linux-kernel - Re: [PATCH] Fix kmem_cache_free performance regression in slab

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Message-ID: <Pine.LNX.4.64.0711301110540.28494@sbz-30.cs.Helsinki.FI>
Date:	Fri, 30 Nov 2007 11:14:01 +0200 (EET)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Fix kmem_cache_free performance regression in slab

Hi,

On Thu, 29 Nov 2007 12:05:13 -0700 Matthew Wilcox <matthew@....cx> wrote:
> > The database performance group have found that half the cycles spent
> > in kmem_cache_free are spent in this one call to BUG_ON.  Moving it
> > into the CONFIG_SLAB_DEBUG-only function cache_free_debugcheck() is a
> > performance win of almost 0.5% on their particular benchmark.
> > 
> > The call was added as part of commit ddc2e812d592457747c4367fb73edcaa8e1e49ff
> > with the comment that "overhead should be minimal".  It may have been
> > minimal at the time, but it isn't now.
> > 

On Thu, 29 Nov 2007, Andrew Morton wrote:
> It is worth noting that the offending commit hit mainline in June 2006.
> 
> It takes a very long time for some performance regressions to be
> discovered.  By which time it is effectively too late to fix it.

What architecture is this? x86_64? I don't think the BUG_ON per se caused 
the performance regression but rather the virt_to_head_page() changes to 
virt_to_cache() that were added later. But reverting the BUG_ON is fine by 
me.

Thanks Matthew and others for tracking this down!

				Pekka
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