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From: Hugh Dickins <hugh@veritas.com>
To: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sudhir Kumar <skumar@linux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@valinux.co.jp>,
	Paul Menage <menage@google.com>,
	lizf@cn.fujitsu.com, linux-kernel@vger.kernel.org,
	taka@valinux.co.jp, linux-mm@kvack.org,
	David Rientjes <rientjes@google.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Subject: Re: [PATCH] Move memory controller allocations to their own slabs (v2)
Date: Tue, 11 Mar 2008 13:05:44 +0000 (GMT)	[thread overview]
Message-ID: <Pine.LNX.4.64.0803111256110.18261@blonde.site> (raw)
In-Reply-To: <47D66865.1080508@linux.vnet.ibm.com>

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

  reply	other threads:[~2008-03-12  0:04 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-11  6:18 Balbir Singh
2008-03-11  8:11 ` Pavel Emelyanov
2008-03-11  8:15   ` Balbir Singh
2008-03-11  8:35     ` Pavel Emelyanov
2008-03-11 11:09       ` Balbir Singh
2008-03-11 13:05         ` Hugh Dickins [this message]
2008-03-12  3:38           ` Nick Piggin
2008-03-12  3:48           ` Balbir Singh
2008-03-11 12:55 ` Hugh Dickins
2008-03-11 18:47   ` Balbir Singh

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