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

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Message-Id: <20071129184539.ba6342b8.akpm@linux-foundation.org>
Date:	Thu, 29 Nov 2007 18:45:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Fix kmem_cache_free performance regression in slab

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.
> 

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.

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