Re: [PATCH] Move memory controller allocations to their own slabs (v2)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Hugh Dickins wrote:
> 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.)
> 

You are right, these numbers are against -rc4 and I do see HWCACHE_ALIGN code
for SLUB in the latest kernel -git tree

> 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.
> 

Very good point. I suppose an overhead proportional to the memory on the system
is too much to digest. I think struct page is a good example for page_cgroup to
follow.


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL