linux-kernel - Re: [PATCH] SLUB use cmpxchg

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Date:	Tue, 21 Aug 2007 19:12:16 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

* Christoph Lameter (clameter@....com) wrote:
> On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
> 
> > - Fixed an erroneous test in slab_free() (logic was flipped from the 
> >   original code when testing for slow path. It explains the wrong 
> >   numbers you have with big free).
> 
> If you look at the numbers that I posted earlier then you will see that 
> even the measurements without free were not up to par.
> 

I seem to get a clear performance improvement in the kmalloc fast path.

> > It applies on top of the 
> > "SLUB Use cmpxchg() everywhere" patch.
> 
> Which one is that?
> 

This one:


SLUB Use cmpxchg() everywhere.

It applies to "SLUB: Single atomic instruction alloc/free using
cmpxchg".

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
---
 mm/slub.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: slab/mm/slub.c
===================================================================
--- slab.orig/mm/slub.c	2007-08-20 18:42:16.000000000 -0400
+++ slab/mm/slub.c	2007-08-20 18:42:28.000000000 -0400
@@ -1682,7 +1682,7 @@ redo:
 
 	object[c->offset] = freelist;
 
-	if (unlikely(cmpxchg_local(&c->freelist, freelist, object) != freelist))
+	if (unlikely(cmpxchg(&c->freelist, freelist, object) != freelist))
 		goto redo;
 	return;
 slow:

> >              |     slab.git HEAD slub (min-max)    |  cmpxchg_local slub
> > kmalloc(8)   |         190 - 201                   |         83
> > kfree(8)     |         351 - 351                   |        363
> > kmalloc(64)  |         224 - 245                   |        115
> > kfree(64)    |         389 - 394                   |        397
> > kmalloc(16384)|        713 - 741                   |        724
> > kfree(16384) |         843 - 856                   |        843
> > 
> > Therefore, there seems to be a repeatable gain on the kmalloc fast path
> > (more than twice faster). No significant performance hit for the kfree
> > case, but no gain neither, same for large kmalloc, as expected.
> 
> There is a consistent loss on slab_free it seems. The 16k numbers are 
> irrelevant since we do not use slab_alloc/slab_free due to the direct pass 
> through patch but call the page allocator directly. That also explains 
> that there is no loss there.
> 

Yes. slab_free in these tests falls mostly into __slab_free() slow path
(I instrumented the number of slow and fast path to get this). The small
performance hit (~10 cycles) can be explained by the added
preempt_disable()/preempt_enable().

> The kmalloc numbers look encouraging. I will check to see if I can 
> reproduce it once I sort out the patches.

Ok.

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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