Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead prepare_to_wait()

[KOSAKI Motohiro]

> 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@xxxxxxxxxxxxxx>
> >>>
> >>>>    		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.
 - if another CPU is busy, woken process don't start to run awhile.
   then, zone_watermark_ok() can calculate correct value.


> > What happens to waiters should running tasks not allocate for a while?
> 
> When a waiter is woken up, it will either:
> 1) see that there is enough free memory and wake up the next guy, or
> 2) run shrink_zone and wake up the next guy
> 
> Either way, the processes that just got woken up will ensure that
> the sleepers behind them in the queue will get woken up.
> 
> -- 
> All rights reversed.



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