Re: [PATCH] General purpose zeroed page slab

Andi Kleen
2004-10-14T18:04:27Z

On Thu, Oct 14, 2004 at 12:50:43PM -0400, Martin K. Petersen wrote:
>
> A while back Bill Irwin converted the page table code on ppc64 to use
> a zeroed page slab. I recently did the same on ia64 and got a
> significant performance improvement in terms of fault time (4 usec ->
> 700 nsec).
>
> This cache needs to be initialized fairly early on and so far we've
> called it from pgtable_cache_init() on both archs. However, Tony Luck
> thought it might be useful to have a general purpose slab cache with
> zeroed pages. And other architectures might decide to use it for
> their page tables too.
>
> Consequently here's a patch that puts this functionality in slab.c.
>
> Signed-off-by: Martin K. Petersen <mkp@wildopensource.com>
>
> --
> Martin K. Petersen Wild Open Source, Inc.
> mkp@wildopensource.com http://www.wildopensource.com/
>
> diff -urN -X /usr/people/mkp/bin/dontdiff linux-pristine/include/linux/slab.h zero-slab/include/linux/slab.h
> --- linux-pristine/include/linux/slab.h 2004-10-11 14:57:20.000000000 -0700
> +++ zero-slab/include/linux/slab.h 2004-10-13 17:49:29.000000000 -0700
> @@ -115,6 +115,7 @@
> extern kmem_cache_t *signal_cachep;
> extern kmem_cache_t *sighand_cachep;
> extern kmem_cache_t *bio_cachep;
> +extern kmem_cache_t *zero_page_cachep;
>
> extern atomic_t slab_reclaim_pages;
>
> diff -urN -X /usr/people/mkp/bin/dontdiff linux-pristine/mm/slab.c zero-slab/mm/slab.c
> --- linux-pristine/mm/slab.c 2004-10-11 14:57:20.000000000 -0700
> +++ zero-slab/mm/slab.c 2004-10-13 17:49:57.000000000 -0700
> @@ -716,6 +716,13 @@
>
> static struct notifier_block cpucache_notifier = { &cpuup_callback, NULL, 0 };
>
> +kmem_cache_t *zero_page_cachep;
> +
> +static void zero_page_ctor(void *pte, kmem_cache_t *cache, unsigned long flags)
> +{
> + memset(pte, 0, PAGE_SIZE);
> +}

The means every user has to memset it to zero before free.
Add a comment for that at least.

Also that's pretty dumb. How about keeping track how much of the
page got non zeroed (e.g. by using a few free words in struct page
for a coarse grained dirty bitmap)

Then you could memset on free only the parts that got actually
changed, and never waste cache lines for anything else.

-Andi