Subject: Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead
 prepare_to_wait()
From: Mike Galbraith <efault@gmx.de>
To: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, lwoodman@redhat.com,
       akpm@linux-foundation.org, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, minchan.kim@gmail.com
In-Reply-To: <4B27A417.3040206@redhat.com>
References: <20091214212936.BBBA.A69D9226@jp.fujitsu.com>
	 <4B264CCA.5010609@redhat.com> <20091215085631.CDAD.A69D9226@jp.fujitsu.com>
	 <1260855146.6126.30.camel@marge.simson.net>  <4B27A417.3040206@redhat.com>
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Date: Tue, 15 Dec 2009 19:43:30 +0100
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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@jp.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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