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Date: Sat, 20 Aug 2011 11:00:37 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Pekka Enberg <penberg@kernel.org>
cc: linux-kernel@vger.kernel.org, rientjes@google.com
Subject: Re: [slub p4 6/7] slub: per cpu cache for partial pages
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On Sat, 20 Aug 2011, Pekka Enberg wrote:

> > @@ -2919,7 +3071,34 @@ static int kmem_cache_open(struct kmem_c
> >  	 * The larger the object size is, the more pages we want on the
> > partial
> >  	 * list to avoid pounding the page allocator excessively.
> >  	 */
> > -	set_min_partial(s, ilog2(s->size));
> > +	set_min_partial(s, ilog2(s->size) / 2);
>
> Why do we want to make minimum size smaller?

Because we are getting additional partial pages cached for each processor.

> > +	 */
> > +	if (s->size >= PAGE_SIZE)
> > +		s->cpu_partial = 2;
> > +	else if (s->size >= 1024)
> > +		s->cpu_partial = 6;
> > +	else if (s->size >= 256)
> > +		s->cpu_partial = 13;
> > +	else
> > +		s->cpu_partial = 30;
>
> How did you come up with these limits?

These are the per cpu queue limits of SLAB.

> > +		struct {		/* slub per cpu partial pages */
> > +			struct page *next;	/* Next partial slab */
> > +#ifdef CONFIG_64BIT
> > +			int pages;	/* Nr of partial slabs left */
> > +			int pobjects;	/* Approximate # of objects */
> > +#else
> > +			short int pages;
> > +			short int pobjects;
> > +#endif
> > +		};
> > +	};
>
> Why are the sizes different on 32-bit and 64-bit? Does this change 'struct
> page' size?

int == long /2 under 64 bit
int == long on 32 bit.

without the ifdef the page struct could get bigger for 32 bit.