + <youpi> instead of making other pages compete with it + <braunr> but then, it's for precious pages + <youpi> I have to say I don't know what a precious page it + <youpi> s + <youpi> does it mean dirty pages? + <braunr> no + <braunr> precious means cached pages + <braunr> "If precious is FALSE, the kernel treats the data as a temporary + and may throw it away if it hasn't been changed. If the precious value is + TRUE, the kernel treats its copy as a data repository and promises to + return it to the manager; the manager may tell the kernel to throw it + away instead by flushing and not cleaning the data" + <braunr> hm no + <braunr> precious means the kernel must keep it + <mcsim> youpi: According to vm_for_every_page. What kind of flag do you + suppose? If object is internal, I suppose not to cross the bound of + object, setting in_end appropriately in vm_calculate_clusters. + <mcsim> If object is external we don't know its actual size, so we should + make mo request first. And for this we should create fictitious pages. + <braunr> mcsim: but how would you implement this "cleaning" with sequential + ? + <youpi> mcsim: ah, ok, I thought you were allocating memory, but it's just + fictitious pages + <youpi> comment "Allocate a new page" should be fixed :) + <mcsim> braunr: I don't now how I will implement this specifically (haven't + tried yet), but I don't think that this is impossible + <youpi> braunr: anyway it's useful as an example where normal and + sequential would be different + <braunr> if it can be done simply + <braunr> because i can see more trouble than gains in there :) + <mcsim> braunr: ok :) + <braunr> mcsim: hm also, why fictitious pages ? + <braunr> fictitious pages should normally be used only when dealing with + memory mapped physically which is not real physical memory, e.g. device + memory + <mcsim> but vm_fault could occur when object represent some device memory. + <braunr> that's exactly why there are fictitious pages + <mcsim> at the moment of allocating of fictitious page it is not know what + backing store of object is. + <braunr> really ? + <braunr> damn, i've got used to UVM too much :/ + <mcsim> braunr: I said something wrong? + <braunr> no no + <braunr> it's just that sometimes, i'm confusing details about the various + BSD implementations i've studied + <braunr> out-of-gsoc-topic question: besides network drivers, do you think + we'll have other drivers that will run in userspace and have to implement + memory mapping ? like framebuffers ? + <braunr> or will there be a translation layer such as storeio that will + handle mapping ? + <youpi> framebuffers typically will, yes + <youpi> that'd be antrik's work on drm + <braunr> hmm + <braunr> ok + <youpi> mcsim: so does the implementation work, and do you see performance + improvement? + <mcsim> youpi: I haven't tested it yet with large ext2 :/ + <mcsim> youpi: I'm going to finish now moving of ext2 to new interface, + than other translators in hurd repository and than finish memory policies + in gnumach. Is it ok? + <youpi> which new interface? + <mcsim> Written by neal. I wrote some temporary code to make ext2 work with + it, but I'm going to change this now. + <youpi> you mean the old unapplied patch? + <mcsim> yes + <youpi> did you have a look at Karim's work? + <youpi> (I have to say I never found the time to check how it related with + neal's patch) + <mcsim> I found only his work in kernel. I didn't see his work in applying + of neal's patch. + <youpi> ok + <youpi> how do they relate with each other? + <youpi> (I have never actually looked at either of them :/) + <mcsim> his work in kernel and neal's patch? + <youpi> yes + <mcsim> They do not correlate with each other. + <youpi> ah, I must be misremembering what each of them do + <mcsim> in kam's patch was changes to support sequential reading in reverse + order (as in OSF/Mach), but posix does not support such behavior, so I + didn't implement this either. + <youpi> I can't find the pointer to neal's patch, do you have it off-hand? + <mcsim> http://comments.gmane.org/gmane.os.hurd.bugs/351 + <youpi> thx + <youpi> I think we are not talking about the same patch from Karim + <youpi> I mean lists.gnu.org/archive/html/bug-hurd/2010-06/msg00023.html + <mcsim> I mean this patch: + http://lists.gnu.org/archive/html/bug-hurd/2010-06/msg00024.html + <mcsim> Oh. + <youpi> ok + <mcsim> seems, this is just the same + <youpi> yes + <youpi> from a non-expert view, I would have thought these patches play + hand in hand, do they really? + <mcsim> this patch is completely for kernel and neal's one is completely + for libpager. + <youpi> i.e. neal's fixes libpager, and karim's fixes the kernel + <mcsim> yes + <youpi> ending up with fixing the whole path? + <youpi> AIUI, karim's patch will be needed so that your increased readahead + will end up with clustered page request? + <mcsim> I will not use kam's patch + <youpi> is it not needed to actually get pages in together? + <youpi> how do you tell libpager to fetch pages together? + <youpi> about the cluster size, I'd say it shouldn't be specified at + vm_advise() level + <youpi> in other OSes, it is usually automatically tuned + <youpi> by ramping it up to a maximum readahead size (which, however, could + be specified) + <youpi> that's important for the normal policy, where there are typically + successive periods of sequential reads, but you don't know in advance for + how long + <mcsim> braunr said that there are legal issues with his code, so I cannot + use it. + <braunr> did i ? + <braunr> mcsim: can you give me a link to the code again please ? + <youpi> see above :) + <braunr> which one ? + <youpi> both + <youpi> they only differ by a typo + <braunr> mcsim: i don't remember saying that, do you have any link ? + <braunr> or log ? + <mcsim> sorry, can you rephrase "ending up with fixing the whole path"? + <mcsim> cluster_size in vm_advise also could be considered as advise + <braunr> no + <braunr> it must be the third time we're talking about this + <youpi> mcsim: I mean both parts would be needed to actually achieve + clustered i/o + <braunr> again, why make cluster_size a per object attribute ? :( + <youpi> wouldn't some objects benefit from bigger cluster sizes, while + others wouldn't? + <youpi> but again, I believe it should rather be autotuned + <youpi> (for each object) + <braunr> if we merely want posix compatibility (and for a first attempt, + it's quite enough), vm_advise is good, and the kernel selects the + implementation (and thus the cluster sizes) + <braunr> if we want finer grained control, perhaps a per pager cluster_size + would be good, although its efficiency depends on several parameters + <braunr> (e.g. where the page is in this cluster) + <braunr> but a per object cluster size is a large waste of memory + considering very few applications (if not none) would use the "feature" + .. + <braunr> (if any*) + <youpi> there must be a misunderstanding + <youpi> why would it be a waste of memory? + <braunr> "per object" + <youpi> so? + <braunr> there can be many memory objects in the kernel + <youpi> so? + <braunr> so such an overhead must be useful to accept it + <youpi> in my understanding, a cluster size per object is just a mere + integer for each object + <youpi> what overhead? + <braunr> yes + <youpi> don't we have just thousands of objects? + <braunr> for now + <braunr> remember we're trying to remove the page cache limit :) + <youpi> that still won't be more than tens of thousands of objects + <youpi> times an integer + <youpi> that's completely neglectible + <mcsim> braunr: Strange, Can't find in logs. Weird things are happening in + my memory :/ Sorry. + <braunr> mcsim: i'm almost sure i never said that :/ + <braunr> but i don't trust my memory too much either + <braunr> youpi: depends + <youpi> mcsim: I mean both parts would be needed to actually achieve + clustered i/o + <mcsim> braunr: I made I call vm_advise that applies policy to memory range + (vm_map_entry to be specific) + <braunr> mcsim: good + <youpi> actually the cluster size should even be per memory range + <mcsim> youpi: In this sense, yes + <youpi> k + <mcsim> sorry, Internet connection lags + <braunr> when changing a structure used to create many objects, keep in + mind one thing + <braunr> if its size gets larger than a threshold (currently, powers of + two), the cache used by the slab allocator will allocate twice the + necessary amount + <youpi> sure + <braunr> this is the case with most object caching allocators, although + some can have specific caches for common sizes such as 96k which aren't + powers of two + <braunr> anyway, an integer is negligible, but the final structure size + must be checked + <braunr> (for both 32 and 64 bits) + <mcsim> braunr: ok. + <mcsim> But I didn't understand what should be done with cluster size in + vm_advise? Should I delete it? + <braunr> to me, the cluster size is a pager property + <youpi> to me, the cluster size is a map property + <braunr> whereas vm_advise indicates what applications want + <youpi> you could have several process accessing the same file in different + ways + <braunr> youpi: that's why there is a policy + <youpi> isn't cluster_size part of the policy? + <braunr> but if the pager abilities are limited, it won't change much + <braunr> i'm not sure + <youpi> cluster_size is the amount of readahead, isn't it? + <braunr> no, it's the amount of data in a single transfer + <mcsim> Yes, it is. + <braunr> ok, i'll have to check your code + <youpi> shouldn't transfers permit unbound amounts of data? + <mcsim> braunr: than I misunderstand what readahead is + <braunr> well then cluster size is per policy :) + <braunr> e.g. random => 0, normal => 3, sequential => 15 + <braunr> why make it per map entry ? + <youpi> because it depends on what the application doezs + <braunr> let me check the code + <youpi> if it's accessing randomly, no need for big transfers + <youpi> just page transfers will be fine + <youpi> if accessing sequentially, rather use whole MiB of transfers + <youpi> and these behavior can be for the same file + <braunr> mcsim: the call is vm_advi*s*e + <braunr> mcsim: the call is vm_advi_s_e + <braunr> not advice + <youpi> yes, he agreed earlier + <braunr> ok + <mcsim> cluster_size is the amount of data that I try to read at one time. + <mcsim> at singe mo_data_request + <mcsim> *single + <youpi> which, to me, will depend on the actual map + <braunr> ok so it is the transfer size + <youpi> and should be autotuned, especially for normal behavior + <braunr> youpi: it makes no sense to have both the advice and the actual + size per map entry + <youpi> to get big readahead with all apps + <youpi> braunr: the size is not only dependent on the advice, but also on + the application behavior + <braunr> youpi: how does this application tell this ? + <youpi> even for sequential, you shouldn't necessarily use very big amounts + of transfers + <braunr> there is no need for the advice if there is a cluster size + <youpi> there can be, in the case of sequential, as we said, to clear + previous pages + <youpi> but otherwise, indeed + <youpi> but for me it's the converse + <youpi> the cluster size should be tuned anyway + <braunr> and i'm against giving the cluster size in the advise call, as we + may want to prefetch previous data as well + <youpi> I don't see how that collides + <braunr> well, if you consider it's the transfer size, it doesn't + <youpi> to me cluster size is just the size of a window + <braunr> if you consider it's the amount of pages following a faulted page, + it will + <braunr> also, if your policy says e.g. "3 pages before, 10 after", and + your cluster size is 2, what happens ? + <braunr> i would find it much simpler to do what other VM variants do: + compute the I/O sizes directly from the policy + <youpi> don't they autotune, and use the policy as a maximum ? + <braunr> depends on the implementations + <youpi> ok, but yes I agree + <youpi> although casting the size into stone in the policy looks bogus to + me + <braunr> but making cluster_size part of the kernel interface looks way too + messy + <braunr> it is + <braunr> that's why i would have thought it as part of the pager properties + <braunr> the pager is the true component besides the kernel that is + actually involved in paging ... + <youpi> well, for me the flexibility should still be per application + <youpi> by pager you mean the whole pager, not each file, right? + <braunr> if a pager can page more because e.g. it's a file system with big + block sizes, why not fetch more ? + <braunr> yes + <braunr> it could be each file + <braunr> but only if we have use for it + <braunr> and i don't see that currently + <youpi> well, posix currently doesn't provide a way to set it + <youpi> so it would be useless atm + <braunr> i was thinking about our hurd pagers + <youpi> could we perhaps say that the policy maximum could be a fraction of + available memory? + <braunr> why would we want that ? + <youpi> (total memory, I mean) + <youpi> to make it not completely cast into stone + <youpi> as have been in the past in gnumach + <braunr> i fail to understand :/ + <youpi> there must be a misunderstanding then + <youpi> (pun not intended) + <braunr> why do you want to limit the policy maximum ? + <youpi> how to decide it? + <braunr> the pager sets it + <youpi> actually I don't see how a pager could decide it + <youpi> on what ground does it make the decision? + <youpi> readahead should ideally be as much as 1MiB + <braunr> 02:02 < braunr> if a pager can page more because e.g. it's a file + system with big block sizes, why not fetch more ? + <braunr> is the example i have in mind + <braunr> otherwise some default values + <youpi> that's way smaller than 1MiB, isn't it? + <braunr> yes + <braunr> and 1 MiB seems a lot to me :) + <youpi> for readahead, not really + <braunr> maybe for sequential + <youpi> that's what we care about! + <braunr> ah, i thought we cared about normal + <youpi> "as much as 1MiB", I said + <youpi> I don't mean normal :) + <braunr> right + <braunr> but again, why limit ? + <braunr> we could have 2 or more ? + <youpi> at some point you don't get more efficiency + <youpi> but eat more memory + <braunr> having the pager set the amount allows us to easily adjust it over + time + <mcsim> braunr: Do you think that readahead should be implemented in + libpager? + <youpi> than needed + <braunr> mcsim: no + <braunr> mcsim: err + <braunr> mcsim: can't answer + <youpi> mcsim: do you read the log of what you have missed during + disconnection? + <braunr> i'm not sure about what libpager does actually + <mcsim> yes + <braunr> for me it's just mutualisation of code used by pagers + <braunr> i don't know the details + <braunr> youpi: yes + <braunr> youpi: that's why we want these values not hardcoded in the kernel + <braunr> youpi: so that they can be adjusted by our shiny user space OS + <youpi> (btw apparently linux uses minimum 16k, maximum 128 or 256k) + <braunr> that's more reasonable + <youpi> that's just 4 times less :) + <mcsim> braunr: You say that pager should decide how much data should be + read ahead, but each pager can't implement it on it's own as there will + be too much overhead. So the only way is to implement this in libpager. + <braunr> mcsim: gni ? + <braunr> why couldn't they ? + <youpi> mcsim: he means the size, not the actual implementation + <youpi> the maximum size, actually + <braunr> actually, i would imagine it as the pager giving per policy + parameters + <youpi> right + <braunr> like how many before and after + <youpi> I agree, then + <braunr> the kernel could limit, sure, to avoid letting pagers use + completely insane values + <youpi> (and that's just a max, the kernel autotunes below that) + <braunr> why not + <youpi> that kernel limit could be a fraction of memory, then? + <braunr> it could, yes + <braunr> i see what you mean now + <youpi> mcsim: did you understand our discussion? + <youpi> don't hesitate to ask for clarification + <mcsim> I supposed cluster_size to be such parameter. And advice will help + to interpret this parameter (whether data should be read after fault page + or some data should be cleaned before) + <youpi> mcsim: we however believe that it's rather the pager than the + application that would tell that + <youpi> at least for the default values + <youpi> posix doesn't have a way to specify it, and I don't think it will + in the future + <braunr> and i don't think our own hurd-specific programs will need more + than that + <braunr> if they do, we can slightly change the interface to make it a per + object property + <braunr> i've checked the slab properties, and it seems we can safely add + it per object + <braunr> cf http://www.sceen.net/~rbraun/slabinfo.out + <braunr> so it would still be set by the pager, but if depending on the + object, the pager could set different values + <braunr> youpi: do you think the pager should just provide one maximum size + ? or per policy sizes ? + <youpi> I'd say per policy size + <youpi> so people can increase sequential size like crazy when they know + their sequential applications need it, without disturbing the normal + behavior + <braunr> right + <braunr> so the last decision is per pager or per object + <braunr> mcsim: i'd say whatever makes your implementation simpler :) + <mcsim> braunr: how kernel knows that object are created by specific pager? + <braunr> that's the kind of things i'm referring to with "whatever makes + your implementation simpler" + <braunr> but usually, vm_objects have an ipc port and some properties + relatedto their pagers + <braunr> -usually + <braunr> the problem i had in mind was the locking protocol but our spin + locks are noops, so it will be difficult to detect deadlocks + <mcsim> braunr: and for every policy there should be variable in vm_object + structure with appropriate cluster_size? + <braunr> if you want it per object, yes + <braunr> although i really don't think we want it + <youpi> better keep it per pager for now + <braunr> let's imagine youpi finishes his 64-bits support, and i can + successfully remove the page cache limit + <braunr> we'd jump from 1.8 GiB at most to potentially dozens of GiB of RAM + <braunr> and 1.8, mostly unused + <braunr> to dozens almost completely used, almost all the times for the + most interesting use cases + <braunr> we may have lots and lots of objects to keep around + <braunr> so if noone really uses the feature ... there is no point + <youpi> but also lots and lots of memory to spend on it :) + <youpi> a lot of objects are just one page, but a lof of them are not + <braunr> sure + <braunr> we wouldn't be doing that otherwise :) + <braunr> i'm just saying there is no reason to add the overhead of several + integers for each object if they're simply not used at all + <braunr> hmm, 64-bits, better page cache, clustered paging I/O :> + <braunr> (and readahead included in the last ofc) + <braunr> good night ! + <mcsim> than, probably, make system-global max-cluster_size? This will save + some memory. Also there is usually no sense in reading really huge chunks + at once. + <youpi> but that'd be tedious to set + <youpi> there are only a few pagers, that's no wasted memory + <youpi> the user being able to set it for his own pager is however a very + nice feature, which can be very useful for databases, image processing, + etc. + <mcsim> In conclusion I have to implement following: 3 memory policies per + object and per vm_map_entry. Max cluster size for every policy should be + set per pager. + <mcsim> So, there should be 2 system calls for setting memory policy and + one for setting cluster sizes. + <mcsim> Also amount of data to transfer should be tuned automatically by + every page fault. + <mcsim> youpi: Correct me, please, if I'm wrong. + <youpi> I believe that's what we ended up to decide, yes + + +## IRC, freenode, #hurd, 2012-07-02 + + <braunr> is it safe to say that all memory objects implemented by external + pagers have "file" semantics ? + <braunr> i wonder if the current memory manager interface is suitable for + device pagers + <mcsim> braunr: What does "file" semantics mean? + <braunr> mcsim: anonymous memory doesn't have the same semantics as a file + for example + <braunr> anonymous memory that is discontiguous in physical memory can be + contiguous in swap + <braunr> and its location can change with time + <braunr> whereas with a memory object, the data exchanged with pagers is + identified with its offset + <braunr> in (probably) all other systems, this way of specifying data is + common to all files, whatever the file system + <braunr> linux uses the struct vm_file name, while in BSD/Solaris they are + called vnodes (the link between a file system inode and virtual memory) + <braunr> my question is : can we implement external device pagers with the + current interface, or is this interface really meant for files ? + <braunr> also + <braunr> mcsim: something about what you said yesterday + <braunr> 02:39 < mcsim> In conclusion I have to implement following: 3 + memory policies per object and per vm_map_entry. Max cluster size for + every policy should be set per pager. + <braunr> not per object + <braunr> one policy per map entry + <braunr> transfer parameters (pages before and after the faulted page) per + policy, defined by pagers + <braunr> 02:39 < mcsim> So, there should be 2 system calls for setting + memory policy and one for setting cluster sizes. + <braunr> adding one call for vm_advise is good because it mirrors the posix + call + <braunr> but for the parameters, i'd suggest changing an already existing + call + <braunr> not sure which one though + <mcsim> braunr: do you know how mo_change_attributes implemented in + OSF/Mach? + <braunr> after a quick reading of the reference manual, i think i + understand why they made it per object + <braunr> mcsim: no + <braunr> did they change the call to include those paging parameters ? + <mcsim> it accept two parameters: flavor and pointer to structure with + parameters. + <mcsim> flavor determines semantics of structure with parameters. + <mcsim> + http://www.darwin-development.org/cgi-bin/cvsweb/osfmk/src/mach_kernel/vm/memory_object.c?rev=1.1 + <mcsim> structure can have 3 different views and what exect view will be is + determined by value of flavor + <mcsim> So, I thought about implementing similar call that could be used + for various purposes. + <mcsim> like ioctl + <braunr> "pointer to structure with parameters" <= which one ? + <braunr> mcsim: don't model anything anywhere like ioctl please + <mcsim> memory_object_info_t attributes + <braunr> ioctl is the very thing we want NOT to have on the hurd + <braunr> ok attributes + <braunr> and what are the possible values of flavour, and what kinds of + attributes ? + <mcsim> and then appears something like this on each case: behave = + (old_memory_object_behave_info_t) attributes; + <braunr> ok i see + <mcsim> flavor could be OLD_MEMORY_OBJECT_BEHAVIOR_INFO, + MEMORY_OBJECT_BEHAVIOR_INFO, MEMORY_OBJECT_PERFORMANCE_INFO etc + <braunr> i don't really see the point of flavour here, other than + compatibility + <braunr> having attributes is nice, but you should probably add it as a + call parameter, not inside a structure + <braunr> as a general rule, we don't like passing structures too much + to/from the kernel, because handling them with mig isn't very clean + <mcsim> ok + <mcsim> What policy parameters should be defined by pager? + <braunr> i'd say number of pages to page-in before and after the faulted + page + <mcsim> Only pages before and after the faulted page? + <braunr> for me yes + <braunr> youpi might have different things in mind + <braunr> the page cleaning in sequential mode is something i wouldn't do + <braunr> 1/ applications might want data read sequentially to remain in the + cache, for other sequential accesses + <braunr> 2/ applications that really don't want to cache anything should + use O_DIRECT + <braunr> 3/ it's complicated, and we're in july + <braunr> i'd rather have a correct and stable result than too many unused + features + <mcsim> braunr: MADV_SEQUENTIAL Expect page references in sequential order. + (Hence, pages in the given range can be aggressively read ahead, and may + be freed soon after they are accessed.) + <mcsim> this is from linux man + <mcsim> braunr: Can I at least make keeping in mind that it could be + implemented? + <mcsim> I mean future rpc interface + <mcsim> braunr: From behalf of kernel pager is just a port. + <mcsim> That's why it is not clear for me how I can make in kernel + per-pager policy + <braunr> mcsim: you can't + <braunr> 15:19 < braunr> after a quick reading of the reference manual, i + think i understand why they made it per object + <braunr> + http://pubs.opengroup.org/onlinepubs/009695399/functions/posix_madvise.html + <braunr> POSIX_MADV_SEQUENTIAL + <braunr> Specifies that the application expects to access the specified + range sequentially from lower addresses to higher addresses. + <braunr> linux might free pages after their access, why not, but this is + entirely up to the implementation + <mcsim> I know, when but applications might want data read sequentially to + remain in the cache, for other sequential accesses this kind of access + could be treated rather normal or random + <braunr> we can do differently + <braunr> mcsim: no + <braunr> sequential means the access will be sequential + <braunr> so aggressive readahead (e.g. 0 pages before, many after), should + be used + <braunr> for better performance + <braunr> from my pov, it has nothing to do with caching + <braunr> i actually sometimes expect data to remain in cache + <braunr> e.g. before playing a movie from sshfs, i sometimes prefetch it + using dd + <braunr> then i use mplayer + <braunr> i'd be very disappointed if my data didn't remain in the cache :) + <mcsim> At least these pages could be placed into inactive list to be first + candidates for pageout. + <braunr> that's what will happen by default + <braunr> mcsim: if we need more properties for memory objects, we'll adjust + the call later, when we actually implement them + <mcsim> so, first call is vm_advise and second is changed + mo_change_attributes? + <braunr> yes + <mcsim> there will appear 3 new parameters in mo_c_a: policy, pages before + and pages after? + <mcsim> braunr: With vm_advise I didn't understand one thing. This call is + defined in defs file, so that should mean that vm_advise is ordinal rpc + call. But on the same time it is defined as syscall in mach internals (in + mach_trap_table). + <braunr> mcsim: what ? + <braunr> were is it "defined" ? (it doesn't exit in gnumach currently) + <mcsim> Ok, let consider vm_map + <mcsim> I define it both in mach_trap_table and in defs file. + <mcsim> But why? + <braunr> uh ? + <braunr> let me see + <mcsim> Why defining in defs file is not enough? + <mcsim> and previous question: there will appear 3 new parameters in + mo_c_a: policy, pages before and pages after? + <braunr> mcsim: give me the exact file paths please + <braunr> mcsim: we'll discuss the new parameters after + <mcsim> kern/syscall_sw.c + <braunr> right i see + <mcsim> here mach_trap_table in defined + <braunr> i think they're not used + <braunr> they were probably introduced for performance + <mcsim> and ./include/mach/mach.defs + <braunr> don't bother adding vm_advise as a syscall + <braunr> about the parameters, it's a bit more complicated + <braunr> you should add 6 parameters + <braunr> before and after, for the 3 policies + <braunr> but + <braunr> as seen in the posix page, there could be more policies .. + <braunr> ok forget what i said, it's stupid + <braunr> yes, the 3 parameters you had in mind are correct + <braunr> don't forget a "don't change" value for the policy though, so the + kernel ignores the before/after values if we don't want to change that + <mcsim> ok + <braunr> mcsim: another reason i asked about "file semantics" is the way we + handle the cache + <braunr> mcsim: file semantics imply data is cached, whereas anonymous and + device memory usually isn't + <braunr> (although having the cache at the vm layer instead of the pager + layer allows nice things like the swap cache) + <mcsim> But this shouldn't affect possibility of implementing of device + pager. + <braunr> yes it may + <braunr> consider how a fault is actually handled by a device + <braunr> mach must use weird fictitious pages for that + <braunr> whereas it would be better to simply let the pager handle the + fault as it sees fit + <mcsim> setting may_cache to false should resolve the issue + <braunr> for the caching problem, yes + <braunr> which is why i still think it's better to handle the cache at the + vm layer, unlike UVM which lets the vnode pager handle its own cache, and + removes the vm cache completely + <mcsim> The only issue with pager interface I see is implementing of + scatter-gather DMA (as current interface does not support non-consecutive + access) + <braunr> right + <braunr> but that's a performance issue + <braunr> my problem with device pagers is correctness + <braunr> currently, i think the kernel just asks pagers for "data" + <braunr> whereas a device pager should really map its device memory where + the fault happen + <mcsim> braunr: You mean that every access to memory should cause page + fault? + <mcsim> I mean mapping of device memory + <braunr> no + <braunr> i mean a fault on device mapped memory should directly access a + shared region + <braunr> whereas file pagers only implement backing store + <braunr> let me explain a bit more + <braunr> here is what happens with file mapped memory + <braunr> you map it, access it (some I/O is done to get the page content in + physical memory), then later it's flushed back + <braunr> whereas with device memory, there shouldn't be any I/O, the device + memory should directly be mapped (well, some devices need the same + caching behaviour, while others provide direct access) + <braunr> one of the obvious consequences is that, when you map device + memory (e.g. a framebuffer), you expect changes in your mapped memory to + be effective right away + <braunr> while with file mapped memory, you need to msync() it + <braunr> (some framebuffers also need to be synced, which suggests greater + control is needed for external pagers) + <mcsim> Seems that I understand you. But how it is implemented in other + OS'es? Do they set something in mmu? + <braunr> mcsim: in netbsd, pagers have a fault operatin in addition to get + and put + <braunr> the device pager sets get and put to null and implements fault + only + <braunr> the fault callback then calls the d_mmap callback of the specific + driver + <braunr> which usually results in the mmu being programmed directly + <braunr> (e.g. pmap_enter or similar) + <braunr> in linux, i think raw device drivers, being implemented as + character device files, must provide raw read/write/mmap/etc.. functions + <braunr> so it looks pretty much similar + <braunr> i'd say our current external pager interface is insufficient for + device pagers + <braunr> but antrik may know more since he worked on ggi + <braunr> antrik: ^ + <mcsim> braunr: Seems he used io_map + <braunr> mcsim: where ar eyou looking at ? the incubator ? + <mcsim> his master's thesis + <braunr> ah the thesis + <braunr> but where ? :) + <mcsim> I'll give you a link + <mcsim> http://dl.dropbox.com/u/36519904/kgi_on_hurd.pdf + <braunr> thanks + <mcsim> see p 158 + <braunr> arg, more than 200 pages, and he says he's lazy :/ + <braunr> mcsim: btw, have a look at m_o_ready + <mcsim> braunr: This is old form of mo_change attributes + <mcsim> I'm not going to change it + <braunr> mcsim: these are actually the default object parameters right ? + <braunr> mcsim: if you don't change it, it means the kernel must set + default values until the pager changes them, if it does + <mcsim> yes. + <antrik> mcsim: madvise() on Linux has a separate flag to indicate that + pages won't be reused. thus I think it would *not* be a good idea to + imply it in SEQUENTIAL + <antrik> braunr: yes, my KMS code relies on mapping memory objects for the + framebuffer + <antrik> (it should be noted though that on "modern" hardware, mapping + graphics memory directly usually gives very poor performance, and drivers + tend to avoid it...) + <antrik> mcsim: BTW, it was most likely me who warned about legal issues + with KAM's work. AFAIK he never managed to get the copyright assignment + done :-( + <antrik> (that's not really mandatory for the gnumach work though... only + for the Hurd userspace parts) + <antrik> also I'd like to point out again that the cluster_size argument + from OSF Mach was probably *not* meant for advice from application + programs, but rather was supposed to reflect the cluster size of the + filesystem in question. at least that sounds much more plausible to me... + <antrik> braunr: I have no idea whay you mean by "device pager". device + memory is mapped once when the VM mapping is established; there is no + need for any fault handling... + <antrik> mcsim: to be clear, I think the cluster_size parameter is mostly + orthogonal to policy... and probably not very useful at all, as ext2 + almost always uses page-sized clusters. I'm strongly advise against + bothering with it in the initial implementation + <antrik> mcsim: to avoid confusion, better use a completely different name + for the policy-decided readahead size + <mcsim> antrik: ok + <antrik> braunr: well, yes, the thesis report turned out HUGE; but the + actual work I did on the KGI port is fairly tiny (not more than a few + weeks of actual hacking... everything else was just brooding) + <antrik> braunr: more importantly, it's pretty much the last (and only + non-trivial) work I did on the Hurd :-( + <antrik> (also, I don't think I used the word "lazy"... my problem is not + laziness per se; but rather inability to motivate myself to do anything + not providing near-instant gratification...) + <braunr> antrik: right + <braunr> antrik: i shouldn't consider myself lazy either + <braunr> mcsim: i agree with antrik, as i told you weeks ago + <braunr> about + <braunr> 21:45 < antrik> mcsim: to be clear, I think the cluster_size + parameter is mostly orthogonal to policy... and probably not very useful + at all, as ext2 almost always uses page-sized clusters. I'm strongly + advise against bothering with it + <braunr> in the initial implementation + <braunr> antrik: but how do you actually map device memory ? + <braunr> also, strangely enough, here is the comment in dragonflys + madvise(2) + <braunr> 21:45 < antrik> mcsim: to be clear, I think the cluster_size + parameter is mostly orthogonal to policy... and probably not very useful + at all, as ext2 almost always uses page-sized clusters. I'm strongly + advise against bothering with it + <braunr> in the initial implementation + <braunr> arg + <braunr> MADV_SEQUENTIAL Causes the VM system to depress the priority of + pages immediately preceding a given page when it is faulted in. + <antrik> braunr: interesting... + <antrik> (about SEQUENTIAL on dragonfly) + <antrik> as for mapping device memory, I just use to device_map() on the + mem device to map the physical address space into a memory object, and + then through vm_map into the driver (and sometimes application) address + space + <antrik> formally, there *is* a pager involved of course (implemented + in-kernel by the mem device), but it doesn't really do anything + interesting + <antrik> thinking about it, there *might* actually be page faults involved + when the address ranges are first accessed... but even then, the handling + is really trivial and not terribly interesting + <braunr> antrik: it does the most interesting part, create the physical + mapping + <braunr> and as trivial as it is, it requires a special interface + <braunr> i'll read about device_map again + <braunr> but yes, the fact that it's in-kernel is what solves the problem + here + <braunr> what i'm interested in is to do it outside the kernel :) + <antrik> why would you want to do that? + <antrik> there is no policy involved in doing an MMIO mapping + <antrik> you ask for the pysical memory region you are interested in, and + that's it + <antrik> whether the kernel adds the page table entries immediately or on + faults is really an implementation detail + <antrik> braunr: ^ + <braunr> yes it's a detail + <braunr> but do we currently have the interface to make such mappings from + userspace ? + <braunr> and i want to do that because i'd like as many drivers as possible + outside the kernel of course + <antrik> again, the userspace driver asks the kernel to establish the + mapping (through device_map() and then vm_map() on the resulting memory + object) + <braunr> hm i'm missing something + <braunr> + http://www.gnu.org/software/hurd/gnumach-doc/Device-Map.html#Device-Map + <= this one ? + <antrik> yes, this one + <braunr> but this implies the device is implemented by the kernel + <antrik> the mem device is, yes + <antrik> but that's not a driver + <braunr> ah + <antrik> it's just the interface for doing MMIO + <antrik> (well, any physical mapping... but MMIO is probably the only real + use case for that) + <braunr> ok + <braunr> i was thinking about completely removing the device interface from + the kernel actually + <braunr> but it makes sense to have such devices there + <antrik> well, in theory, specific kernel drivers can expose their own + device_map() -- but IIRC the only one that does (besides mem of course) + is maptime -- which is not a real driver either... + <braunr> oh btw, i didn't know you had a blog :) + <antrik> well, it would be possible to replace the device interface by + specific interfaces for the generic pseudo devices... I'm not sure how + useful that would be + <braunr> there are lots of interesting stuff there + <antrik> hehe... another failure ;-) + <braunr> failure ? + <antrik> well, when I realized that I'm speding a lot of time pondering + things, and never can get myself to actually impelemnt any of them, I had + the idea that if I write them down, there might at least be *some* good + from it... + <antrik> unfortunately it turned out that I need so much effort to write + things down, that most of the time I can't get myself to do that either + :-( + <braunr> i see + <braunr> well it's still nice to have it + <antrik> (notice that the latest entry is two years old... and I haven't + even started describing most of my central ideas :-( ) + <braunr> antrik: i tried to create a blog once, and found what i wrote so + stupid i immediately removed it + <antrik> hehe + <antrik> actually some of my entries seem silly in retrospect as well... + <antrik> but I guess that's just the way it is ;-) + <braunr> :) + <braunr> i'm almost sure other people would be interested in what i had to + say + <antrik> BTW, I'm actually not sure whether the Mach interfaces are + sufficient to implement GEM/TTM... we would certainly need kernel support + for GART (as for any other kind IOMMU in fact); but beyond that it's not + clear to me + <braunr> GEM ? TTM ? GART ? + <antrik> GEM = Graphics Execution Manager. part of the "new" DRM interface, + closely tied with KMS + <antrik> TTM = Translation Table Manager. does part of the background work + for most of the GEM drivers + <braunr> "The Graphics Execution Manager (GEM) is a computer software + system developed by Intel to do memory management for device drivers for + graphics chipsets." hmm + <antrik> (in fact it was originally meant to provide the actual interface; + but the Inter folks decided that it's not useful for their UMA graphics) + <antrik> GART = Graphics Aperture + <antrik> kind of an IOMMU for graphics cards + <antrik> allowing the graphics card to work with virtual mappings of main + memory + <antrik> (i.e. allowing safe DMA) + <braunr> ok + <braunr> all this graphics stuff looks so complex :/ + <antrik> it is + <antrik> I have a whole big chapter on that in my thesis... and I'm not + even sure I got everything right + <braunr> what is nvidia using/doing (except for getting the finger) ? + <antrik> flushing out all the details for KMS, GEM etc. took the developers + like two years (even longer if counting the history of TTM) + <antrik> Nvidia's proprietary stuff uses a completely own kernel interface, + which is of course not exposed or docuemented in any way... but I guess + it's actually similar in what it does) + <braunr> ok + <antrik> (you could ask the nouveau guys if you are truly + interested... they are doing most of their reverse engineering at the + kernel interface level) + <braunr> it seems graphics have very special needs, and a lot of them + <braunr> and the interfaces are changing often + <braunr> so it's not that much interesting currently + <braunr> it just means we'll probably have to change the mach interface too + <braunr> like you said + <braunr> so the answer to my question, which was something like "do mach + external pagers only implement files ?", is likely yes + <antrik> well, KMS/GEM had reached some stability; but now there are + further changes ahead with the embedded folks coming in with all their + dedicated hardware, calling for unified buffer management across the + whole pipeline (from capture to output) + <antrik> and yes: graphics hardware tends to be much more complex regarding + the interface than any other hardware. that's because it's a combination + of actual I/O (like most other devices) with a very powerful coprocessor + <antrik> and the coprocessor part is pretty much unique amongst peripherial + devices + <antrik> (actually, the I/O part is also much more complex than most other + hardware... but that alone would only require a more complex driver, not + special interfaces) + <antrik> embedded hardware makes it more interesting in that the I/O + part(s) are separate from the coprocessor ones; and that there are often + several separate specialised ones of each... the DRM/KMS stuff is not + prepared to deal with this + <antrik> v4l over time has evolved to cover such things; but it's not + really the right place to implement graphics drivers... which is why + there are not efforts to unify these frameworks. funny times... + + +## IRC, freenode, #hurd, 2012-07-03 + + <braunr> mcsim: vm_for_every_page should be static + <mcsim> braunr: ok + <braunr> mcsim: see http://gcc.gnu.org/onlinedocs/gcc/Inline.html + <braunr> and it looks big enough that you shouldn't make it inline + <braunr> let the compiler decide for you (which is possible only if the + function is static) + <braunr> (otherwise a global symbol needs to exist) + <braunr> mcsim: i don't know where you copied that comment from, but you + should review the description of the vm_advice call in mach.Defs + <mcsim> braunr: I see + <mcsim> braunr: It was vm_inherit :) + <braunr> mcsim: why isn't NORMAL defined in vm_advise.h ? + <braunr> mcsim: i figured actually ;) + <mcsim> braunr: I was going to do it later when. + <braunr> mcsim: for more info on inline, see + http://www.kernel.org/doc/Documentation/CodingStyle + <braunr> arg that's an old one + <mcsim> braunr: I know that I do not follow coding style + <braunr> mcsim: this one is about linux :p + <braunr> mcsim: http://lxr.linux.no/linux/Documentation/CodingStyle should + have it + <braunr> mcsim: "Chapter 15: The inline disease" + <mcsim> I was going to fix it later during refactoring when I'll merge + mplaneta/gsoc12/working to mplaneta/gsoc12/master + <braunr> be sure not to forget :p + <braunr> and the best not to forget is to do it asap + <braunr> +way + <mcsim> As to inline. I thought that even if I specify function as inline + gcc makes final decision about it. + <mcsim> There was a specifier that made function always inline, AFAIR. + <braunr> gcc can force a function not to be inline, yes + <braunr> but inline is still considered as a strong hint + + +## IRC, freenode, #hurd, 2012-07-05 + + <mcsim1> braunr: hello. You've said that pager has to supply 2 values to + kernel to give it an advice how execute page fault. These two values + should be number of pages before and after the page where fault + occurred. But for sequential policy number of pager before makes no + sense. For random policy too. For normal policy it would be sane to make + readahead symmetric. Probably it would be sane to make pager supply + cluster_size (if it is necessary to supply any) that w + <mcsim1> *that will be advice for kernel of least sane value? And maximal + value will be f(free_memory, map_entry_size)? + <antrik> mcsim1: I doubt symmetric readahead would be a good default + policy... while it's hard to estimate an optimum over all typical use + cases, I'm pretty sure most situtations will benefit almost exclusively + from reading following pages, not preceeding ones + <antrik> I'm not even sure it's useful to read preceding pages at all in + the default policy -- the use cases are probably so rare that the penalty + in all other use cases is not justified. I might be wrong on that + though... + <antrik> I wonder how other systems handle that + <LarstiQ> antrik: if there is a mismatch between pages and the underlying + store, like why changing small bits of data on an ssd is slow? + <braunr> mcsim1: i don't see why not + <braunr> antrik: netbsd reads a few pages before too + <braunr> actually, what netbsd does vary on the version, some only mapped + in resident pages, later versions started asynchronous transfers in the + hope those pages would be there + <antrik> LarstiQ: not sure what you are trying to say + <braunr> in linux : + <braunr> 321 * MADV_NORMAL - the default behavior is to read clusters. + This + <braunr> 322 * results in some read-ahead and read-behind. + <braunr> not sure if it's actually what the implementation does + <antrik> well, right -- it's probably always useful to read whole clusters + at a time, especially if they are the same size as pages... that doesn't + mean it always reads preceding pages; only if the read is in the middle + of the cluster AIUI + <LarstiQ> antrik: basically what braunr just pasted + <antrik> and in most cases, we will want to read some *following* clusters + as well, but probably not preceding ones + * LarstiQ nods + <braunr> antrik: the default policy is usually rather sequential + <braunr> here are the numbers for netbsd + <braunr> 166 static struct uvm_advice uvmadvice[] = { + <braunr> 167 { MADV_NORMAL, 3, 4 }, + <braunr> 168 { MADV_RANDOM, 0, 0 }, + <braunr> 169 { MADV_SEQUENTIAL, 8, 7}, + <braunr> 170 }; + <braunr> struct uvm_advice { + <braunr> int advice; + <braunr> int nback; + <braunr> int nforw; + <braunr> }; + <braunr> surprising isn't it ? + <braunr> they may suggest sequential may be backwards too + <braunr> makes sense + <antrik> braunr: what are these numbers? pages? + <braunr> yes + <antrik> braunr: I suspect the idea behind SEQUENTIAL is that with typical + sequential access patterns, you will start at one end of the file, and + then go towards the other end -- so the extra clusters in the "wrong" + direction do not actually come into play + <antrik> only situation where some extra clusters are actually read is when + you start in the middle of a file, and thus do not know yet in which + direction the sequential read will go... + <braunr> yes, there are similar comments in the linux code + <braunr> mcsim1: so having before and after numbers seems both + straightforward and in par with other implementations + <antrik> I'm still surprised about the almost symmetrical policy for NORMAL + though + <antrik> BTW, is it common to use heuristics for automatically recognizing + random and sequential patterns in the absence of explicit madise? + <braunr> i don't know + <braunr> netbsd doesn't use any, linux seems to have different behaviours + for anonymous and file memory + <antrik> when KAM was working on this stuff, someone suggested that... + <braunr> there is a file_ra_state struct in linux, for per file read-ahead + policy + <braunr> now the structure is of course per file system, since they all use + the same address + <braunr> (which is why i wanted it to be per pager in the first place) + <antrik> mcsim1: as I said before, it might be useful for the pager to + supply cluster size, if it's different than page size. but right now I + don't think this is something worth bothering with... + <antrik> I seriously doubt it would be useful for the pager to supply any + other kind of policy + <antrik> braunr: I don't understand your remark about using the same + address... + <antrik> braunr: pre-mapping seems the obvious way to implement readahead + policy + <antrik> err... per-mapping + <braunr> the ra_state (read ahead state) isn't the policy + <braunr> the policy is per mapping, parts of the implementation of the + policy is per file system + <mcsim1> braunr: How do you look at following implementation of NORMAL + policy: We have fault page that is current. Than we have maximal size of + readahead block. First we find first absent pages before and after + current. Than we try to fit block that will be readahead into this + range. Here could be following situations: in range RBS/2 (RBS -- size of + readahead block) there is no any page, so readahead will be symmetric; if + current page is first absent page than all + <mcsim1> RBS block will consist of pages that are after current; on the + contrary if current page is last absent than readahead will go backwards. + <mcsim1> Additionally if current page is approximately in the middle of the + range we can decrease RBS, supposing that access is random. + <braunr> mcsim1: i think your gsoc project is about readahead, we're in + july, and you need to get the job done + <braunr> mcsim1: grab one policy that works, pages before and after are + good enough + <braunr> use sane default values, let the pagers decide if they want + something else + <braunr> and concentrate on the real work now + <antrik> braunr: I still don't see why pagers should mess with that... only + complicates matters IMHO + <braunr> antrik: probably, since they almost all use the default + implementation + <braunr> mcsim1: just use sane values inside the kernel :p + <braunr> this simplifies things by only adding the new vm_advise call and + not change the existing external pager interface diff --git a/open_issues/pfinet_vs_system_time_changes.mdwn b/open_issues/pfinet_vs_system_time_changes.mdwn index 513cbc73..46705047 100644 --- a/open_issues/pfinet_vs_system_time_changes.mdwn +++ b/open_issues/pfinet_vs_system_time_changes.mdwn @@ -1,4 +1,5 @@ -[[!meta copyright="Copyright © 2010, 2011 Free Software Foundation, Inc."]] +[[!meta copyright="Copyright © 2010, 2011, 2012 Free Software Foundation, +Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this @@ -58,3 +59,24 @@ IRC, freenode, #hurd, 2011-10-27: <antrik> it's really fascinating that only the pfinet on the Hurd instance where I set the date is affected, and not the pfinet in the other instance + +IRC, freenode, #hurd, 2012-06-28: + + <bddebian> great, now setting the date/time fucked my machine + <pinotree> yes, we lack a monotonic clock + <pinotree> there are select() loops that use gettimeofday to determine how + much time to wait + <pinotree> thus if the time changes (eg goes back), the calculation goes + crazy + <antrik> pinotree: didn't you implement a monotonic clock?... + <pinotree> started to + <antrik> bddebian: did it really fuck the machine? normally it only resets + TCP connections... + <pinotree> yeah, i remember such gettimeofay-based select-loops at least in + pfinet + <antrik> I don't think it's a loop. it just drops the connections, + believing they have timed out + <bddebian> antrik: Well in this case I don't know because I am at work but + it fucked me because I now cannot get to it.. :) + <antrik> bddebian: that's odd... you should be able to just log in again + IIRC diff --git a/open_issues/qemu_writeback.mdwn b/open_issues/qemu_writeback.mdwn new file mode 100644 index 00000000..ab881705 --- /dev/null +++ b/open_issues/qemu_writeback.mdwn @@ -0,0 +1,18 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_documentation]] + + +# IRC, freenode, #hurdfr, 2012-07-01 + + <braunr> remplace "-hda file.img" par "-drive + cache=writeback,index=0,media=disk,file=file.img" + <braunr> tu sentiras tout de suite la différence diff --git a/open_issues/strict_aliasing.mdwn b/open_issues/strict_aliasing.mdwn new file mode 100644 index 00000000..01019372 --- /dev/null +++ b/open_issues/strict_aliasing.mdwn @@ -0,0 +1,21 @@ +[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +[[!tag open_issue_glibc open_issue_gnumach open_issue_hurd open_issue_mig]] + + +# IRC, freenode, #hurd, 2012-07-04 + + <braunr> we should perhaps build the hurd with -fno-strict-aliasing, + considering the number of warnings i can see during the build :/ + <pinotree> braunr: wouldn't be better to "just" fix the mig-generated stubs + instead? + <braunr> pinotree: if we can rely on gcc for the warnings, yes + <braunr> but i suspect there might be other silent issues in very old code |