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diff --git a/open_issues/gnumach_memory_management.mdwn b/open_issues/gnumach_memory_management.mdwn deleted file mode 100644 index b36c674a..00000000 --- a/open_issues/gnumach_memory_management.mdwn +++ /dev/null @@ -1,2391 +0,0 @@ -[[!meta copyright="Copyright © 2011, 2012, 2013, 2014 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]] - -There is a [[!FF_project 266]][[!tag bounty]] on this task. - -[[!toc]] - - -# IRC, freenode, #hurd, 2011-04-12 - - <antrik> braunr: do you think the allocator you wrote for x15 could be used - for gnumach? and would you be willing to mentor this? :-) - <braunr> antrik: to be willing to isn't my current problem - <braunr> antrik: and yes, I think my allocator can be used - <braunr> it's a slab allocator after all, it only requires reap() and - grow() - <braunr> or mmap()/munmap() whatever you want to call it - <braunr> a backend - <braunr> antrik: although i've been having other ideas recently - <braunr> that would have more impact on our usage patterns I think - <antrik> mcsim: have you investigated how the zone allocator works and how - it's hooked into the system yet? - <braunr> mcsim: now let me give you a link - <braunr> mcsim: - http://git.sceen.net/rbraun/libbraunr.git/?a=blob;f=mem.c;h=330436e799f322949bfd9e2fedf0475660309946;hb=HEAD - <braunr> mcsim: this is an implementation of the slab allocator i've been - working on recently - <braunr> mcsim: i haven't made it public because i reworked the per - processor layer, and this part isn't complete yet - <braunr> mcsim: you could use it as a reference for your project - <mcsim> braunr: ok - <braunr> it used to be close to the 2001 vmem paper - <braunr> but after many tests, fragmentation and accounting issues have - been found - <braunr> so i rewrote it to be closer to the linux implementation (cache - filling/draining in bukl transfers) - <braunr> bulk* - <braunr> they actually use the word draining in linux too :) - <mcsim> antrik: not complete yet. - <antrik> braunr: oh, it's unfinished? that's unfortunate... - <braunr> antrik: only the per processor part - <braunr> antrik: so it doesn't matter much for gnumach - <braunr> and it's not difficult to set up - <antrik> mcsim: hm, OK... but do you think you will have a fairly good - understanding in the next couple of days?... - <antrik> I'm asking because I'd really like to see a proposal a bit more - specific than "I'll look into things..." - <antrik> i.e. you should have an idea which things you will actually have - to change to hook up a new allocator etc. - <antrik> braunr: OK. will the interface remain unchanged, so it could be - easily replaced with an improved implementation later? - <braunr> the zone allocator in gnumach is a badly written bare object - allocator actually, there aren't many things to understand about it - <braunr> antrik: yes - <antrik> great :-) - <braunr> and the per processor part should be very close to the phys - allocator sitting next to it - <braunr> (with the slight difference that, as per cpu caches have variable - sizes, they are allocated on the free path rather than on the allocation - path) - <braunr> this is a nice trick in the vmem paper i've kept in mind - <braunr> and the interface also allows to set a "source" for caches - <antrik> ah, good point... do you think we should replace the physmem - allocator too? and if so, do it in one step, or one piece at a time?... - <braunr> no - <braunr> too many drivers currently depend on the physical allocator and - the pmap module as they are - <braunr> remember linux 2.0 drivers need a direct virtual to physical - mapping - <braunr> (especially true for dma mappings) - <antrik> OK - <braunr> the nice thing about having a configurable memory source is that - <antrik> whot do you mean by "allocated on the free path"? - <braunr> even if most caches will use the standard vm_kmem module as their - backend - <braunr> there is one exception in the vm_map module, allowing us to get - rid of either a static limit, or specific allocation code - <braunr> antrik: well, when you allocate a page, the allocator will lookup - one in a per cpu cache - <braunr> if it's empty, it fills the cache - <braunr> (called pools in my implementations) - <braunr> it then retries - <braunr> the problem in the slab allocator is that per cpu caches have - variable sizes - <braunr> so per cpu pools are allocated from their own pools - <braunr> (remember the magazine_xx caches in the output i showed you, this - is the same thing) - <braunr> but if you allocate them at allocation time, you could end up in - an infinite loop - <braunr> so, in the slab allocator, when a per cpu cache is empty, you just - fall back to the slab layer - <braunr> on the free path, when a per cpu cache doesn't exist, you allocate - it from its own cache - <braunr> this way you can't have an infinite loop - <mcsim> antrik: I'll try, but I have exams now. - <mcsim> As I understand amount of elements which could be allocated we - determine by zone initialization. And at this time memory for zone is - reserved. I'm going to change this. And make something similar to kmalloc - and vmalloc (support for pages consecutive physically and virtually). And - pages in zones consecutive always physically. - <mcsim> Am I right? - <braunr> mcsim: don't try to do that - <mcsim> why? - <braunr> mcsim: we just need a slab allocator with an interface close to - the zone allocator - <antrik> mcsim: IIRC the size of the complete zalloc map is fixed; but not - the number of elements per zone - <braunr> we don't need two allocators like kmalloc and vmalloc - <braunr> actually we just need vmalloc - <braunr> IIRC the limits are only present because the original developers - wanted to track leaks - <braunr> they assumed zones would be large enough, which isn't true any - more today - <braunr> but i didn't see any true reservation - <braunr> antrik: i'm not sure i was clear enough about the "allocation of - cpu caches on the free path" - <braunr> antrik: for a better explanation, read the vmem paper ;) - <antrik> braunr: you mean there is no fundamental reason why the zone map - has a limited maximal size; and it was only put in to catch cases where - something eats up all memory with kernel object creation?... - <antrik> braunr: I think I got it now :-) - <braunr> antrik: i'm pretty certin of it yes - <antrik> I don't see though how it is related to what we were talking - about... - <braunr> 10:55 < braunr> and the per processor part should be very close to - the phys allocator sitting next to it - <braunr> the phys allocator doesn't have to use this trick - <braunr> because pages have a fixed size, so per cpu caches all have the - same size too - <braunr> and the number of "caches", that is, physical segments, is limited - and known at compile time - <braunr> so having them statically allocated is possible - <antrik> I see - <braunr> it would actually be very difficult to have a phys allocator - requiring dynamic allocation when the dynamic allocator isn't yet ready - <antrik> hehe :-) - <mcsim> total size of all zone allocations is limited to 12 MB. And is "was - only put in to catch cases where something eats up all memory with kernel - object creation?" - <braunr> mcsim: ah right, there could be a kernel submap backing all the - zones - <braunr> but this can be increased too - <braunr> submaps are kind of evil :/ - <antrik> mcsim: I think it's actually 32 MiB or something like that in the - Debian version... - <antrik> braunr: I'm not sure I ever fully understood what the zalloc map - is... I looked through the code once, and I think I got a rough - understading, but I was still pretty uncertain about some bits. and I - don't remember the details anyways :-) - <braunr> antrik: IIRC, it's a kernel submap - <braunr> it's named kmem_map in x15 - <antrik> don't know what a submap is - <braunr> submaps are vm_map objects - <braunr> in a top vm_map, there are vm_map_entries - <braunr> these entries usually point to vm_objects - <braunr> (for the page cache) - <braunr> but they can point to other maps too - <braunr> the goal is to reduce fragmentation by isolating allocations - <braunr> this also helps reducing contention - <braunr> for exemple, on BSD, there is a submap for mbufs, so that the - network code doesn't interfere too much with other kernel allocations - <braunr> antrik: they are similar to spans in vmem, but vmem has an elegant - importing mechanism which eliminates the static limit problem - <antrik> so memory is not directly allocated from the physical allocator, - but instead from another map which in turn contains physical memory, or - something like that?... - <braunr> no, this is entirely virtual - <braunr> submaps are almost exclusively used for the kernel_map - <antrik> you are using a lot of identifies here, but I don't remember (or - never knew) what most of them mean :-( - <braunr> sorry :) - <braunr> the kernel map is the vm_map used to represent the ~1 GiB of - virtual memory the kernel has (on i386) - <braunr> vm_map objects are simple virtual space maps - <braunr> they contain what you see in linux when doing /proc/self/maps - <braunr> cat /proc/self/maps - <braunr> (linux uses entirely different names but it's roughly the same - structure) - <braunr> each line is a vm_map_entry - <braunr> (well, there aren't submaps in linux though) - <braunr> the pmap tool on netbsd is able to show the kernel map with its - submaps, but i don't have any image around - <mcsim> braunr: is limit for zones is feature and shouldn't be changed? - <braunr> mcsim: i think we shouldn't have fixed limits for zones - <braunr> mcsim: this should be part of the debugging facilities in the slab - allocator - <braunr> is this fixed limit really a major problem ? - <braunr> i mean, don't focus on that too much, there are other issues - requiring more attention - <antrik> braunr: at 12 MiB, it used to be, causing a lot of zalloc - panics. after increasing, I don't think it's much of a problem anymore... - <antrik> but as memory sizes grow, it might become one again - <antrik> that's the problem with a fixed size... - <braunr> yes, that's the issue with submaps - <braunr> but gnumach is full of those, so let's fix them by order of - priority - <antrik> well, I'm still trying to digest what you wrote about submaps :-) - <braunr> i'm downloading netbsd, so you can have a good view of all this - <antrik> so, when the kernel allocates virtual address space regions - (mostly for itself), instead of grabbing chunks of the address space - directly, it takes parts out of a pre-reserved region? - <braunr> not exactly - <braunr> both statements are true - <mcsim> antrik: only virtual addresses are reserved - <braunr> it grabs chunks of the address space directly, but does so in a - reserved region of the address space - <braunr> a submap is like a normal map, it has a start address, a size, and - is empty, then it's populated with vm_map_entries - <braunr> so instead of allocating from 3-4 GiB, you allocate from, say, - 3.1-3.2 GiB - <antrik> yeah, that's more or less what I meant... - <mcsim> braunr: I see two problems: limited zones and absence of caching. - <mcsim> with caching absence of readahead paging will be not so significant - <braunr> please avoid readahead - <mcsim> ok - <braunr> and it's not about paging, it's about kernel memory, which is - wired - <braunr> (well most of it) - <braunr> what about limited zones ? - <braunr> the whole kernel space is limited, there has to be limits - <braunr> the problem is how to handle them - <antrik> braunr: almost all. I looked through all zones once, and IIRC I - found exactly one that actually allows paging... - <braunr> currently, when you reach the limit, you have an OOM error - <braunr> antrik: yes, there are - <braunr> i don't remember which implementation does that but, when - processes haven't been active for a minute or so, they are "swapedout" - <braunr> completely - <braunr> even the kernel stack - <braunr> and the page tables - <braunr> (most of the pmap structures are destroyed, some are retained) - <antrik> that might very well be true... at least inactive processes often - show up with 0 memory use in top on Hurd - <braunr> this is done by having a pageable kernel map, with wired entries - <braunr> when the swapper thread swaps tasks out, it unwires them - <braunr> but i think modern implementations don't do that any more - <antrik> well, I was talking about zalloc only :-) - <braunr> oh - <braunr> so the zalloc_map must be pageable - <braunr> or there are two submaps ? - <antrik> not sure whether "morden implementations" includes Linux ;-) - <braunr> no, i'm talking about the bsd family only - <antrik> but it's certainly true that on Linux even inactive processes - retain some memory - <braunr> linux doesn't make any difference between processor-bound and - I/O-bound processes - <antrik> braunr: I have no idea how it works. I just remember that when - creating zones, one of the optional flags decides whether the zone is - pagable. but as I said, IIRC there is exactly one that actually is... - <braunr> zone_map = kmem_suballoc(kernel_map, &zone_min, &zone_max, - zone_map_size, FALSE); - <braunr> kmem_suballoc(parent, min, max, size, pageable) - <braunr> so the zone_map isn't - <antrik> IIRC my conclusion was that pagable zones do not count in the - fixed zone map limit... but I'm not sure anymore - <braunr> zinit() has a memtype parameter - <braunr> with ZONE_PAGEABLE as a possible flag - <braunr> this is wierd :) - <mcsim> There is no any zones which use ZONE_PAGEABLE flag - <antrik> mcsim: are you sure? I think I found one... - <braunr> if (zone->type & ZONE_PAGEABLE) { - <antrik> admittedly, it is several years ago that I looked into this, so my - memory is rather dim... - <braunr> if (kmem_alloc_pageable(zone_map, &addr, ... - <braunr> calling kmem_alloc_pageable() on an unpageable submap seems wrong - <mcsim> I've greped gnumach code and there is no any zinit procedure call - with ZONE_PAGEABLE flag - <braunr> good - <antrik> hm... perhaps it was in some code that has been removed - alltogether since ;-) - <antrik> actually I think it would be pretty neat to have pageable kernel - objects... but I guess it would require considerable effort to implement - this right - <braunr> mcsim: you also mentioned absence of caching - <braunr> mcsim: the zone allocator actually is a bare caching object - allocator - <braunr> antrik: no, it's easy - <braunr> antrik: i already had that in x15 0.1 - <braunr> antrik: the problem is being sure the objects you allocate from a - pageable backing store are never used when resolving a page fault - <braunr> that's all - <antrik> I wouldn't expect that to be easy... but surely you know better - :-) - <mcsim> braunr: indeed. I was wrong. - <antrik> braunr: what is a caching object allocator?... - <braunr> antrik: ok, it's not easy - <braunr> antrik: but once you have vm_objects implemented, having pageable - kernel object is just a matter of using the right options, really - <braunr> antrik: an allocator that caches its buffers - <braunr> some years ago, the term "object" would also apply to - preconstructed buffers - <antrik> I have no idea what you mean by "caches its buffers" here :-) - <braunr> well, a memory allocator which doesn't immediately free its - buffers caches them - <mcsim> braunr: but can it return objects to system? - <braunr> mcsim: which one ? - <antrik> yeah, obviously the *implementation* of pageable kernel objects is - not hard. the tricky part is deciding which objects can be pageable, and - which need to be wired... - <mcsim> Can zone allocator return cached objects to system as in slab? - <mcsim> I mean reap() - <braunr> well yes, it does so, and it does that too often - <braunr> the caching in the zone allocator is actually limited to the - pagesize - <braunr> once page is completely free, it is returned to the vm - <mcsim> this is bad caching - <braunr> yes - <mcsim> if object takes all page than there is now caching at all - <braunr> caching by side effect - <braunr> true - <braunr> but the linux slab allocator does the same thing :p - <braunr> hm - <braunr> no, the solaris slab allocator does so - <mcsim> linux's slab returns objects only when system ask - <antrik> without preconstructed objects, is there actually any point in - caching empty slabs?... - <mcsim> Once I've changed my allocator to slab and it cached more than 1GB - of my memory) - <braunr> ok wait, need to fix a few mistakes first - <mcsim> s/ask/asks - <braunr> the zone allocator (in gnumach) actually has a garbage collector - <antrik> braunr: well, the Solaris allocator follows the slab/magazine - paper, right? so there is caching at the magazine layer... in that case - caching empty slabs too would be rather redundant I'd say... - <braunr> which is called when running low on memory, similar to the slab - allocaotr - <braunr> antrik: yes - <antrik> (or rather the paper follows the Solaris allocator ;-) ) - <braunr> mcsim: the zone allocator reap() is zone_gc() - <antrik> braunr: hm, right, there is a "collectable" flag for zones... but - I never understood what it means - <antrik> braunr: BTW, I heard Linux has yet another allocator now called - "slob"... do you happen to know what that is? - <braunr> slob is a very simple allocator for embedded devices - <mcsim> AFAIR this is just heap allocator - <braunr> useful when you have a very low amount of memory - <braunr> like 1 MiB - <braunr> yes - <antrik> just googled it :-) - <braunr> zone and slab are very similar - <antrik> sounds like a simple heap allocator - <mcsim> there is another allocator that calls slub, and it better than slab - in many cases - <braunr> the main difference is the data structures used to store slabs - <braunr> mcsim: i disagree - <antrik> mcsim: ah, you already said that :-) - <braunr> mcsim: slub is better for systems with very large amounts of - memory and processors - <braunr> otherwise, slab is better - <braunr> in addition, there are accounting issues with slub - <braunr> because of cache merging - <mcsim> ok. This strange that slub is default allocator - <braunr> well both are very good - <braunr> iirc, linus stated that he really doesn't care as long as its - works fine - <braunr> he refused slqb because of that - <braunr> slub is nice because it requires less memory than slab, while - still being as fast for most cases - <braunr> it gets slower on the free path, when the cpu performing the free - is different from the one which allocated the object - <braunr> that's a reasonable cost - <mcsim> slub uses heap for large object. Are there any tests that compare - what is better for large objects? - <antrik> well, if slub requires less memory, why do you think slab is - better for smaller systems? :-) - <braunr> antrik: smaller is relative - <antrik> mcsim: for large objects slab allocation is rather pointless, as - you don't have multiple objects in a page anyways... - <braunr> antrik: when lameter wrote slub, it was intended for systems with - several hundreds processors - <antrik> BTW, was slqb really refused only because the other ones are "good - enough"?... - <braunr> yes - <antrik> wow, that's a strange argument... - <braunr> linus is already unhappy of having "so many" allocators - <antrik> well, if the new one is better, it could replace one of the others - :-) - <antrik> or is it useful only in certain cases? - <braunr> that's the problem - <braunr> nobody really knows - <antrik> hm, OK... I guess that should be tested *before* merging ;-) - <antrik> is anyone still working on it, or was it abandonned? - <antrik> mcsim: back to caching... - <antrik> what does caching in the kernel object allocator got to do with - readahead (i.e. clustered paging)?... - <mcsim> if we cached some physical pages we don't need to find new ones for - allocating new object. And that's why there will not be a page fault. - <mcsim> antrik: Regarding kam. Hasn't he finished his project? - <antrik> err... what? - <antrik> one of us must be seriously confused - <antrik> I totally fail to see what caching of physical pages (which isn't - even really a correct description of what slab does) has to do with page - faults - <antrik> right, KAM didn't finish his project - <mcsim> If we free the physical page and return it to system we need - another one for next allocation. But if we keep it, we don't need to find - new physical page. - <mcsim> And physical page is allocated only then when page fault - occurs. Probably, I'm wrong - <antrik> what does "return to system" mean? we are talking about the - kernel... - <antrik> zalloc/slab are about allocating kernel objects. this doesn't have - *anything* to do with paging of userspace processes - <antrik> only thing the have in common is that they need to get pages from - the physical page allocator. but that's yet another topic - <mcsim> Under "return to system" I mean ability to use this page for other - needs. - <braunr> mcsim: consider kernel memory to be wired - <braunr> here, return to system means releasing a page back to the vm - system - <braunr> the vm_kmem module then unmaps the physical page and free its - virtual address in the kernel map - <mcsim> ok - <braunr> antrik: the problem with new allocators like slqb is that it's - very difficult to really know if they're better, even with extensive - testing - <braunr> antrik: there are papers (like wilson95) about the difficulties in - making valuable results in this field - <braunr> see - http://www.sceen.net/~rbraun/dynamic_storage_allocation_a_survey_and_critical_review.pdf - <mcsim> how can be allocated physically continuous object now? - <braunr> mcsim: rephrase please - <mcsim> what is similar to kmalloc in Linux to gnumach? - <braunr> i know memory is reserved for dma in a direct virtual to physical - mapping - <braunr> so even if the allocation is done similarly to vmalloc() - <braunr> the selected region of virtual space maps physical memory, so - memory is physically contiguous too - <braunr> for other allocation types, a block large enough is allocated, so - it's contiguous too - <mcsim> I don't clearly understand. If we have fragmentation in physical - ram, so there aren't 2 free pages in a row, but there are able apart, we - can't to allocate these 2 pages along? - <braunr> no - <braunr> but every system has this problem - <mcsim> But since we have only 12 or 32 MB of memory the problem becomes - more significant - <braunr> you're confusing virtual and physical memory - <braunr> those 32 MiB are virtual - <braunr> the physical pages backing them don't have to be contiguous - <mcsim> Oh, indeed - <mcsim> So the only problem are limits? - <braunr> and performance - <braunr> and correctness - <braunr> i find the zone allocator badly written - <braunr> antrik: mcsim: here is the content of the kernel pmap on NetBSD - (which uses a virtual memory system close to the Mach VM) - <braunr> antrik: mcsim: http://www.sceen.net/~rbraun/pmap.out - -[[pmap.out]] - - <braunr> you can see the kmem_map (which is used for most general kernel - allocations) is 128 MiB large - <braunr> actually it's not the kernel pmap, it's the kernel_map - <antrik> braunr: why is it called pmap.out then? ;-) - <braunr> antrik: because the tool is named pmap - <braunr> for process map - <braunr> it also exists under Linux, although direct access to - /proc/xx/maps gives more info - <mcsim> braunr: I've said that this is kernel_map. Can I see kernel_map for - Linux? - <braunr> mcsim: I don't know how to do that - <mcsim> s/I've/You've - <braunr> but Linux doesn't have submaps, and uses a direct virtual to - physical mapping, so it's used differently - <antrik> how are things (such as zalloc zones) entered into kernel_map? - <braunr> in zone_init() you have - <braunr> zone_map = kmem_suballoc(kernel_map, &zone_min, &zone_max, - zone_map_size, FALSE); - <braunr> so here, kmem_map is named zone_map - <braunr> then, in zalloc() - <braunr> kmem_alloc_wired(zone_map, &addr, zone->alloc_size) - <antrik> so, kmem_alloc just deals out chunks of memory referenced directly - by the address, and without knowing anything about the use? - <braunr> kmem_alloc() gives virtual pages - <braunr> zalloc() carves them into buffers, as in the slab allocator - <braunr> the difference is essentially the lack of formal "slab" object - <braunr> which makes the zone code look like a mess - <antrik> so kmem_suballoc() essentially just takes a bunch of pages from - the main kernel_map, and uses these to back another map which then in - turn deals out pages just like the main kernel_map? - <braunr> no - <braunr> kmem_suballoc creates a vm_map_entry object, and sets its start - and end address - <braunr> and creates a vm_map object, which is then inserted in the new - entry - <braunr> maybe that's what you meant with "essentially just takes a bunch - of pages from the main kernel_map" - <braunr> but there really is no allocation at this point - <braunr> except the map entry and the new map objects - <antrik> well, I'm trying to understand how kmem_alloc() manages things. so - it has map_entry structures like the maps of userspace processes? do - these also reference actual memory objects? - <braunr> kmem_alloc just allocates virtual pages from a vm_map, and backs - those with physical pages (unless the user requested pageable memory) - <braunr> it's not "like the maps of userspace processes" - <braunr> these are actually the same structures - <braunr> a vm_map_entry can reference a memory object or a kernel submap - <braunr> in netbsd, it can also referernce nothing (for pure wired kernel - memory like the vm_page array) - <braunr> maybe it's the same in mach, i don't remember exactly - <braunr> antrik: this is actually very clear in vm/vm_kern.c - <braunr> kmem_alloc() creates a new kernel object for the allocation - <braunr> allocates a new entry (or uses a previous existing one if it can - be extended) through vm_map_find_entry() - <braunr> then calls kmem_alloc_pages() to back it with wired memory - <antrik> "creates a new kernel object" -- what kind of kernel object? - <braunr> kmem_alloc_wired() does roughly the same thing, except it doesn't - need a new kernel object because it knows the new area won't be pageable - <braunr> a simple vm_object - <braunr> used as a container for anonymous memory in case the pages are - swapped out - <antrik> vm_object is the same as memory object/pager? or yet something - different? - <braunr> antrik: almost - <braunr> antrik: a memory_object is the user view of a vm_object - <braunr> as in the kernel/user interfaces used by external pagers - <braunr> vm_object is a more internal name - <mcsim> Is fragmentation a big problem in slab allocator? - <mcsim> I've tested it on my computer in Linux and for some caches it - reached 30-40% - <antrik> well, fragmentation is a major problem for any allocator... - <antrik> the original slab allocator was design specifically with the goal - of reducing fragmentation - <antrik> the revised version with the addition of magazines takes a step - back on this though - <antrik> have you compared it to slub? would be pretty interesting... - <mcsim> I have an idea how can it be decreased, but it will hurt by - performance... - <mcsim> antrik: no I haven't, but there will be might the same, I think - <mcsim> if each cache will handle two types of object: with sizes that will - fit cache sizes (or I bit smaller) and with sizes which are much smaller - than maximal cache size. For first type of object will be used standard - slab allocator and for latter type will be used (within page) heap - allocator. - <mcsim> I think that than fragmentation will be decreased - <antrik> not at all. heap allocator has much worse fragmentation. that's - why slab allocator was invented - <antrik> the problem is that in a long-running program (such an the - kernel), objects tend to have vastly varying lifespans - <mcsim> but we use heap only for objects of specified sizes - <antrik> so often a few old objects will keep a whole page hostage - <mcsim> for example for 32 byte cache it could be 20-28 byte objects - <antrik> that's particularily visible in programs such as firefox, which - will grow the heap during use even though actual needs don't change - <antrik> the slab allocator groups objects in a fashion that makes it more - likely adjacent objects will be freed at similar times - <antrik> well, that's pretty oversimplyfied, but I hope you get the - idea... it's about locality - <mcsim> I agree, but I speak not about general heap allocation. We have - many heaps for objects with different sizes. - <mcsim> Could it be better? - <antrik> note that this has been a topic of considerable research. you - shouldn't seek to improve the actual algorithms -- you would have to read - up on the existing research at least before you can contribute anything - to the field :-) - <antrik> how would that be different from the slab allocator? - <mcsim> slab will allocate 32 byte for both 20 and 32 byte requests - <mcsim> And if there was request for 20 bytes we get 12 unused - <antrik> oh, you mean the implementation of the generic allocator on top of - slabs? well, that might not be optimal... but it's not an often used case - anyways. mostly the kernel uses constant-sized objects, which get their - own caches with custom tailored size - <antrik> I don't think the waste here matters at all - <mcsim> affirmative. So my idea is useless. - <antrik> does the statistic you refer to show the fragmentation in absolute - sizes too? - <mcsim> Can you explain what is absolute size? - <mcsim> I've counted what were requested (as parameter of kmalloc) and what - was really allocated (according to best fit cache size). - <antrik> how did you get that information? - <mcsim> I simply wrote a hook - <antrik> I mean total. i.e. how many KiB or MiB are wasted due to - fragmentation alltogether - <antrik> ah, interesting. how does it work? - <antrik> BTW, did you read the slab papers? - <mcsim> Do you mean articles from lwn.net? - <antrik> no - <antrik> I mean the papers from the Sun hackers who invented the slab - allocator(s) - <antrik> Bonwick mostly IIRC - <mcsim> Yes - <antrik> hm... then you really should know the rationale behind it... - <mcsim> There he says about 11% percent of memory waste - <antrik> you didn't answer my other questions BTW :-) - <mcsim> I've corrupted kernel tree with patch, and tomorrow I'm going to - read myself up for exam (I have it on Thursday). But than I'll send you a - module which I've used for testing. - <antrik> OK - <mcsim> I can send you module now, but it will not work without patch. - <mcsim> It would be better to rewrite it using debugfs, but when I was - writing this test I didn't know about trace_* macros - - -# IRC, freenode, #hurd, 2011-04-15 - - <mcsim> There is a hack in zone_gc when it allocates and frees two - vm_map_kentry_zone elements to make sure the gc will be able to allocate - two in vm_map_delete. Isn't it better to allocate memory for these - entries statically? - <youpi> mcsim: that's not the point of the hack - <youpi> mcsim: the point of the hack is to make sure vm_map_delete will be - able to allocate stuff - <youpi> allocating them statically will just work once - <youpi> it may happen several times that vm_map_delete needs to allocate it - while it's empty (and thus zget_space has to get called, leading to a - hang) - <youpi> funnily enough, the bug is also in macos X - <youpi> it's still in my TODO list to manage to find how to submit the - issue to them - <braunr> really ? - <braunr> eh - <braunr> is that because of map entry splitting ? - <youpi> it's git commit efc3d9c47cd744c316a8521c9a29fa274b507d26 - <youpi> braunr: iirc something like this, yes - <braunr> netbsd has this issue too - <youpi> possibly - <braunr> i think it's a fundamental problem with the design - <braunr> people think of munmap() as something similar to free() - <braunr> whereas it's really unmap - <braunr> with a BSD-like VM, unmap can easily end up splitting one entry in - two - <braunr> but your issue is more about harmful recursion right ? - <youpi> I don't remember actually - <youpi> it's quite some time ago :) - <braunr> ok - <braunr> i think that's why i have "sources" in my slab allocator, the - default source (vm_kern) and a custom one for kernel map entries - - -# IRC, freenode, #hurd, 2011-04-18 - - <mcsim> braunr: you've said that once page is completely free, it is - returned to the vm. - <mcsim> who else, besides zone_gc, can return free pages to the vm? - <braunr> mcsim: i also said i was wrong about that - <braunr> zone_gc is the only one - - -# IRC, freenode, #hurd, 2011-04-19 - - <braunr> antrik: mcsim: i added back a new per-cpu layer as planned - <braunr> - http://git.sceen.net/rbraun/libbraunr.git/?a=blob;f=mem.c;h=c629b2b9b149f118a30f0129bd8b7526b0302c22;hb=HEAD - <braunr> mcsim: btw, in mem_cache_reap(), you can clearly see there are two - loops, just as in zone_gc, to reduce contention and avoid deadlocks - <braunr> this is really common in memory allocators - - -# IRC, freenode, #hurd, 2011-04-23 - - <mcsim> I've looked through some allocators and all of them use different - per cpu cache policy. AFAIK gnuhurd doesn't support multiprocessing, but - still multiprocessing must be kept in mind. So, what do you think what - kind of cpu caches is better? As for me I like variant with only per-cpu - caches (like in slqb). - <antrik> mcsim: well, have you looked at the allocator braunr wrote - himself? :-) - <antrik> I'm not sure I suggested that explicitly to you; but probably it - makes most sense to use that in gnumach - - -# IRC, freenode, #hurd, 2011-04-24 - - <mcsim> antrik: Yes, I have. He uses both global and per cpu caches. But he - also suggested to look through slqb, where there are only per cpu - caches.\ - <braunr> i don't remember slqb in detail - <braunr> what do you mean by "only per-cpu caches" ? - <braunr> a whole slab sytem for each cpu ? - <mcsim> I mean that there are no global queues in caches, but there are - special queues for each cpu. - <mcsim> I've just started investigating slqb's code, but I've read an - article on lwn about it. And I've read that it is used for zen kernel. - <braunr> zen ? - <mcsim> Here is this article http://lwn.net/Articles/311502/ - <mcsim> Yes, this is linux kernel with some patches which haven't been - approved to torvald's tree - <mcsim> http://zen-kernel.org/ - <braunr> i see - <braunr> well it looks nice - <braunr> but as for slub, the problem i can see is cross-CPU freeing - <braunr> and I think nick piggins mentions it - <braunr> piggin* - <braunr> this means that sometimes, objects are "burst-free" from one cpu - cache to another - <braunr> which has the same bad effects as in most other allocators, mainly - fragmentation - <mcsim> There is a special list for freeing object allocated for another - CPU - <mcsim> And garbage collector frees such object on his own - <braunr> so what's your question ? - <mcsim> It is described in the end of article. - <mcsim> What cpu-cache policy do you think is better to implement? - <braunr> at this point, any - <braunr> and even if we had a kernel that perfectly supports - multiprocessor, I wouldn't care much now - <braunr> it's very hard to evaluate such allocators - <braunr> slqb looks nice, but if you have the same amount of fragmentation - per slab as other allocators do (which is likely), you have tat amount of - fragmentation multiplied by the number of processors - <braunr> whereas having shared queues limit the problem somehow - <braunr> having shared queues mean you have a bit more contention - <braunr> so, as is the case most of the time, it's a tradeoff - <braunr> by the way, does pigging say why he "doesn't like" slub ? :) - <braunr> piggin* - <mcsim> http://lwn.net/Articles/311093/ - <mcsim> here he describes what slqb is better. - <braunr> well it doesn't describe why slub is worse - <mcsim> but not very particularly - <braunr> except for order-0 allocations - <braunr> and that's a form of fragmentation like i mentioned above - <braunr> in mach those problems have very different impacts - <braunr> the backend memory isn't physical, it's the kernel virtual space - <braunr> so the kernel allocator can request chunks of higher than order-0 - pages - <braunr> physical pages are allocated one at a time, then mapped in the - kernel space - <mcsim> Doesn't order of page depend on buffer size? - <braunr> it does - <mcsim> And why does gnumach allocates higher than order-0 pages more? - <braunr> why more ? - <braunr> i didn't say more - <mcsim> And why in mach those problems have very different impact? - <braunr> ? - <braunr> i've just explained why :) - <braunr> 09:37 < braunr> physical pages are allocated one at a time, then - mapped in the kernel space - <braunr> "one at a time" means order-0 pages, even if you allocate higher - than order-0 chunks - <mcsim> And in Linux they allocated more than one at time because of - prefetching page reading? - <braunr> do you understand what virtual memory is ? - <braunr> linux allocators allocate "physical memory" - <braunr> mach kernel allocator allocates "virtual memory" - <braunr> so even if you allocate a big chunk of virtual memory, it's backed - by order-0 physical pages - <mcsim> yes, I understand this - <braunr> you don't seem to :/ - <braunr> the problem of higher than order-0 page allocations is - fragmentation - <braunr> do you see why ? - <mcsim> yes - <braunr> so - <braunr> fragmentation in the kernel space is less likely to create issues - than it does in physical memory - <braunr> keep in mind physical memory is almost always full because of the - page cache - <braunr> and constantly under some pressure - <braunr> whereas the kernel space is mostly empty - <braunr> so allocating higher then order-0 pages in linux is more dangerous - than it is in Mach or BSD - <mcsim> ok - <braunr> on the other hand, linux focuses pure performance, and not having - to map memory means less operations, less tlb misses, quicker allocations - <braunr> the Mach VM must map pages "one at a time", which can be expensive - <braunr> it should be adapted to handle multiple page sizes (e.g. 2 MiB) so - that many allocations can be made with few mappings - <braunr> but that's not easy - <braunr> as always: tradeoffs - <mcsim> There are other benefits of physical allocating. In big DMA - transfers can be needed few continuous physical pages. How does mach - handles such cases? - <braunr> gnumach does that awfully - <braunr> it just reserves the whole DMA-able memory and uses special - allocation functions on it, IIRC - <braunr> but kernels which have a MAch VM like memory sytem such as BSDs - have cleaner methods - <braunr> NetBSD provides a function to allocate contiguous physical memory - <braunr> with many constraints - <braunr> FreeBSD uses a binary buddy system like Linux - <braunr> the fact that the kernel allocator uses virtual memory doesn't - mean the kernel has no mean to allocate contiguous physical memory ... - - -# IRC, freenode, #hurd, 2011-05-02 - - <braunr> hm nice, my allocator uses less memory than glibc (squeeze - version) on both 32 and 64 bits systems - <braunr> the new per-cpu layer is proving effective - <neal> braunr: Are you reimplementation malloc? - <braunr> no - <braunr> it's still the slab allocator for mach, but tested in userspace - <braunr> so i wrote malloc wrappers - <neal> Oh. - <braunr> i try to heavily test most of my code in userspace now - <neal> it's easier :-) - <neal> I agree - <braunr> even the physical memory allocator has been implemented this way - <neal> is this your mach version? - <braunr> virtual memory allocation will follow - <neal> or are you working on gnu mach? - <braunr> for now it's my version - <braunr> but i intend to spend the summer working on ipc port names - management - -[[rework_gnumach_IPC_spaces]]. - - <braunr> and integrate the result in gnu mach - <neal> are you keeping the same user-space API? - <neal> Or are you experimenting with something new? - <antrik> braunr: to be fair, it's not terribly hard to use less memory than - glibc :-) - <braunr> yes - <braunr> antrik: well ptmalloc3 received some nice improvements - <braunr> neal: the goal is to rework some of the internals only - <braunr> neal: namely, i simply intend to replace the splay tree with a - radix tree - <antrik> braunr: the glibc allocator is emphasising performace, unlike some - other allocators that trade some performance for much better memory - utilisation... - <antrik> ptmalloc3? - <braunr> that's the allocator used in glibc - <braunr> http://www.malloc.de/en/ - <antrik> OK. haven't seen any recent numbers... the comparision I have in - mind is many years old... - <braunr> i also made some additions to my avl and red-black trees this week - end, which finally make them suitable for almost all generic uses - <braunr> the red-black tree could be used in e.g. gnu mach to augment the - linked list used in vm maps - <braunr> which is what's done in most modern systems - <braunr> it could also be used to drop the overloaded (and probably over - imbalanced) page cache hash table - -[[gnumach_vm_map_red-black_trees]]. - - -# IRC, freenode, #hurd, 2011-05-03 - - <mcsim> antrik: How should I start porting? Have I just include rbraun's - allocator to gnumach and make it compile? - <antrik> mcsim: well, basically yes I guess... but you will have to look at - the code in question first before we know anything more specific :-) - <antrik> I guess braunr might know better how to start, but he doesn't - appear to be here :-( - <braunr> mcsim: you can't juste put my code into gnu mach and make it run, - it really requires a few careful changes - <braunr> mcsim: you will have to analyse how the current zone allocator - interacts with regard to locking - <braunr> if it is used in interrupt handlers - <braunr> what kind of locks it should use instead of the pthread stuff - available in userspace - <braunr> you will have to change the reclamiing policy, so that caches are - reaped on demand - <braunr> (this basically boils down to calling the new reclaiming function - instead of zone_gc()) - <braunr> you must be careful about types too - <braunr> there is work to be done ;) - <braunr> (not to mention the obvious about replacing all the calls to the - zone allocator, and testing/debugging afterwards) - - -# IRC, freenode, #hurd, 2011-07-14 - - <braunr> can you make your patch available ? - <mcsim> it is available in gnumach repository at savannah - <mcsim> tree mplaneta/libbraunr/master - <braunr> mcsim: i'll test your branch - <mcsim> ok. I'll give you a link in a minute - <braunr> hm why balloc ? - <mcsim> Braun's allocator - <braunr> err - <braunr> - http://git.sceen.net/rbraun/x15mach.git/?a=blob;f=kern/kmem.c;h=37173fa0b48fc9d7e177bf93de531819210159ab;hb=HEAD - <braunr> mcsim: this is the interface i had in mind for a kernel version :) - <braunr> very similar to the original slab allocator interface actually - <braunr> well, you've been working - <mcsim> But I have a problem with this patch. When I apply it to gnumach - code from debian repository. I have to make a change in file ramdisk.c - with sed -i 's/kernel_map/\&kernel_map/' device/ramdisk.c - <mcsim> because in git repository there is no such file - <braunr> mcsim: how do you configure the kernel before building ? - <braunr> mcsim: you should keep in touch more often i think, so that you - get feedback from us and don't spend too much time "off course" - <mcsim> I didn't configure it. I just run dpkg-buildsource -b. - <braunr> oh you build the debian package - <braunr> well my version was by configure --enable-kdb --enable-rtl8139 - <braunr> and it seems stuck in an infinite loop during bootstrap - <mcsim> and printf doesn't work. The first function called by c_boot_entry - is printf(version). - <braunr> mcsim: also, you're invited to get the x15mach version of my - files, which are gplv2+ licensed - <braunr> be careful of my macros.h file, it can conflict with the - macros_help.h file from gnumach iirc - <mcsim> There were conflicts with MACRO_BEGIN and MACRO_END. But I solved - it - <braunr> ok - <braunr> it's tricky - <braunr> mcsim: try to find where the first use of the allocator is made - - -# IRC, freenode, #hurd, 2011-07-22 - - <mcsim> braunr, hello. Kernel with your allocator already compiles and - runs. There still some problems, but, certainly, I'm on the final stage - already. I hope I'll finish in a few days. - <tschwinge> mcsim: Oh, cool! Have you done some measurements already? - <mcsim> Not yet - <tschwinge> OK. - <tschwinge> But if it able to run a GNU/Hurd system, then that already is - something, a big milestone! - <braunr> nice - <braunr> although you'll probably need to tweak the garbage collecting - process - <mcsim> tschwinge: thanks - <mcsim> braunr: As back-end for allocating memory I use - kmem_alloc_wired. But in zalloc was an opportunity to use as back-end - kmem_alloc_pageable. Although there was no any zone that used - kmem_alloc_pageable. Do I need to implement this functionality? - <braunr> mcsim: do *not* use kmem_alloc_pageable() - <mcsim> braunr: Ok. This is even better) - <braunr> mcsim: in x15, i've taken this even further: there is *no* kernel - vm object, which means all kernel memory is wired and unmanaged - <braunr> making it fast and safe - <braunr> pageable kernel memory was useful back when RAM was really scarce - <braunr> 20 years ago - <braunr> but it's a source of deadlock - <mcsim> Indeed. I'll won't use kmem_alloc_pageable. - - -# IRC, freenode, #hurd, 2011-08-09 - - < braunr> mcsim: what's the "bug related to MEM_CF_VERIFY" you refer to in - one of your commits ? - < braunr> mcsim: don't use spin_lock_t as a member of another structure - < mcsim> braunr: I confused with types in *_verify functions, so they - didn't work. Than I fixed it in the commit you mentioned. - < braunr> in gnumach, most types are actually structure pointers - < braunr> use simple_lock_data_t - < braunr> mcsim: ok - < mcsim> > use simple_lock_data_t - < mcsim> braunr: ok - < braunr> mcsim: don't make too many changes to the code base, and if - you're unsure, don't hesitate to ask - < braunr> also, i really insist you rename the allocator, as done in x15 - for example - (http://git.sceen.net/rbraun/x15mach.git/?a=blob;f=vm/kmem.c), instead of - a name based on mine :/ - < mcsim> braunr: Ok. It was just work name. When I finish I'll rename the - allocator. - < braunr> other than that, it's nice to see progress - < braunr> although again, it would be better with some reports along - < braunr> i won't be present at the meeting tomorrow unfortunately, but you - should use those to report the status of your work - < mcsim> braunr: You've said that I have to tweak gc process. Did you mean - to call mem_gc() when physical memory ends instead of calling it every x - seconds? Or something else? - < braunr> there are multiple topics, alhtough only one that really matters - < braunr> study how zone_gc was called - < braunr> reclaiming memory should happen when there is pressure on the VM - subsystem - < braunr> but it shouldn't happen too ofte, otherwise there is trashing - < braunr> and your caches become mostly useless - < braunr> the original slab allocator uses a 15-second period after a - reclaim during which reclaiming has no effect - < braunr> this allows having a somehow stable working set for this duration - < braunr> the linux slab allocator uses 5 seconds, but has a more - complicated reclaiming mechanism - < braunr> it releases memory gradually, and from reclaimable caches only - (dentry for example) - < braunr> for x15 i intend to implement the original 15 second interval and - then perform full reclaims - < mcsim> In zalloc mem_gc is called by vm_pageout_scan, but not often than - once a second. - < mcsim> In balloc I've changed interval to once in 15 seconds. - < braunr> don't use the code as it is - < braunr> the version you've based your work on was meant for userspace - < braunr> where there isn't memory pressure - < braunr> so a timer is used to trigger reclaims at regular intervals - < braunr> it's different in a kernel - < braunr> mcsim: where did you see vm_pageout_scan call the zone gc once a - second ? - < mcsim> vm_pageout_scan calls consider_zone_gc and consider_zone_gc checks - if second is passed. - < braunr> where ? - < mcsim> Than zone_gc can be called. - < braunr> ah ok, it's in zaclloc.c then - < braunr> zalloc.c - < braunr> yes this function is fine - < mcsim> so old gc didn't consider vm pressure. Or I missed something. - < braunr> it did - < mcsim> how? - < braunr> well, it's called by the pageout daemon - < braunr> under memory pressure - < braunr> so it's fine - < mcsim> so if mem_gc is called by pageout daemon is it fine? - < braunr> it must be changed to do something similar to what - consider_zone_gc does - < mcsim> It does. mem_gc does the same work as consider_zone_gc and - zone_gc. - < braunr> good - < mcsim> so gc process is fine? - < braunr> should be - < braunr> i see mem.c only includes mem.h, which then includes other - headers - < braunr> don't do that - < braunr> always include all the headers you need where you need them - < braunr> if you need avltree.h in both mem.c and mem.h, include it in both - files - < braunr> and by the way, i recommend you use the red black tree instead of - the avl type - < braunr> (it's the same interface so it shouldn't take long) - < mcsim> As to report. If you won't be present at the meeting, I can tell - you what I have to do now. - < braunr> sure - < braunr> in addition, use GPLv2 as the license, teh BSD one is meant for - the userspace version only - < braunr> GPLv2+ actually - < braunr> hm you don't need list.c - < braunr> it would only add dead code - < braunr> "Zone for dynamical allocator", don't mix terms - < braunr> this comment refers to a vm_map, so call it a map - < mcsim> 1. Change constructor for kentry_alloc_cache. - < mcsim> 2. Make measurements. - < mcsim> + - < mcsim> 3. Use simple_lock_data_t - < mcsim> 4. Replace license - < braunr> kentry_alloc_cache <= what is that ? - < braunr> cache for kernel map entries in vm_map ? - < braunr> the comment for mem_cpu_pool_get doesn't apply in gnumach, as - there is no kernel preemption - -[[microkernel/mach/gnumach/preemption]]. - - < braunr> "Don't attempt mem GC more frequently than hz/MEM_GC_INTERVAL - times a second. - < braunr> " - < mcsim> sorry. I meant vm_map_kentry_cache - < braunr> hm nothing actually about this comment - < braunr> mcsim: ok - < braunr> yes kernel map entries need special handling - < braunr> i don't know how it's done in gnumach though - < braunr> static preallocation ? - < mcsim> yes - < braunr> that's ugly :p - < mcsim> but it uses dynamic allocation further even for vm_map kernel - entries - < braunr> although such bootstrapping issues are generally difficult to - solve elegantly - < braunr> ah - < mcsim> now I use only static allocation, but I'll add dynamic allocation - too - < braunr> when you have time, mind the coding style (convert everything to - gnumach style, which mostly implies using tabs instead of 4-spaces - indentation) - < braunr> when you'll work on dynamic allocation for the kernel map - entries, you may want to review how it's done in x15 - < braunr> the mem_source type was originally intended for that purpose, but - has slightly changed once the allocator was adapted to work in my kernel - < mcsim> ok - < braunr> vm_map_kentry_zone is the only zone created with ZONE_FIXED - < braunr> and it is zcram()'ed immediately after - < braunr> so you can consider it a statically allocated zone - < braunr> in x15 i use another strategy: there is a special kernel submap - named kentry_map which contains only one map entry (statically allocated) - < braunr> this map is the backend (mem_source) for the kentry_cache - < braunr> the kentry_cache is created with a special flag that tells it - memory can't be reclaimed - < braunr> when the cache needs to grow, the single map entry is extended to - cover the allocated memory - < braunr> it's similar to the way pmap_growkernel() works for kernel page - table pages - < braunr> (and is actually based on that idea) - < braunr> it's a compromise between full static and dynamic allocation - types - < braunr> the advantage is that the allocator code can be used (so there is - no need for a special allocator like in netbsd) - < braunr> the drawback is that some resources can never be returned to - their source (and under peaks, the amount of unfreeable resources could - become large, but this is unexpected) - < braunr> mcsim: for now you shouldn't waste your time with this - < braunr> i see the number of kernel map entries is fixed at 256 - < braunr> and i've never seen the kernel use more than around 30 entries - < mcsim> Do you think that I have to left this problem to the end? - < braunr> yes - - -# IRC, freenode, #hurd, 2011-08-11 - - < mcsim> braunr: Hello. Can you give me an advice how can I make - measurements better? - < braunr> mcsim: what kind of measurements - < mcsim> braunr: How much is your allocator better than zalloc. - < braunr> slightly :p - < braunr> that's why i never took the time to put it in gnumach - < mcsim> braunr: Just I thought that there are some rules or - recommendations of such measurements. Or I can do them any way I want? - < braunr> mcsim: i don't know - < braunr> mcsim: benchmarking is an art of its own, and i don't even know - how to use the bits of profiling code available in gnumach (if it still - works) - < antrik> mcsim: hm... are you saying you already have a running system - with slab allocator?... :-) - < braunr> mcsim: the main advantage i can see is the removal of many - arbitrary hard limits - < mcsim> antrik: yes - < antrik> \o/ - < antrik> nice work! - < braunr> :) - < braunr> the cpu layer should also help a bit, but it's hard to measure - < braunr> i guess it could be seen on the ipc path for very small buffers - < mcsim> antrik: Thanks. But I still have to 1. Change constructor for - kentry_alloc_cache. and 2. Make measurements. - < braunr> and polish the whole thing :p - < antrik> mcsim: I'm not sure this can be measured... the performance - differente in any real live usage is probably just a few percent at most - -- it's hard to construct a benchmark giving enough precision so it's not - drowned in noise... - < antrik> perhaps it conserves some memory -- but that too would be hard to - measure I fear - < braunr> yes - < braunr> there *should* be better allocation times, less fragmentation, - better accounting ... :) - < braunr> and no arbitrary limits ! - < antrik> :-) - < braunr> oh, and the self debugging features can be nice too - < mcsim> But I need to prove that my work wasn't useless - < braunr> well it wasn't, but that's hard to measure - < braunr> it's easy to prove though, since there are additional features - that weren't present in the zone allocator - < mcsim> Ok. If there are some profiling features in gnumach can you give - me a link with their description? - < braunr> mcsim: sorry, no - < braunr> mcsim: you could still write the basic loop test, which counts - the number of allocations performed in a fixed time interval - < braunr> but as it doesn't match many real life patterns, it won't be very - useful - < braunr> and i'm afraid that if you consider real life patterns, you'll - see how negligeable the improvement can be compared to other operations - such as memory copies or I/O (ouch) - < mcsim> Do network drivers use this allocator? - < mcsim> ok. I'll scrape up some test and than I'll report results. - - -# IRC, freenode, #hurd, 2011-08-26 - - < mcsim> hello. Are there any analogs of copy_to_user and copy_from_user in - linux for gnumach? - < mcsim> Or how can I determine memory map if I know address? I need this - for vm_map_copyin - < guillem> mcsim: vm_map_lookup_entry? - < mcsim> guillem: but I need to transmit map to this function and it will - return an entry which contains specified address. - < mcsim> And I don't know what map have I transmit. - < mcsim> I need to transfer static array from kernel to user. What map - contains static data? - < antrik> mcsim: Mach doesn't have copy_{from,to}_user -- instead, large - chunks of data are transferred as out-of-line data in IPC messages - (i.e. using VM magic) - < mcsim> antrik: can you give me an example? I just found using - vm_map_copyin in host_zone_info. - < antrik> no idea what vm_map_copyin is to be honest... - - -# IRC, freenode, #hurd, 2011-08-27 - - < braunr> mcsim: the primitives are named copyin/copyout, and they are used - for messages with inline data - < braunr> or copyinmsg/copyoutmsg - < braunr> vm_map_copyin/out should be used for chunks larger than a page - (or roughly a page) - < braunr> also, when writing to a task space, see which is better suited: - vm_map_copyout or vm_map_copy_overwrite - < mcsim> braunr: and what will be src_map for vm_map_copyin/out? - < braunr> the caller map - < braunr> which you can get with current_map() iirc - < mcsim> braunr: thank you - < braunr> be careful not to leak anything in the transferred buffers - < braunr> memset() to 0 if in doubt - < mcsim> braunr:ok - < braunr> antrik: vm_map_copyin() is roughly vm_read() - < antrik> braunr: what is it used for? - < braunr> antrik: 01:11 < antrik> mcsim: Mach doesn't have - copy_{from,to}_user -- instead, large chunks of data are transferred as - out-of-line data in IPC messages (i.e. using VM magic) - < braunr> antrik: that "VM magic" is partly implemented using vm_map_copy* - functions - < antrik> braunr: oh, you mean it doesn't actually copy data, but only page - table entries? if so, that's *not* really comparable to - copy_{from,to}_user()... - - -# IRC, freenode, #hurd, 2011-08-28 - - < braunr> antrik: the equivalent of copy_{from,to}_user are - copy{in,out}{,msg} - < braunr> antrik: but when the data size is about a page or more, it's - better not to copy, of course - < antrik> braunr: it's actually not clear at all that it's really better to - do VM magic than to copy... - - -# IRC, freenode, #hurd, 2011-08-29 - - < braunr> antrik: at least, that used to be the general idea, and with a - simpler VM i suspect it's still true - < braunr> mcsim: did you progress on your host_zone_info replacement ? - < braunr> mcsim: i think you should stick to what the original - implementation did - < braunr> which is making an inline copy if caller provided enough space, - using kmem_alloc_pageable otherwise - < braunr> specify ipc_kernel_map if using kmem_alloc_pageable - < mcsim> braunr: yes. And it works. But I use kmem_alloc, not pageable. Is - it worse? - < mcsim> braunr: host_zone_info replacement is pushed to savannah - repository. - < braunr> mcsim: i'll have a look - < mcsim> braunr: I've pushed one more commit just now, which has attitude - to host_zone_info. - < braunr> mem_alloc_early_init should be renamed mem_bootstrap - < mcsim> ok - < braunr> mcsim: i don't understand your call to kmem_free - < mcsim> braunr: It shouldn't be there? - < braunr> why should it be there ? - < braunr> you're freeing what the copy object references - < braunr> it's strange that it even works - < braunr> also, you shouldn't pass infop directly as the copy object - < braunr> i guess you get a warning for that - < braunr> do what the original code does: use an intermediate copy object - and a cast - < mcsim> ok - < braunr> another error (without consequence but still, you should mind it) - < braunr> simple_lock(&mem_cache_list_lock); - < braunr> [...] - < braunr> kr = kmem_alloc(ipc_kernel_map, &info, info_size); - < braunr> you can't hold simple locks while allocating memory - < braunr> read how the original implementation works around this - < mcsim> ok - < braunr> i guess host_zone_info assumes the zone list doesn't change much - while unlocked - < braunr> or that's it's rather unimportant since it's for debugging - < braunr> a strict snapshot isn't required - < braunr> list_for_each_entry(&mem_cache_list, cache, node) max_caches++; - < braunr> you should really use two separate lines for readability - < braunr> also, instead of counting each time, you could just maintain a - global counter - < braunr> mcsim: use strncpy instead of strcpy for the cache names - < braunr> not to avoid overflow but rather to clear the unused bytes at the - end of the buffer - < braunr> mcsim: about kmem_alloc vs kmem_alloc_pageable, it's a minor - issue - < braunr> you're handing off debugging data to a userspace application - < braunr> a rather dull reporting tool in most cases, which doesn't require - wired down memory - < braunr> so in order to better use available memory, pageable memory - should be used - < braunr> in the future i guess it could become a not-so-minor issue though - < mcsim> ok. I'll fix it - < braunr> mcsim: have you tried to run the kernel with MC_VERIFY always on - ? - < braunr> MEM_CF_VERIFY actually - < mcsim1> yes. - < braunr> oh - < braunr> nothing wrong - < braunr> ? - < mcsim1> it is always set - < braunr> ok - < braunr> ah, you set it in macros.h .. - < braunr> don't - < braunr> put it in mem.c if you want, or better, make it a compile-time - option - < braunr> macros.h is a tiny macro library, it shouldn't define such - unrelated options - < mcsim1> ok. - < braunr> mcsim1: did you try fault injection to make sure the checking - code actually works and how it behaves when an error occurs ? - < mcsim1> I think that when I finish I'll merge files cpu.h and macros.h - with mem.c - < braunr> yes that would simplify things - < mcsim1> Yes. When I confused with types mem_buf_fill worked wrong and - panic occurred. - < braunr> very good - < braunr> have you progressed concerning the measurements you wanted to do - ? - < mcsim1> not much. - < braunr> ok - < mcsim1> I think they will be ready in a few days. - < antrik> what measurements are these? - < mcsim1> braunr: What maximal size for static data and stack in kernel? - < braunr> what do you mean ? - < braunr> kernel stacks are one page if i'm right - < braunr> static data (rodata+data+bss) are limited by grub bugs only :) - < mcsim1> braunr: probably they are present, because when I created too big - array I couldn't boot kernel - < braunr> local variable or static ? - < mcsim1> static - < braunr> how large ? - < mcsim1> 4Mb - < braunr> hm - < braunr> it's not a grub bug then - < braunr> i was able to embed as much as 32 MiB in x15 while doing this - kind of tests - < braunr> I guess it's the gnu mach boot code which only preallocates one - page for the initial kernel mapping - < braunr> one PTP (page table page) maps 4 MiB - < braunr> (x15 does this completely dynamically, unlike mach or even - current BSDs) - < mcsim1> antrik: First I want to measure time of each cache - creation/allocation/deallocation and then compile kernel. - < braunr> cache creation is irrelevant - < braunr> because of the cpu pools in the new allocator, you should test at - least two different allocation patterns - < braunr> one with quick allocs/frees - < braunr> the other with large numbers of allocs then their matching frees - < braunr> (larger being at least 100) - < braunr> i'd say the cpu pool layer is the real advantage over the - previous zone allocator - < braunr> (from a performance perspective) - < mcsim1> But there is only one cpu - < braunr> it doesn't matter - < braunr> it's stil a very effective cache - < braunr> in addition to reducing contention - < braunr> compare mem_cpu_pool_pop() against mem_cache_alloc_from_slab() - < braunr> mcsim1: work is needed to polish the whole thing, but getting it - actually working is a nice achievement for someone new on the project - < braunr> i hope it helped you learn about memory allocation, virtual - memory, gnu mach and the hurd in general :) - < antrik> indeed :-) - - -# IRC, freenode, #hurd, 2011-09-06 - - [some performance testing] - <braunr> i'm not sure such long tests are relevant but let's assume balloc - is slower - <braunr> some tuning is needed here - <braunr> first, we can see that slab allocation occurs more often in balloc - than page allocation does in zalloc - <braunr> so yes, as slab allocation is slower (have you measured which part - actually is slow ? i guess it's the kmem_alloc call) - <braunr> the whole process gets a bit slower too - <mcsim> I used alloc_size = 4096 for zalloc - <braunr> i don't know what that is exactly - <braunr> but you can't hold 500 16 bytes buffers in a page so zalloc must - have had free pages around for that - <mcsim> I use kmem_alloc_wired - <braunr> if you have time, measure it, so that we know how much it accounts - for - <braunr> where are the results for dealloc ? - <mcsim> I can't give you result right now because internet works very - bad. But for first DEALLOC result are the same, exept some cases when it - takes balloc for more than 1000 ticks - <braunr> must be the transfer from the cpu layer to the slab layer - <mcsim> as to kmem_alloc_wired. I think zalloc uses this function too for - allocating objects in zone I test. - <braunr> mcsim: yes, but less frequently, which is why it's faster - <braunr> mcsim: another very important aspect that should be measured is - memory consumption, have you looked into that ? - <mcsim> I think that I made too little iterations in test SMALL - <mcsim> If I increase constant SMALL_TESTS will it be good enough? - <braunr> mcsim: i don't know, try both :) - <braunr> if you increase the number of iterations, balloc average time will - be lower than zalloc, but this doesn't remove the first long - initialization step on the allocated slab - <mcsim> SMALL_TESTS to 500, I mean - <braunr> i wonder if maintaining the slabs sorted through insertion sort is - what makes it slow - <mcsim> braunr: where do you sort slabs? I don't see this. - <braunr> mcsim: mem_cache_alloc_from_slab and its free counterpart - <braunr> mcsim: the mem_source stuff is useless in gnumach, you can remove - it and directly call the kmem_alloc/free functions - <mcsim> But I have to make special allocator for kernel map entries. - <braunr> ah right - <mcsim> btw. It turned out that 256 entries are not enough. - <braunr> that's weird - <braunr> i'll make a patch so that the mem_source code looks more like what - i have in x15 then - <braunr> about the results, i don't think the slab layer is that slow - <braunr> it's the cpu_pool_fill/drain functions that take time - <braunr> they preallocate many objects (64 for your objects size if i'm - right) at once - <braunr> mcsim: look at the first result page: some times, a number around - 8000 is printed - <braunr> the common time (ticks, whatever) for a single object is 120 - <braunr> 8132/120 is 67, close enough to the 64 value - <mcsim> I forgot about SMALL tests here are they: - http://paste.debian.net/128533/ (balloc) http://paste.debian.net/128534/ - (zalloc) - <mcsim> braunr: why do you divide 8132 by 120? - <braunr> mcsim: to see if it matches my assumption that the ~8000 number - matches the cpu_pool_fill call - <mcsim> braunr: I've got it - <braunr> mcsim: i'd be much interested in the dealloc results if you can - paste them too - <mcsim> dealloc: http://paste.debian.net/128589/ - http://paste.debian.net/128590/ - <braunr> mcsim: thanks - <mcsim> second dealloc: http://paste.debian.net/128591/ - http://paste.debian.net/128592/ - <braunr> mcsim: so the main conclusion i retain from your tests is that the - transfers from the cpu and the slab layers are what makes the new - allocator a bit slower - <mcsim> OPERATION_SMALL dealloc: http://paste.debian.net/128593/ - http://paste.debian.net/128594/ - <braunr> mcsim: what needs to be measured now is global memory usage - <mcsim> braunr: data from /proc/vmstat after kernel compilation will be - enough? - <braunr> mcsim: let me check - <braunr> mcsim: no it won't do, you need to measure kernel memory usage - <braunr> the best moment to measure it is right after zone_gc is called - <mcsim> Are there any facilities in gnumach for memory measurement? - <braunr> it's specific to the allocators - <braunr> just count the number of used pages - <braunr> after garbage collection, there should be no free page, so this - should be rather simple - <mcsim> ok - <mcsim> braunr: When I measure memory usage in balloc, what formula is - better cache->nr_slabs * cache->bufs_per_slab * cache->buf_size or - cache->nr_slabs * cache->slab_size? - <braunr> the latter - - -# IRC, freenode, #hurd, 2011-09-07 - - <mcsim> braunr: I've disabled calling of mem_cpu_pool_fill and allocator - became faster - <braunr> mcsim: sounds nice - <braunr> mcsim: i suspect the free path might not be as fast though - <mcsim> results for first calling: http://paste.debian.net/128639/ second: - http://paste.debian.net/128640/ and with many alloc/free: - http://paste.debian.net/128641/ - <braunr> mcsim: thanks - <mcsim> best result are for second call: average time decreased from 159.56 - to 118.756 - <mcsim> First call slightly worse, but this is because I've added some - profiling code - <braunr> i still see some ~8k lines in 128639 - <braunr> even some around ~12k - <mcsim> I think this is because of mem_cache_grow I'm investigating it now - <braunr> i guess so too - <mcsim> I've measured time for first call in cache and from about 22000 - mem_cache_grow takes 20000 - <braunr> how did you change the code so that it doesn't call - mem_cpu_pool_fill ? - <braunr> is the cpu layer still used ? - <mcsim> http://paste.debian.net/128644/ - <braunr> don't forget the free path - <braunr> mcsim: anyway, even with the previous slightly slower behaviour we - could observe, the performance hit is negligible - <mcsim> Is free path a compilation? (I'm sorry for my english) - <braunr> mcsim: mem_cache_free - <braunr> mcsim: the last two measurements i'd advise are with big (>4k) - object sizes and, really, kernel allocator consumption - <mcsim> http://paste.debian.net/128648/ http://paste.debian.net/128646/ - http://paste.debian.net/128649/ (first, second, small) - <braunr> mcsim: these numbers are closer to the zalloc ones, aren't they ? - <mcsim> deallocating slighty faster too - <braunr> it may not be the case with larger objects, because of the use of - a tree - <mcsim> yes, they are closer - <braunr> but then, i expect some space gains - <braunr> the whole thing is about compromise - <mcsim> ok. I'll try to measure them today. Anyway I'll post result and you - could read them in the morning - <braunr> at least, it shows that the zone allocator was actually quite good - <braunr> i don't like how the code looks, there are various hacks here and - there, it lacks self inspection features, but it's quite good - <braunr> and there was little room for true improvement in this area, like - i told you :) - <braunr> (my allocator, like the current x15 dev branch, focuses on mp - machines) - <braunr> mcsim: thanks again for these numbers - <braunr> i wouldn't have had the courage to make the tests myself before - some time eh - <mcsim> braunr: hello. Look at the small_4096 results - http://paste.debian.net/128692/ (balloc) http://paste.debian.net/128693/ - (zalloc) - <braunr> mcsim: wow, what's that ? :) - <braunr> mcsim: you should really really include your test parameters in - the report - <braunr> like object size, purpose, and other similar details - <mcsim> for balloc I specified only object_size = 4096 - <mcsim> for zalloc object_size = 4096, alloc_size = 4096, memtype = 0; - <braunr> the results are weird - <braunr> apart from the very strange numbers (e.g. 0 or 4429543648), none - is around 3k, which is the value matching a kmem_alloc call - <braunr> happy to see balloc behaves quite good for this size too - <braunr> s/good/well/ - <mcsim> Oh - <mcsim> here is significant only first 101 lines - <mcsim> I'm sorry - <braunr> ok - <braunr> what does the test do again ? 10 loops of 10 allocs/frees ? - <mcsim> yes - <braunr> ok, so the only slowdown is at the beginning, when the slabs are - created - <braunr> the two big numbers (31844 and 19548) are strange - <mcsim> on the other hand time of compilation is - <mcsim> balloc zalloc - <mcsim> 38m28.290s 38m58.400s - <mcsim> 38m38.240s 38m42.140s - <mcsim> 38m30.410s 38m52.920s - <braunr> what are you compiling ? - <mcsim> gnumach kernel - <braunr> in 40 mins ? - <mcsim> yes - <braunr> you lack hvm i guess - <mcsim> is it long? - <mcsim> I use real PC - <braunr> very - <braunr> ok - <braunr> so it's normal - <mcsim> in vm it was about 2 hours) - <braunr> the difference really is negligible - <braunr> ok i can explain the big numbers - <braunr> the slab size depends on the object size, and for 4k, it is 32k - <braunr> you can store 8 4k buffers in a slab (lines 2 to 9) - <mcsim> so we need use kmem_alloc_* 8 times? - <braunr> on line 10, the ninth object is allocated, which adds another slab - to the cache, hence the big number - <braunr> no, once for a size of 32k - <braunr> and then the free list is initialized, which means accessing those - pages, which means tlb misses - <braunr> i guess the zone allocator already has free pages available - <mcsim> I see - <braunr> i think you can stop performance measurements, they show the - allocator is slightly slower, but so slightly we don't care about that - <braunr> we need numbers on memory usage now (at the page level) - <braunr> and this isn't easy - <mcsim> For balloc I can get numbers if I summarize nr_slabs*slab_size for - each cache, isn't it? - <braunr> yes - <braunr> you can have a look at the original implementation, function - mem_info - <mcsim> And for zalloc I have to summarize of cur_size and then add - zalloc_wasted_space? - <braunr> i don't know :/ - <braunr> i think the best moment to obtain accurate values is after zone_gc - removes the collected pages - <braunr> for both allocators, you could fill a stats structure at that - moment, and have an rpc copy that structure when a client tool requests - it - <braunr> concerning your tests, there is another point to have in mind - <braunr> the very first loop in your code shows a result of 31844 - <braunr> although you disabled the call to cpu_pool_fill - <braunr> but the reason why it's so long is that the cpu layer still exists - <braunr> and if you look carefully, the cpu pools are created as needed on - the free path - <mcsim> I removed cpu_pool_drain - <braunr> but not cpu_pool_push/pop i guess - <mcsim> http://paste.debian.net/128698/ - <braunr> see, you still allocate the cpu pool array on the free path - <mcsim> but I don't fill it - <braunr> that's not the point - <braunr> it uses mem_cache_alloc - <braunr> so in a call to free, you can also have an allocation, that can - potentially create a new slab - <mcsim> I see, so I have to create cpu_pool at the initialization stage? - <braunr> no, you can't - <braunr> there is a reason why they're allocated on the free path - <braunr> but since you don't have the fill/drain functions, i wonder if you - should just comment out the whole cpu layer code - <braunr> but hmm - <braunr> no really, it's not worth the effort - <braunr> even with drains/fills, the results are really good enough - <braunr> it makes the allocator smp ready - <braunr> we should just keep it that way - <braunr> mcsim: fyi, the reason why cpu pool arrays are allocated on the - free path is to avoid recursion - <braunr> because cpu pool arrays are allocated from caches just as almost - everything else - <mcsim> ok - <mcsim> summ of cur_size and then adding zalloc_wasted_space gives 0x4e1954 - <mcsim> but this value isn't even page aligned - <mcsim> For balloc I've got 0x4c6000 0x4aa000 0x48d000 - <braunr> hm can you report them in decimal, >> 10 so that values are in KiB - ? - <mcsim> 4888 4776 4660 for balloc - <mcsim> 4998 for zalloc - <braunr> when ? - <braunr> after boot ? - <mcsim> boot, compile, zone_gc - <mcsim> and then measure - <braunr> ? - <mcsim> I call garbage collector before measuring - <mcsim> and I measure after kernel compilation - <braunr> i thought it took you 40 minutes - <mcsim> for balloc I got results at night - <braunr> oh so you already got them - <braunr> i can't beleive the kernel only consumes 5 MiB - <mcsim> before gc it takes about 9052 Kib - <braunr> can i see the measurement code ? - <braunr> oh, and how much ram does your machine have ? - <mcsim> 758 mb - <mcsim> 768 - <braunr> that's really weird - <braunr> i'd expect the kernel to consume much more space - <mcsim> http://paste.debian.net/128703/ - <mcsim> it's only dynamically allocated data - <braunr> yes - <braunr> ipc ports, rights, vm map entries, vm objects, and lots of other - hanging buffers - <braunr> about how much is zalloc_wasted_space ? - <braunr> if it's small or constant, i guess you could ignore it - <mcsim> about 492 - <mcsim> KiB - <braunr> well it's another good point, mach internal structures don't imply - much overhead - <braunr> or, the zone allocator is underused - - <tschwinge> mcsim, braunr: The memory allocator project is coming along - good, as I get from your IRC messages? - <braunr> tschwinge: yes, but as expected, improvements are minor - <tschwinge> But at the very least it's now well-known, maintainable code. - <braunr> yes, it's readable, easier to understand, provides self inspection - and is smp ready - <braunr> there also are less hacks, but a few less features (there are no - way to avoid sleeping so it's unusable - and unused - in interrupt - handlers) - <braunr> is* no way - <braunr> tschwinge: mcsim did a good job porting and measuring it - - -# IRC, freenode, #hurd, 2011-09-08 - - <antrik> braunr: note that the zalloc map used to be limited to 8 MiB or - something like that a couple of years ago... so it doesn't seems - surprising that the kernel uses "only" 5 MiB :-) - <antrik> (yes, we had a *lot* of zalloc panics back then...) - - -# IRC, freenode, #hurd, 2011-09-14 - - <mcsim> braunr: hello. I've written a constructor for kernel map entries - and it can return resources to their source. Can you have a look at it? - http://paste.debian.net/130037/ If all be OK I'll push it tomorrow. - <braunr> mcsim: send the patch through mail please, i'll apply it on my - copy - <braunr> are you sure the cache is reapable ? - <mcsim> All slabs, except first I allocate with kmem_alloc_wired. - <braunr> how can you be sure ? - <mcsim> First slab I allocate during bootstrap and use pmap_steal_memory - and further I use only kmem_alloc_wired - <braunr> no, you use kmem_free - <braunr> in kentry_dealloc_cache() - <braunr> which probably creates a recursion - <braunr> using the constructor this way isn't a good idea - <braunr> constructors are good for preconstructed state (set counters to 0, - init lists and locks, that kind of things, not allocating memory) - <braunr> i don't think you should try to make this special cache reapable - <braunr> mcsim: keep in mind constructors are applied on buffers at *slab* - creation, not at object allocation - <braunr> so if you allocate a single slab with, say, 50 or 100 objects per - slab, kmem_alloc_wired would be called that number of times - <mcsim> why kentry_dealloc_cache can create recursion? kentry_dealloc_cache - is called only by mem_cache_reap. - <braunr> right - <braunr> but are you totally sure mem_cache_reap() can't be called by - kmem_free() ? - <braunr> i think you're right, it probably can't - - -# IRC, freenode, #hurd, 2011-09-25 - - <mcsim> braunr: hello. I rewrote constructor for kernel entries and seems - that it works fine. I think that this was last milestone. Only moving of - memory allocator sources to more appropriate place and merge with main - branch left. - <braunr> mcsim: it needs renaming and reindenting too - <mcsim> for reindenting C-x h Tab in emacs will be enough? - <braunr> mcsim: make sure which style must be used first - <mcsim> and what should I rename and where better to place allocator? For - example, there is no lib directory, like in x15. Should I create it and - move list.* and rbtree.* to lib/ or move these files to util/ or - something else? - <braunr> mcsim: i told you balloc isn't a good name before, use something - more meaningful (kmem is already used in gnumach unfortunately if i'm - right) - <braunr> you can put the support files in kern/ - <mcsim> what about vm_alloc? - <braunr> you should prefix it with vm_ - <braunr> shouldn't - <braunr> it's a top level allocator - <braunr> on top of the vm system - <braunr> maybe mcache - <braunr> hm no - <braunr> maybe just km_ - <mcsim> kern/km_alloc.*? - <braunr> no - <braunr> just km - <mcsim> ok. - - -# IRC, freenode, #hurd, 2011-09-27 - - <mcsim> braunr: hello. When I've tried to speed of new allocator and bad - I've removed function mem_cpu_pool_fill. But you've said to undo this. I - don't understand why this function is necessary. Can you explain it, - please? - <mcsim> When I've tried to compare speed of new allocator and old* - <braunr> i'm not sure i said that - <braunr> i said the performance overhead is negligible - <braunr> so it's better to leave the cpu pool layer in place, as it almost - doesn't hurt - <braunr> you can implement the KMEM_CF_NO_CPU_POOL I added in the x15 mach - version - <braunr> so that cpu pools aren't used by default, but the code is present - in case smp is implemented - <mcsim> I didn't remove cpu pool layer. I've just removed filling of cpu - pool during creation of slab. - <braunr> how do you fill the cpu pools then ? - <mcsim> If object is freed than it is added to cpu poll - <braunr> so you don't fill/drain the pools ? - <braunr> you try to get/put an object and if it fails you directly fall - back to the slab layer ? - <mcsim> I drain them during garbage collection - <braunr> oh - <mcsim> yes - <braunr> you shouldn't touch the cpu layer during gc - <braunr> the number of objects should be small enough so that we don't care - much - <mcsim> ok. I can drain cpu pool at any other time if it is prohibited to - in mem_gc. - <mcsim> But why do we need to fill cpu poll during slab creation? - <mcsim> In this case allocation consist of: get object from slab -> put it - to cpu pool -> get it from cpu pool - <mcsim> I've just remove last to stages - <braunr> hm cpu pools aren't filled at slab creation - <braunr> they're filled when they're empty, and drained when they're full - <braunr> so that the number of objects they contain is increased/reduced to - a value suitable for the next allocations/frees - <braunr> the idea is to fall back as little as possible to the slab layer - because it requires the acquisition of the cache lock - <mcsim> oh. You're right. I'm really sorry. The point is that if cpu pool - is empty we don't need to fill it first - <braunr> uh, yes we do :) - <mcsim> Why cache locking is so undesirable? If we have free objects in - slabs locking will not take a lot if time. - <braunr> mcsim: it's undesirable on a smp system - <mcsim> ok. - <braunr> mcsim: and spin locks are normally noops on a up system - <braunr> which is the case in gnumach, hence the slightly better - performances without the cpu layer - <braunr> but i designed this allocator for x15, which only supports mp - systems :) - <braunr> mcsim: sorry i couldn't look at your code, sick first, busy with - server migration now (new server almost ready for xen hurds :)) - <mcsim> ok. - <mcsim> I ended with allocator if didn't miss anything important:) - <braunr> i'll have a look soon i hope :) - - -# IRC, freenode, #hurd, 2011-09-27 - - <antrik> braunr: would it be realistic/useful to check during GC whether - all "used" objects are actually in a CPU pool, and if so, destroy them so - the slab can be freed?... - <antrik> mcsim: BTW, did you ever do any measurements of memory - use/fragmentation? - <mcsim> antrik: I couldn't do this for zalloc - <antrik> oh... why not? - <antrik> (BTW, I would be interested in a comparision between using the CPU - layer, and bare slab allocation without CPU layer) - <mcsim> Result I've got were strange. It wasn't even aligned to page size. - <mcsim> Probably is it better to look into /proc/vmstat? - <mcsim> Because I put hooks in the code and probably I missed something - <antrik> mcsim: I doubt vmstat would give enough information to make any - useful comparision... - <braunr> antrik: isn't this draining cpu pools at gc time ? - <braunr> antrik: the cpu layer was found to add a slight overhead compared - to always falling back to the slab layer - <antrik> braunr: my idea is only to drop entries from the CPU cache if they - actually prevent slabs from being freed... if other objects in the slab - are really in use, there is no point in flushing them from the CPU cache - <antrik> braunr: I meant comparing the fragmentation with/without CPU - layer. the difference in CPU usage is probably negligable anyways... - <antrik> you might remember that I was (and still am) sceptical about CPU - layer, as I suspect it worsens the good fragmentation properties of the - pure slab allocator -- but it would be nice to actually check this :-) - <braunr> antrik: right - <braunr> antrik: the more i think about it, the more i consider slqb to be - a better solution ...... :> - <braunr> an idea for when there's time - <braunr> eh - <antrik> hehe :-) - - -# IRC, freenode, #hurd, 2011-10-13 - - <braunr> mcsim: what's the current state of your gnumach branch ? - <mcsim> I've merged it with master in September - <braunr> yes i've seen that, but does it build and run fine ? - <mcsim> I've tested it on gnumach from debian repository, but for building - I had to make additional change in device/ramdisk.c, as I mentioned. - <braunr> mcsim: why ? - <mcsim> And it runs fine for me. - <braunr> mcsim: why did you need to make other changes ? - <mcsim> because there is a patch which comes with from-debian-repository - kernel and it addes some code, where I have to make changes. Earlier - kernel_map was a pointer to structure, but I change that and now - kernel_map is structure. So handling to it should be by taking the - address (&kernel_map) - <braunr> why did you do that ? - <braunr> or put it another way: what made you do that type change on - kernel_map ? - <mcsim> Earlier memory for kernel_map was allocating with zalloc. But now - salloc can't allocate memory before it's initialisation - <braunr> that's not a good reason - <braunr> a simple workaround for your problem is this : - <braunr> static struct vm_map kernel_map_store; - <braunr> vm_map_t kernel_map = &kernel_map_store; - <mcsim> braunr: Ok. I'll correct this. - - -# IRC, freenode, #hurd, 2011-11-01 - - <braunr> etenil: but mcsim's work is, for one, useful because the allocator - code is much clearer, adds some debugging support, and is smp-ready - - -# IRC, freenode, #hurd, 2011-11-14 - - <braunr> i've just realized that replacing the zone allocator removes most - (if not all) static limit on allocated objects - <braunr> as we have nothing similar to rlimits, this means kernel resources - are actually exhaustible - <braunr> and i'm not sure every allocation is cleanly handled in case of - memory shortage - <braunr> youpi: antrik: tschwinge: is this acceptable anyway ? - <braunr> (although IMO, it's also a good thing to get rid of those limits - that made the kernel panic for no valid reason) - <youpi> there are actually not many static limits on allocated objects - <youpi> only a few have one - <braunr> those defined in kern/mach_param.h - <youpi> most of them are not actually enforced - <braunr> ah ? - <braunr> they are used at zinit() time - <braunr> i thought they were - <youpi> yes, but most zones are actually fine with overcoming the max - <braunr> ok - <youpi> see zone->max_size += (zone->max_size >> 1); - <youpi> you need both !EXHAUSTIBLE and FIXED - <braunr> ok - <pinotree> making having rlimits enforced would be nice... - <pinotree> s/making// - <braunr> pinotree: the kernel wouldn't handle many standard rlimits anyway - - <braunr> i've just committed my final patch on mcsim's branch, which will - serve as the starting point for integration - <braunr> which means code in this branch won't change (or only last minute - changes) - <braunr> you're invited to test it - <braunr> there shouldn't be any noticeable difference with the master - branch - <braunr> a bit less fragmentation - <braunr> more memory can be reclaimed by the VM system - <braunr> there are debugging features - <braunr> it's SMP ready - <braunr> and overall cleaner than the zone allocator - <braunr> although a bit slower on the free path (because of what's - performed to reduce fragmentation) - <braunr> but even "slower" here is completely negligible - - -# IRC, freenode, #hurd, 2011-11-15 - - <mcsim> I enabled cpu_pool layer and kentry cache exhausted at "apt-get - source gnumach && (cd gnumach-* && dpkg-buildpackage)" - <mcsim> I mean kernel with your last commit - <mcsim> braunr: I'll make patch how I've done it in a few minutes, ok? It - will be more specific. - <braunr> mcsim: did you just remove the #if NCPUS > 1 directives ? - <mcsim> no. I replaced macro NCPUS > 1 with SLAB_LAYER, which equals NCPUS - > 1, than I redefined macro SLAB_LAYER - <braunr> ah, you want to make the layer optional, even on UP machines - <braunr> mcsim: can you give me the commands you used to trigger the - problem ? - <mcsim> apt-get source gnumach && (cd gnumach-* && dpkg-buildpackage) - <braunr> mcsim: how much ram & swap ? - <braunr> let's see if it can handle a quite large aptitude upgrade - <mcsim> how can I check swap size? - <braunr> free - <braunr> cat /proc/meminfo - <braunr> top - <braunr> whatever - <mcsim> total used free shared buffers - cached - <mcsim> Mem: 786368 332296 454072 0 0 - 0 - <mcsim> -/+ buffers/cache: 332296 454072 - <mcsim> Swap: 1533948 0 1533948 - <braunr> ok, i got the problem too - <mcsim> braunr: do you run hurd in qemu? - <braunr> yes - <braunr> i guess the cpu layer increases fragmentation a bit - <braunr> which means more map entries are needed - <braunr> hm, something's not right - <braunr> there are only 26 kernel map entries when i get the panic - <braunr> i wonder why the cache gets that stressed - <braunr> hm, reproducing the kentry exhaustion problem takes quite some - time - <mcsim> braunr: what do you mean? - <braunr> sometimes, dpkg-buildpackage finishes without triggering the - problem - <mcsim> the problem is in apt-get source gnumach - <braunr> i guess the problem happens because of drains/fills, which - allocate/free much more object than actually preallocated at boot time - <braunr> ah ? - <braunr> ok - <braunr> i've never had it at that point, only later - <braunr> i'm unable to trigger it currently, eh - <mcsim> do you use *-dbg kernel? - <braunr> yes - <braunr> well, i use the compiled kernel, with the slab allocator, built - with the in kernel debugger - <mcsim> when you run apt-get source gnumach, you run it in clean directory? - Or there are already present downloaded archives? - <braunr> completely empty - <braunr> ah just got it - <braunr> ok the limit is reached, as expected - <braunr> i'll just bump it - <braunr> the cpu layer drains/fills allocate several objects at once (64 if - the size is small enough) - <braunr> the limit of 256 (actually 252 since the slab descriptor is - embedded in its slab) is then easily reached - <antrik> mcsim: most direct way to check swap usage is vmstat - <braunr> damn, i can't live without slabtop and the amount of - active/inactive cache memory any more - <braunr> hm, weird, we have active/inactive memory in procfs, but not - buffers/cached memory - <braunr> we could set buffers to 0 and everything as cached memory, since - we're currently unable to communicate the purpose of cached memory - (whether it's used by disk servers or file system servers) - <braunr> mcsim: looks like there are about 240 kernel map entries (i forgot - about the ones used in kernel submaps) - <braunr> so yes, addin the cpu layer is what makes the kernel reach the - limit more easily - <mcsim> braunr: so just increasing limit will solve the problem? - <braunr> mcsim: yes - <braunr> slab reclaiming looks very stable - <braunr> and unfrequent - <braunr> (which is surprising) - <pinotree> braunr: "unfrequent"? - <braunr> pinotree: there isn't much memory pressure - <braunr> slab_collect() gets called once a minute on my hurd - <braunr> or is it infrequent ? - <braunr> :) - <pinotree> i have no idea :) - <braunr> infrequent, yes - - -# IRC, freenode, #hurd, 2011-11-16 - - <braunr> for those who want to play with the slab branch of gnumach, the - slabinfo tool is available at http://git.sceen.net/rbraun/slabinfo.git/ - <braunr> for those merely interested in numbers, here is the output of - slabinfo, for a hurd running in kvm with 512 MiB of RAM, an unused swap, - and a short usage history (gnumach debian packages built, aptitude - upgrade for a dozen of packages, a few git commands) - <braunr> http://www.sceen.net/~rbraun/slabinfo.out - <antrik> braunr: numbers for a long usage history would be much more - interesting :-) - - -## IRC, freenode, #hurd, 2011-11-17 - - <braunr> antrik: they'll come :) - <etenil> is something going on on darnassus? it's mighty slow - <braunr> yes - <braunr> i've rebooted it to run a modified kernel (with the slab - allocator) and i'm building stuff on it to stress it - <braunr> (i don't have any other available machine with that amount of - available physical memory) - <etenil> ok - <antrik> braunr: probably would be actually more interesting to test under - memory pressure... - <antrik> guess that doesn't make much of a difference for the kernel object - allocator though - <braunr> antrik: if ram is larger, there can be more objects stored in - kernel space, then, by building something large such as eglibc, memory - pressure is created, causing caches to be reaped - <braunr> our page cache is useless because of vm_object_cached_max - <braunr> it's a stupid arbitrary limit masking the inability of the vm to - handle pressure correctly - <braunr> if removing it, the kernel freezes soon after ram is filled - <braunr> antrik: it may help trigger the "double swap" issue you mentioned - <antrik> what may help trigger it? - <braunr> not checking this limit - <antrik> hm... indeed I wonder whether the freezes I see might have the - same cause - - -## IRC, freenode, #hurd, 2011-11-19 - - <braunr> http://www.sceen.net/~rbraun/slabinfo.out <= state of the slab - allocator after building the debian libc packages and removing all files - once done - <braunr> it's mostly the same as on any other machine, because of the - various arbitrary limits in mach (most importantly, the max number of - objects in the page cache) - <braunr> fragmentation is still quite low - <antrik> braunr: actually fragmentation seems to be lower than on the other - run... - <braunr> antrik: what makes you think that ? - <antrik> the numbers of currently unused objects seem to be in a similar - range IIRC, but more of them are reclaimable I think - <antrik> maybe I'm misremembering the other numbers - <braunr> there had been more reclaims on the other run - - -# IRC, freenode, #hurd, 2011-11-25 - - <braunr> mcsim: i've just updated the slab branch, please review my last - commit when you have time - <mcsim> braunr: Do you mean compilation/tests? - <braunr> no, just a quick glance at the code, see if it matches what you - intended with your original patch - <mcsim> braunr: everything is ok - <braunr> good - <braunr> i think the branch is ready for integration - - -# IRC, freenode, #hurd, 2011-12-17 - - <braunr> in the slab branch, there now is no use for the defines in - kern/mach_param.h - <braunr> should the file be removed or left empty as a placeholder for - future arbitrary limits ? - <braunr> (i'd tend ro remove it as a way of indicating we don't want - arbitrary limits but there may be a good reason to keep it around .. :)) - <youpi> I'd just drop it - <braunr> ok - <braunr> hmm maybe we do want to keep that one : - <braunr> #define IMAR_MAX (1 << 10) /* Max number of - msg-accepted reqs */ - <antrik> whatever that is... - <braunr> it gets returned in ipc_marequest_info - <braunr> but the mach_debug interface has never been used on the hurd - <braunr> there now is a master-slab branch in the gnumach repo, feel free - to test it - - -# IRC, freenode, #hurd, 2011-12-22 - - <youpi> braunr: does the new gnumach allocator has profiling features? - <youpi> e.g. to easily know where memory leaks reside - <braunr> youpi: you mean tracking call traces to allocated blocks ? - <youpi> not necessarily traces - <youpi> but at least means to know what kind of objects is filling memory - <braunr> it's very close to the zone allocator - <braunr> but instead of zones, there are caches - <braunr> each named after the type they store - <braunr> see http://www.sceen.net/~rbraun/slabinfo.out - <youpi> ok, so we can know, per-type, how much memory is used - <braunr> yes - <youpi> good - <braunr> if backtraces can easily be forged, it wouldn't be hard to add - that feature too - <youpi> does it dump such info when memory goes short? - <braunr> no but it can - <braunr> i've done this during tests - <youpi> it'd be good - <youpi> because I don't know in advance when a buildd will crash due to - that :) - <braunr> each time slab_collect() is called for example - <youpi> I mean not on collect, but when it's too late - <youpi> and thus always enabled - <braunr> ok - <youpi> (because there's nothing better to do than at least give infos) - <braunr> you just have to define "when it's too late", and i can add that - <youpi> when there is no memory left - <braunr> you mean when the number of free pages strictly reaches 0 ? - <youpi> yes - <braunr> ok - <youpi> i.e. just before crashing the kernel - <braunr> i see - - -# IRC, freenode, #hurdfr, 2012-01-02 - - <youpi> braunr: le code du slab allocator, il est écrit from scratch ? - <youpi> il y a encore du copyright carnegie mellon - <youpi> (dans slab_info.h du moins) - <youpi> ipc_hash_global_size = 256; - <youpi> il faudrait mettre 256 comme constante dans un header - <youpi> sinon c'est encore une valeur arbitraire cachée dans du code - <youpi> de même pour ipc_marequest_size etc. - <braunr> youpi: oui, from scratch - <braunr> slab_info.h est à l'origine zone_info.h - <braunr> pour les valeurs fixes, elles étaient déjà présentes de cette - façon, j'ai pensé qu'il valait mieux laisser comme ça pour faciliter la - lecture des diffs - <braunr> je ferai des macros à la place - <braunr> du coup il faudra peut-être remettre mach_param.h - <braunr> ou alors dans les .h ipc - - -# IRC, freenode, #hurd, 2012-01-18 - - <braunr> does the slab branch need other reviews/reports before being - integrated ? - - -# IRC, freenode, #hurd, 2012-01-30 - - <braunr> youpi: do you have some idea about when you want to get the slab - branch in master ? - <youpi> I was considering as soon as mcsim gets his paper - <braunr> right - - -# IRC, freenode, #hurd, 2012-02-22 - - <mcsim> Do I understand correct, that real memory page should be - necessarily in one of following lists: vm_page_queue_active, - vm_page_queue_inactive, vm_page_queue_free? - <braunr> cached pages are - <braunr> some special pages used only by the kernel aren't - <braunr> pages can be both wired and cached (i.e. managed by the page - cache), so that they can be passed to external applications and then - unwired (as is the case with your host_slab_info() function if you - remember) - <braunr> use "physical" instead of "real memory" - <mcsim> braunr: thank you. - - -# IRC, freenode, #hurd, 2012-04-22 - - <braunr> youpi: tschwinge: when the slab code was added, a few new files - made into gnumach that come from my git repo and are used in other - projects as well - <braunr> they're licensed under BSD upstream and GPL in gnumach, and though - it initially didn't disturb me, now it does - <braunr> i think i should fix this by leaving the original copyright and - adding the GPL on top - <youpi> sure, submit a patch - <braunr> hm i have direct commit acces if im right - <youpi> then fix it :) - <braunr> do you want to review ? - <youpi> I don't think there is any need to - <braunr> ok - - -# IRC, freenode, #hurd, 2012-12-08 - - <mcsim> braunr: hi. Do I understand correct that merely the same technique - is used in linux to determine the slab where, the object to be freed, - resides? - <braunr> yes but it's faster on linux since it uses a direct mapping of - physical memory - <braunr> it just has to shift the virtual address to obtain the physical - one, whereas x15 has to walk the pages tables - <braunr> of course it only works for kmalloc, vmalloc is entirely different - <mcsim> btw, is there sense to use some kind of B-tree instead of AVL to - decrease number of cache misses? AFAIK, in modern processors size of L1 - cache line is at least 64 bytes, so in one node we can put at least 4 - leafs (key + pointer to data) making search faster. - <braunr> that would be a b-tree - <braunr> and yes, red-black trees were actually developed based on - properties observed on b-trees - <braunr> but increasing the size of the nodes also increases memory - overhead - <braunr> and code complexity - <braunr> that's why i have a radix trees for cases where there are a large - number of entries with keys close to each other :) - <braunr> a radix-tree is basically a b-tree using the bits of the key as - indexes in the various arrays it walks instead of comparing keys to each - other - <braunr> the original avl tree used in my slab allocator was intended to - reduce the average height of the tree (avl is better for that) - <braunr> avl trees are more suited for cases where there are more lookups - than inserts/deletions - <braunr> they make the tree "flatter" but the maximum complexity of - operations that change the tree is 2log2(n), since rebalancing the tree - can make the algorithm reach back to the tree root - <braunr> red-black trees have slightly bigger heights but insertions are - limited to 2 rotations and deletions to 3 - <mcsim> there should be not much lookups in slab allocators - <braunr> which explains why they're more generally found in generic - containers - <mcsim> or do I misunderstand something? - <braunr> well, there is a lookup for each free() - <braunr> whereas there are insertions/deletions when a slab becomes - non-empty/empty - <mcsim> I see - <braunr> so it was very efficient for caches of small objects, where slabs - have many of them - <braunr> also, i wrote the implementation in userspace, without - functionality pmap provides (although i could have emulated it - afterwards) - - -# IRC, freenode, #hurd, 2013-01-06 - - <youpi> braunr: panic: vm_map: kentry memory exhausted - <braunr> youpi: ouch - <youpi> that's what I usually get - <braunr> ok - <braunr> the kentry area is a preallocated memory area that is used to back - the vm_map_kentry cache - <braunr> objects from this cache are used to describe kernel virtual memory - <braunr> so in this case, i simply assume the kentry area must be enlarged - <braunr> (currently, both virtual and physical memory is preallocated, an - improvement could be what is now done in x15, to preallocate virtual - memory only - <braunr> ) - <youpi> Mmm, why do we actually have this limit? - <braunr> the kentry area must be described by one entry - <youpi> ah, sorry, vm/vm_resident.c: kentry_data = - pmap_steal_memory(kentry_data_size); - <braunr> a statically allocated one - <youpi> I had missed that one - <braunr> previously, the zone allocator would do that - <braunr> the kentry area is required to avoid recursion when allocating - memory - <braunr> another solution would be a custom allocator in vm_map, but i - wanted to use a common cache for those objects too - <braunr> youpi: you could simply try doubling KENTRY_DATA_SIZE - <youpi> already doing that - <braunr> we might even consider a much larger size until it's reworked - <youpi> well, it's rare enough on buildds already - <youpi> doubling should be enough - <youpi> or else we have leaks - <braunr> right - <braunr> it may not be leaks though - <braunr> it may be poor map entry merging - <braunr> i'd expected the kernel map entries to be easier to merge, but it - may simply not be the case - <braunr> (i mean, when i made my tests, it looked like there were few - kernel map entries, but i may have missed corner cases that could cause - more of them to be needed) - - -## IRC, freenode, #hurd, 2014-02-11 - - <braunr> youpi: what's the issue with kentry_data_size ? - <youpi> I don't know - <braunr> so back to 64pages from 256 ? - <youpi> in debian for now yes - <braunr> :/ - <braunr> from what i recall with x15, grub is indeed allowed to put modules - and command lines around as it likes - <braunr> restricted to 4G - <braunr> iirc, command lines were in the first 1M while modules could be - loaded right after the kernel or at the end of memory, depending on the - versions - <youpi> braunr: possibly VM_KERNEL_MAP_SIZE is then not big enough - <braunr> youpi: what's the size of the ramdisk ? - <braunr> youpi: or kmem_map too big - <braunr> we discussed this earlier with teythoon - -[[user-space_device_drivers]], *Open Issues*, *System Boot*, *IRC, freenode, -\#hurd, 2011-07-27*, *IRC, freenode, #hurd, 2014-02-10* - - <braunr> or maybe we want to remove kmem_map altogether and directly use - kernel_map - <youpi> it's 6.2MiB big - <braunr> hm - <youpi> err no - <braunr> looks small - <youpi> 70MiB - <braunr> ok yes - <youpi> (uncompressed) - <braunr> well - <braunr> kernel_map is supposed to have 64M on i386 ... - <braunr> it's 192M large, with kmem_map taking 128M - <braunr> so at most 64M, with possible fragmentation - <teythoon> i believe the compressed initrd is stored in the ramdisk - <youpi> ah, right it's ext2fs which uncompresses it - <braunr> uncompresses it where - <braunr> ? - <teythoon> libstore does that - <youpi> module --nounzip /boot/${gtk}initrd.gz - <youpi> braunr: in userland memory - <youpi> it's not grub which uncompresses it for sure - <teythoon> braunr: so my ramdisk isn't 64 megs either - <braunr> which explains why it sometimes works - <teythoon> yes - <teythoon> mine is like 15 megs - <braunr> kentry_data_size calls pmap_steal_memory, an early allocation - function which changes virtual_space_start, which is later used to create - the first kernel map entry - <braunr> err, pmap_steal_memory is called with kentry_data_size as its - argument - <braunr> this first kernel map entry is installed inside kernel_map and - reduces the amount of available virtual memory there - <braunr> so yes, it all points to a layout problem - <braunr> i suggest reducing kmem_map down to 64M - <youpi> that's enough to get d-i back to boot - <youpi> what would be the downside? - <youpi> (why did you raise it to 128 actually? :) ) - <braunr> i merged the map used by generic kalloc allocations into kmem_map - <braunr> both were 64M - <braunr> i don't see any downside for the moment - <braunr> i rarely see more than 50M used by the slab allocator - <braunr> and with the recent code i added to collect reclaimable memory on - kernel allocation failures, it's unlikely the slab allocator will be - starved - <youpi> but then we need that patch too - <braunr> no - <braunr> it would be needed if kmem_map gets filled - <braunr> this very rarely happens - <youpi> is "very rarely" enough ? :) - <braunr> actualy i've never seen it happen - <braunr> i added it because i had port leaks with fakeroot - <braunr> port rights are a bit special because they're stored in a table in - kernel space - <braunr> this table is enlarged with kmem_realloc - <braunr> when an ipc space gets very large, fragmentation makes it very - difficult to successfully resize it - <braunr> that should be the only possible issue - <braunr> actually, there is another submap that steals memory from - kernel_map: device_io_map is 16M large - <braunr> so kernel_map gets down to 48M - <braunr> if the initial entry (that is, kentry_data_size + the physical - page table size) gets a bit large, kernel_map may have very little - available room - <braunr> the physical page table size obviously varies depending on the - amount of physical memory loaded, which may explain why the installer - worked on some machines - <youpi> well, it works up to 1855M - <youpi> at 1856 it doesn't work any more :) - <braunr> heh :) - <youpi> and that's about the max gnumach can handle anyway - <braunr> then reducing kmem_map down to 96M should be enough - <youpi> it works indeed - <braunr> could you check the amount of available space in kernel_map ? - <braunr> the value of kernel_map->size should do - <youpi> printing it "multiboot modules" print should be fine I guess? - - -### IRC, freenode, #hurd, 2014-02-12 - - <braunr> probably - <teythoon> ? - <braunr> i expect a bit more than 160M - <braunr> (for the value of kernel_map->size) - <braunr> teythoon: ? - <youpi> well, it's 2110210048 - <teythoon> what is multiboot modules printing ? - <youpi> almost last in gnumach bootup - <braunr> humm - <braunr> it must account directly mapped physical pages - <braunr> considering the kernel has exactly 2G, this means there is 36M - available in kernel_map - <braunr> youpi: is the ramdisk loaded at that moment ? - <youpi> what do you mean by "loaded" ? :) - <braunr> created - <youpi> where? - <braunr> allocated in kernel memory - <youpi> the script hasn't started yet - <braunr> ok - <braunr> its size was 6M+ right ? - <braunr> so it leaves around 30M - <youpi> something like this yes - <braunr> and changing kmem_map from 128M to 96M gave us 32M - <braunr> so that's it - - -# IRC, freenode, #hurd, 2013-04-18 - - <braunr> oh nice, i've found a big scalability issue with my slab allocator - <braunr> it shouldn't affect gnumach much though - - -## IRC, freenode, #hurd, 2013-04-19 - - <ArneBab> braunr: is it fixable? - <braunr> yes - <braunr> well, i'll do it in x15 for a start - <braunr> again, i don't think gnumach is much affected - <braunr> it's a scalability issue - <braunr> when millions of objects are in use - <braunr> gnumach rarely has more than a few hundred thousands - <braunr> it's also related to heavy multithreading/smp - <braunr> and by multithreading, i also mean preemption - <braunr> gnumach isn't preemptible and uniprocessor - <braunr> if the resulting diff is clean enough, i'll push it to gnumach - though :) - - -### IRC, freenode, #hurd, 2013-04-21 - - <braunr> ArneBab_: i fixed the scalability problems btw - - -## IRC, freenode, #hurd, 2013-04-20 - - <braunr> well, there is also a locking error in the slab allocator, - although not a problem for a non preemptible kernel like gnumach - <braunr> non preemptible / uniprocessor |