linux-kernel - Re: [PATCH] vmscan: stop meaningless loop iteration when no reclaimable slab

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Date:	Tue, 13 Jul 2010 15:33:57 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Mel Gorman <mel@....ul.ie>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] vmscan: stop meaningless loop iteration when no 
	reclaimable slab

On Tue, Jul 13, 2010 at 1:48 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
> Hi
>
>>
>> old shrink_slab
>>
>> shrinker->nr += delta; /* Add delta to previous shrinker's remained count */
>> total_scan = shrinker->nr;
>>
>> while(total_scan >= SHRINK_BATCH) {
>>       nr_before = shrink(xxx);
>>       total_scan =- this_scan;
>> }
>>
>> shrinker->nr += total_scan;
>>
>> The total_scan can always be the number < SHRINK_BATCH.
>> So, when next shrinker calcuates loop count, the number can affect.
>
> Correct.
>
>
>>
>> new shrink_slab
>>
>> shrinker->nr += delta; /* nr is always zero by your patch */
>
> no.
> my patch don't change delta calculation at all.
>
>
>> total_scan = shrinker->nr;
>>
>> while(total_scan >= SHRINK_BATCH) {
>>       nr_before = shrink(xxx);
>>       if (nr_before == 0) {
>>               total_scan = 0;
>>               break;
>>       }
>> }
>>
>> shrinker->nr += 0;
>>
>> But after your patch, total_scan is always zero. It never affect
>> next shrinker's loop count.
>
> No. after my patch this loop has two exiting way
>  1) total_scan are less than SHRINK_BATCH.
>      -> no behavior change.  we still pass shrinker->nr += total_scan code.
>  2) (*shrinker->shrink)(0, gfp_mask) return 0
>      don't increase shrinker->nr.  because two reason,
>      a) if total_scan are 10000,  we shouldn't carry over such big number.
>      b) now, we have zero slab objects, then we have been freed form the guilty of keeping
>          balance page and slab reclaim. shrinker->nr += 0; have zero side effect.

Totally, I agree with you.
Thanks for good explanation, Kosaki.

Reviewed-by: Minchan kim <minchan.kim@...il.com>


-- 
Kind regards,
Minchan Kim
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