Re: [PATCH] Fix kmem_cache_free performance regression in slab

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On Thu, 29 Nov 2007 12:05:13 -0700 Matthew Wilcox <[email protected]> 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 

Thanks Matthew and others for tracking this down!

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