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

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Date:	Wed, 16 Dec 2009 06:43:44 +0100
From:	Mike Galbraith <efault@....de>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Rik van Riel <riel@...hat.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 Wed, 2009-12-16 at 09:48 +0900, KOSAKI Motohiro 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.
> 
> if we really need wait a bit, Mike's wake_up_batch is best, I think.
> It mean
>  - if another CPU is idle, wake up one process soon. iow, it don't
>    make meaningless idle.

Along those lines, there's also NEWIDLE balancing considerations.  That
idle may result in a task being pulled, which may or may not hurt a bit.

'course, if you're jamming up on memory allocation, that's the least of
your worries, but every idle avoided is potentially a pull avoided.

Just a thought.

	-Mike

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