Messages in this thread | | | Date | Tue, 11 Mar 2008 13:05:44 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: [PATCH] Move memory controller allocations to their own slabs (v2) |
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On Tue, 11 Mar 2008, Balbir Singh wrote: > > On my 64 bit powerpc system (structure size could be different on other systems) > > 1. sizeof page_cgroup is 40 bytes > which means kmalloc will allocate 64 bytes > 2. With 4K pagesize SLAB with HWCACHE_ALIGN, 59 objects are packed per slab > 3. With SLUB the value is 102 per slab
I expect you got those numbers with 2.6.25-rc4? Towards the end of -rc5 there's a patch from Nick to make SLUB's treatment of HWCACHE_ALIGN the same as SLAB's, so I expect you'd be back to a similar poor density with SLUB too. (But I'm replying without actually testing it out myself.)
I think you'd need a strong reason to choose HWCACHE_ALIGN for these.
Consider: the (normal configuration) x86_64 struct page size was 56 bytes for a long time (and still is without MEM_RES_CTLR), but we've never inserted padding to make that a round 64 bytes (and they would benefit additionally from some simpler arithmetic, not the case with page_cgroups). Though it's good to avoid unnecessary sharing and multiple cacheline accesses, it's not so good as to justify almost doubling the size of a very very common structure. I think.
Hugh
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