[PATCH] Fix kmem_cache_free performance regression in slab

[PATCH] Fix kmem_cache_free performance regression in slab

[Matthew Wilcox]

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.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>

diff --git a/mm/slab.c b/mm/slab.c
index cfa6be4..6e16431 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2881,6 +2881,8 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
 	unsigned int objnr;
 	struct slab *slabp;
 
+	BUG_ON(virt_to_cache(objp) != cachep);
+
 	objp -= obj_offset(cachep);
 	kfree_debugcheck(objp);
 	page = virt_to_head_page(objp);
@@ -3759,8 +3761,6 @@ void kmem_cache_free(struct kmem_cache *cachep, void *objp)
 {
 	unsigned long flags;
 
-	BUG_ON(virt_to_cache(objp) != cachep);
-
 	local_irq_save(flags);
 	debug_check_no_locks_freed(objp, obj_size(cachep));
 	__cache_free(cachep, objp);

-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

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Re: [PATCH] Fix kmem_cache_free performance regression in slab

[Andrew Morton]

On Thu, 29 Nov 2007 12:05:13 -0700 Matthew Wilcox <matthew@wil.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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Re: [PATCH] Fix kmem_cache_free performance regression in slab

[Pekka J Enberg]

Hi,

On Thu, 29 Nov 2007 12:05:13 -0700 Matthew Wilcox <matthew@wil.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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