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diff --git a/open_issues/gnumach_vm_map_red-black_trees.mdwn b/open_issues/gnumach_vm_map_red-black_trees.mdwn deleted file mode 100644 index 53ff66c5..00000000 --- a/open_issues/gnumach_vm_map_red-black_trees.mdwn +++ /dev/null @@ -1,346 +0,0 @@ -[[!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, 2012-04-23 - - <braunr> btw, i'm running a gnumach version using red-black trees for vm - map entries - <antrik> braunr: sounds fashionable ;-) - <youpi> braunr: with some perf improvement? - <braunr> looks promising for our ext2fs instances showing several thousands - of map entries - <braunr> youpi: i'm not using it for lookups yet - <braunr> but the tree is there, maintained, used for both regular and copy - maps, and it doesn't crash - <youpi> good :) - <braunr> antrik: isn't it ? :) - <braunr> youpi: and the diff stat is like 50/15 - <antrik> braunr: what's the goal of using the fashionable trees? - <braunr> antrik: speeding up lookups in address spaces - <antrik> braunr: so the idea is that if we have a heavily fragmented - address space, the performance penalty is smaller? - <braunr> yes - <antrik> OK - <antrik> I take it you gave up on attempts to actually decrease - fragmentation?... - <braunr> it's not as good as reducing fragmentation, which requires - implementing a powerful merge, but it's still better - <braunr> yes - <braunr> it's too messy for my brain :/ - <antrik> I see - <antrik> oh - <braunr> it will add some overhead though - <youpi> I guess log(n) ? - <braunr> but if there is a significant performance gain, it'll be worth it - <braunr> yes - <braunr> i was more thinking about the memory overhead - <antrik> right now it's a linear list? - <youpi> I don't think we care nowadays :) - <braunr> antrik: yes - <antrik> ouch - <braunr> antrik: yes ... :> - <braunr> the original authors expected vm maps to have like 30 entries - <braunr> so they used a list for the maps, and a hash table for the - object/offset to physical page lookups - <braunr> there is a small lookup cache though, which is a nice optimization - <braunr> my code now uses it first, and falls back to the RB tree if the - hint didn't help - <antrik> braunr: well, don't forget to check whether it actually *is* still - an optimisation, when using fashionable trees ;-) - <braunr> antrik: i checked that already :) - <braunr> i did the same in x15 - <antrik> I see - <braunr> both bsd and linux uses a similar technique - <braunr> use* - <braunr> (well, bsd actually use what is done in mach :) - <antrik> (or perhaps the other way around... ;-) ) - <braunr> i don't think so, as the bsd vm is really the mach vm - <braunr> but we don't care much - <antrik> oh, right... that part actually went full circle - <braunr> youpi: i have a patch ready for test on machines with significant - amounts of map entries (e.g. the buildds ..) - <braunr> youpi: i won't have time for tests tonight, are you interested ? - <braunr> (i've been running it for 15 minutes without any issue for now) - <youpi> I'd say post to the list - <braunr> ok - <youpi> braunr: your patch uses the rb tree for lookups, right? - <youpi> braunr: the buildd using rbtree seems swift - <youpi> but maybe it's just a psychologic effect :) - <youpi> the chroot ext2fs already has 1392 lines in vminfo - <youpi> an rbtree can't hurt there :) - <youpi> braunr: it really seems faster - <youpi> the reboot might have helped too - <youpi> benchmarks shall say - <youpi> for now, I'll just let ironforge use it - <antrik> youpi: it's always fast after a reboot ;-) - <youpi> sure - <youpi> but still - <youpi> I mean - <youpi> *obviously* the reboot has helped - <youpi> but it might not be all - <youpi> at least it feels so - <youpi> and obviously only benchmarks can say - <antrik> the major benefit AIUI is rather that the slowdown happening over - time will be less noticable - -[[performance/degradation]]. - - <youpi> "over time" is actually quite fast - <youpi> ext2 fills up very quickly when you build a package - <youpi> it went up to 1700 lines very quickly - <youpi> and stabilized around there - <antrik> yeah, it can be very fast under heavy load - <youpi> that's why I say reboot seems not all - <youpi> it's already not so fresh - <youpi> with 1700 lines in vminfo - <antrik> well, I don't know how much of the slowdown I'm experiencing - (after doing stuff under memory pressure) is actually related to VM map - fragmentation... - <antrik> might be all, might be none, might be something in between - <youpi> then try his patch - <antrik> guess I should play a bit with vminfo... - <antrik> well, currently I actually experience pretty little slowdown, as - for certain reasons (only indirectly related to the Hurd) I'm not running - mutt on this machine, so I don't really have memory pressure... - - -## IRC, freenode, #hurd, 2012-04-24 - - <braunr> youpi: yes, it uses bst lookups - <braunr> youpi: FYI, the last time i checked, one ext2fs instance had 4k+ - map entries, and another around 7.5k - <braunr> (on ironforge) - - -## IRC, freenode, #hurd, 2012-04-24 - - <youpi> braunr: $ sudo vminfo 624 | wc -l - <youpi> 22957 - <youpi> there's no way it can not help :) - <braunr> youpi: 23k, that's really huge - - -## IRC, freenode, #hurd, 2012-04-26 - - <braunr> youpi: any new numbers wrt the rbtree patch ? - <youpi> well, buildd times are not really accurate :) - <youpi> but what I can tell is that it managed to build qtwebkit fine - <braunr> ok - <youpi> so the patch is probably safe :) - <braunr> i'll commit it soon then - <youpi> I'd say feel free to, yes - <braunr> thanks - - -## IRC, freenode, #hurd, 2012-04-27 - - <braunr> elmig: don't expect anything grand though, this patch is mostly - useful when address spaces get really fragmented, which mainly happens on - buildds - <braunr> (and it only speeds lookups, which isn't as good as reducing - fragmentation; things like fork still have to copy thousands of map - 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) - - -### IRC, freenode, #hurd, 2012-07-15 - - <bddebian> I get errors in vm_map.c whenever I try to "mount" a CD - <bddebian> Hmm, this time it rebooted the machine - <bddebian> braunr: The translator set this time and the machine reboots - before I can get the full message about vm_map, but here is some of the - crap I get: http://paste.debian.net/179191/ - <braunr> oh - <braunr> nice - <braunr> that may be the bug youpi saw with my redblack tree patch - <braunr> bddebian: assert(diff != 0); ? - <bddebian> Aye - <braunr> good - <braunr> it means we're trying to insert a vm_map_entry at a region in a - map which is already occupied - <bddebian> Oh - <braunr> and unlike the previous code, the tree actually checks that - <braunr> it has to - <braunr> so you just simply use the iso9660fs translator and it crashes ? - <bddebian> Well it used to on just trying to set the translator. This time - I was able to set the translator but as soon as I cd to the mount point I - get all that crap - <braunr> that's very good - <braunr> more test cases to fix the vm - - -### IRC, freenode, #hurd, 2012-11-01 - - <youpi> braunr: Assertion `diff != 0' failed in file "vm/vm_map.c", line - 1002 - <youpi> that's in rbtree_insert - <braunr> youpi: the problem isn't the tree, it's the map entries - <braunr> some must overlap - <braunr> if you can inspect that, it would be helpful - <youpi> I have a kdb there - <youpi> it's within a port_name_to_task system call - <braunr> this assertion basically means there already is an item in the - tree where the new item is supposed to be inserted - <youpi> this port_name_to_task presence in the stack is odd - <braunr> it's in vm_map_enter - <youpi> there's a vm_map just after that (and the assembly trap code - before) - <youpi> I know - <youpi> I'm wondering about the caller - <braunr> do you have a way to inspect the inserted map entry ? - <youpi> I'm actually wondering whether I have the right kernel in gdb - <braunr> oh - <youpi> better - <youpi> with the right kernel :) - <youpi> 0x80039acf (syscall_vm_map) - (target_map=d48b6640,address=d3b63f90,size=0,mask=0,anywhere=1) - <youpi> size == 0 seems odd to me - <youpi> (same parameters for vm_map) - <braunr> right - <braunr> my code does assume an entry has a non null size - <braunr> (in the entry comparison function) - <braunr> EINVAL (since Linux 2.6.12) length was 0. - <braunr> that's a quick glance at mmap(2) - <braunr> might help track bugs from userspace (e.g. in exec .. :)) - <braunr> posix says the saem - <braunr> same* - <braunr> the gnumach manual isn't that precise - <youpi> I don't seem to manage to read the entry - <youpi> but I guess size==0 is the problem anyway - <mcsim> youpi, braunr: Is there another kernel fault? Was that in my - kernel? - <braunr> no that's another problem - <braunr> which became apparent following the addition of red black trees in - the vm_map code - <braunr> (but which was probably present long before) - <mcsim> braunr: BTW, do you know if there where some specific circumstances - that led to memory exhaustion in my code? Or it just aggregated over - time? - <braunr> mcsim: i don't know - <mcsim> s/where/were - <mcsim> braunr: ok - - -### IRC, freenode, #hurd, 2012-11-05 - - <tschwinge> braunr: I have now also hit the diff != 0 assertion error; - sitting in KDB, waiting for your commands. - <braunr> tschwinge: can you check the backtrace, have a look at the system - call and its parameters like youpi did ? - <tschwinge> If I manage to figure out how to do that... :-) - * tschwinge goes read scrollback. - <braunr> "trace" i suppose - <braunr> if running inside qemu, you can use the integrated gdb server - <tschwinge> braunr: No, hardware. And work intervened. And mobile phone - <-> laptop via bluetooth didn't work. But now: - <tschwinge> Pretty similar to Samuel's: - <tschwinge> Assert([...]) - <tschwinge> vm_map_enter(0xc11de6c8, 0xc1785f94, 0, 0, 1) - <tschwinge> vm_map(0xc11de6c8, 0xc1785f94, 0, 0, 1) - <tschwinge> syscall_vm_map(1, 0x1024a88, 0, 0, 1) - <tschwinge> mach_call_call(1, 0x1024a88, 0, 0, 1) - <braunr> thanks - <braunr> same as youpi observed, the requested size for the mapping is 0 - <braunr> tschwinge: thanks - <tschwinge> braunr: Anything else you'd like to see before I reboot? - <braunr> tschwinge: no, that's enough for now, and the other kind of info - i'd like are much more difficult to obtain - <braunr> if we still have the problem once a small patch to prevent null - size is applied, then it'll be worth looking more into it - <pinotree> isn't it possible to find out who called with that size? - <braunr> not easy, no - <braunr> it's also likely that the call that fails isn't the first one - <pinotree> ah sure - <pinotree> braunr: making mmap reject 0 size length could help? posix says - such size should be rejected straight away - <braunr> 17:09 < braunr> if we still have the problem once a small patch to - prevent null size is applied, then it'll be worth looking more into it - <braunr> that's the idea - <braunr> making faulty processes choke on it should work fine :) - <pinotree> «If len is zero, mmap() shall fail and no mapping shall be - established.» - <pinotree> braunr: should i cook up such patch for mmap? - <braunr> no, the change must be applied in gnumach - <pinotree> sure, but that could simply such condition in mmap (ie avoiding - to call io_map on a file) - <braunr> such calls are erroneous and rare, i don't see the need - <pinotree> ok - <braunr> i bet it comes from the exec server anyway :p - <tschwinge> braunr: Is the mmap with size 0 already a reproducible testcase - you can use for the diff != 0 assertion? - <tschwinge> Otherwise I'd have a reproducer now. - <braunr> tschwinge: i'm not sure but probably yes - <tschwinge> braunr: Otherwise, take GDB sources, then: gcc -fsplit-stack - gdb/testsuite/gdb.base/morestack.c && ./a.out - <tschwinge> I have not looked what exactly this does; I think -fsplit-stack - is not really implemented for us (needs something in libgcc we might not - have), is on my GCC TODO list already. - <braunr> tschwinge: interesting too :) - - -### IRC, freenode, #hurd, 2012-11-19 - - <tschwinge> braunr: Hmm, I have now hit the diff != 0 GNU Mach assertion - failure during some GCC invocation (GCC testsuite) that does not relate - to -fsplit-stack (as the others before always have). - <tschwinge> Reproduced: - /media/erich/home/thomas/tmp/gcc/hurd/master.build/gcc/xgcc - -B/media/erich/home/thomas/tmp/gcc/hurd/master.build/gcc/ - /home/thomas/tmp/gcc/hurd/master/gcc/testsuite/gcc.dg/torture/pr42878-1.c - -fno-diagnostics-show-caret -O2 -flto -fuse-linker-plugin - -fno-fat-lto-objects -fcompare-debug -S -o pr42878-1.s - <tschwinge> Will check whether it's the same backtrace in GNU Mach. - <tschwinge> Yes, same. - <braunr> tschwinge: as youpi seems quite busy these days, i'll cook a patch - and commit it directly - <tschwinge> braunr: Thanks! I have, by the way, confirmed that the - following is enough to trigger the issue: vm_map(mach_task_self(), 0, 0, - 0, 1, 0, 0, 0, 0, 0, 0); - <tschwinge> ... and before the allocator patch, GNU Mach did accept that - and return 0 -- though I did not check what effect it actually has. (And - I don't think it has any useful one.) I'm also reading that as of lately - (Linux 2.6.12), mmap (length = 0) is to return EINVAL, which I think is - the foremost user of vm_map. - <pinotree> tschwinge: posix too says to return EINVAL for length = 0 - <braunr> yes, we checked that earlier with youpi - -[[!message-id "87sj8522zx.fsf@kepler.schwinge.homeip.net"]]. - - <braunr> tschwinge: well, actually your patch is what i had in mind - (although i'd like one in vm_map_enter to catch wrong kernel requests - too) - <braunr> tschwinge: i'll work on it tonight, and do some testing to make - sure we don't regress critical stuff (exec is another major direct user - of vm_map iirc) - <tschwinge> braunr: Oh, OK. :-) |