linux-kernel - Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead prepare_to

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Date:	Tue, 15 Dec 2009 19:43:30 +0100
From:	Mike Galbraith <efault@....de>
To:	Rik van Riel <riel@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	lwoodman@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, minchan.kim@...il.com
Subject: Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead
 prepare_to_wait()

On Tue, 2009-12-15 at 09:58 -0500, Rik van Riel wrote:
> On 12/15/2009 12:32 AM, Mike Galbraith wrote:
> > On Tue, 2009-12-15 at 09:45 +0900, KOSAKI Motohiro wrote:
> >>> On 12/14/2009 07:30 AM, KOSAKI Motohiro wrote:
> >>>> if we don't use exclusive queue, wake_up() function wake _all_ waited
> >>>> task. This is simply cpu wasting.
> >>>>
> >>>> Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@...fujitsu.com>
> >>>
> >>>>    		if (zone_watermark_ok(zone, sc->order, low_wmark_pages(zone),
> >>>>    					0, 0)) {
> >>>> -			wake_up(wq);
> >>>> +			wake_up_all(wq);
> >>>>    			finish_wait(wq,&wait);
> >>>>    			sc->nr_reclaimed += sc->nr_to_reclaim;
> >>>>    			return -ERESTARTSYS;
> >>>
> >>> I believe we want to wake the processes up one at a time
> >>> here.
> 
> >> Actually, wake_up() and wake_up_all() aren't different so much.
> >> Although we use wake_up(), the task wake up next task before
> >> try to alloate memory. then, it's similar to wake_up_all().
> 
> That is a good point.  Maybe processes need to wait a little
> in this if() condition, before the wake_up().  That would give
> the previous process a chance to allocate memory and we can
> avoid waking up too many processes.

Pondering, I think I'd at least wake NR_CPUS.  If there's not enough to
go round, oh darn, but if there is, you have full utilization quicker.

$.02.

	-Mike

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