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From: Mike Galbraith on 15 Dec 2009 13:50
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(a)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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From: Mike Galbraith on 16 Dec 2009 00:50
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(a)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.
>
> 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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