linux-kernel - Re: No, really, stop trying to delete slab until you've finished making slub perform as well

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Date:	Wed, 13 Aug 2008 23:14:30 +0900
From:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>
To:	"Christoph Lameter" <cl@...ux-foundation.org>
Cc:	"Matthew Wilcox" <matthew@....cx>,
	"Pekka Enberg" <penberg@...helsinki.fi>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	"Mel Gorman" <mel@...net.ie>, andi@...stfloor.org,
	"Rik van Riel" <riel@...hat.com>
Subject: Re: No, really, stop trying to delete slab until you've finished making slub perform as well

>> :t-0000128               28739     128     1.3G  20984/20984/8  512 0  99   0 *
>
> Argh. Most slabs contain a single object. Probably due to the conflict resolution.

agreed with the issue exist in lock contention code.


> The obvious fix is to avoid allocating another slab on conflict but how will
> this impact performance?
>
>
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c    2008-08-13 08:06:00.000000000 -0500
> +++ linux-2.6/mm/slub.c 2008-08-13 08:07:59.000000000 -0500
> @@ -1253,13 +1253,11 @@
>  static inline int lock_and_freeze_slab(struct kmem_cache_node *n,
>                                                        struct page *page)
>  {
> -       if (slab_trylock(page)) {
> -               list_del(&page->lru);
> -               n->nr_partial--;
> -               __SetPageSlubFrozen(page);
> -               return 1;
> -       }
> -       return 0;
> +       slab_lock(page);
> +       list_del(&page->lru);
> +       n->nr_partial--;
> +       __SetPageSlubFrozen(page);
> +       return 1;
>  }

I don't mesure it yet. I don't like this patch.
maybe, it decrease other typical benchmark.

So, I think better way is

1. slab_trylock(), if success goto 10.
2. check fragmentation ratio, if low goto 10
3. slab_lock()
10. return func

I think this way doesn't cause performance regression.
because high fragmentation cause defrag and compaction lately.
So, prevent fragmentation often increase performance.

Thought?
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