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

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Date:	Tue, 11 Mar 2008 13:05:44 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	Paul Menage <menage@...gle.com>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, taka@...inux.co.jp,
	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] Move memory controller allocations to their own slabs
 (v2)

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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