linux-kernel - Re: [PATCH] mm-slab: allocate kmem_cache with __GFP

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Date:	Wed, 20 Jul 2011 18:31:05 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mel Gorman <mgorman@...e.de>, Pekka Enberg <penberg@...nel.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH] mm-slab: allocate kmem_cache with __GFP_REPEAT

Le mercredi 20 juillet 2011 à 11:17 -0500, Christoph Lameter a écrit :
> On Wed, 20 Jul 2011, Eric Dumazet wrote:
> 
> > Note that adding ____cacheline_aligned_in_smp on nodelists[] actually
> > helps performance, as all following fields are readonly after kmem_cache
> > setup.
> 
> Well but that is not addresssing the same issue. Could you separate that
> out?
> 

I would like this patch not being a performance regression. I know some
people really want fast SLAB/SLUB ;)

> The other question that follows from this is then: Does that
> alignment compensate for the loss of performance due to the additional
> lookup in hot code paths and the additional cacheline reference required?
> 

In fact resulting code is smaller, because most fields are now with <
127 offset (x86 assembly code can use shorter instructions)

Before patch :
# size mm/slab.o
   text	   data	    bss	    dec	    hex	filename
  22605	 361665	     32	 384302	  5dd2e	mm/slab.o

After patch :
# size mm/slab.o
   text	   data	    bss	    dec	    hex	filename
  22347	 328929	  32800	 384076	  5dc4c	mm/slab.o

> The per node pointers are lower priority in terms of performance than the
> per cpu pointers. I'd rather have the per node pointers requiring an
> additional lookup. Less impact on hot code paths.
> 

Sure. I'll post a V2 to have CPU array before NODE array.


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