Messages in this thread | | | Date | Fri, 06 Jan 2006 07:35:47 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] Shrinks sizeof(files_struct) and better layout |
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David Lang a écrit : > On Wed, 4 Jan 2006, Eric Dumazet wrote: >> Andi Kleen a écrit : >>> Eric Dumazet <dada1@cosmosbay.com> writes: >>>> 1) Reduces the size of (struct fdtable) to exactly 64 bytes on 32bits >>>> platforms, lowering kmalloc() allocated space by 50%. >>> >>> It should be probably a kmem_cache_alloc() instead of a kmalloc >>> in the first place anyways. This would reduce fragmentation. >> >> Well in theory yes, if you really expect thousand of tasks running... >> But for most machines, number of concurrent tasks is < 200, and using >> a special cache for this is not a win. > > is it enough of a win on machines with thousands of concurrent tasks > that it would be a useful config option?
Well..., not if NR_CPUS is big too. (We just saw a thread on lkml about raising NR_CPUS to 1024 on ia64).
On a 1024 CPUS machine, a kmem cache could use at least 1 MB for its internal structures, plus 1024 pages (PAGE_SIZE) for holding the caches (one cache per CPU), if you assume at least one task was created on behalf each cpu.
if PAGE_SIZE is 64KB, you end up with 65 MB of ram for the cache. Even with 100.000 tasks running on the machine, its not a win.
slab caches are very complex machinery that consume O(NR_CPUS) ram.
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