netdev - Re: [rfc][patch 1/3] slub: fix small HWCACHE

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Message-ID: <20080305000820.GB1510@wotan.suse.de>
Date:	Wed, 5 Mar 2008 01:08:20 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Lameter <clameter@....com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	yanmin_zhang@...ux.intel.com, David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: [rfc][patch 1/3] slub: fix small HWCACHE_ALIGN alignment

On Mon, Mar 03, 2008 at 01:32:54PM -0800, Christoph Lameter wrote:
> On Mon, 3 Mar 2008, Pekka Enberg wrote:
> 
> > Well, not my definition either but SLAB has guaranteed that for small
> > objects in the past, so I think Nick has a point here. However, with
> > all this back and forth, I've lost track why this matters. I suppose
> > it causes regression on some workload?
> 
> Well the guarantee can only be exploited if you would check the cacheline 
> sizes and the object size from the code that creates the slab cache. 
> Basically you would have to guestimate what the slab allocator is doing.
> 
> So the guarantee is basically meaningless. If the object is larger than a 
> cacheline then this will never work.

Of course it works. It fits the object into the fewest number of cachelines
possible. If you need to be accessing such objects in a random manner, then
for highest performance you want to touch as few cachelines as possible.

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