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Diffstat (limited to 'open_issues/gnumach_memory_management.mdwn')
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diff --git a/open_issues/gnumach_memory_management.mdwn b/open_issues/gnumach_memory_management.mdwn index 1fe2f9be..fb3d6895 100644 --- a/open_issues/gnumach_memory_management.mdwn +++ b/open_issues/gnumach_memory_management.mdwn @@ -1412,3 +1412,368 @@ There is a [[!FF_project 266]][[!tag bounty]] on this task. 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 :-) |