Re: [RFC][PATCH 3/3] mm: reserve max drift pages at boot time instead using zone_page_state_snapshot()

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Oct 14, 2010 at 11:39:34AM +0900, KOSAKI Motohiro wrote:
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 53627fa..194bdaa 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4897,6 +4897,15 @@ static void setup_per_zone_wmarks(void)
> > >  	for_each_zone(zone) {
> > >  		u64 tmp;
> > >  
> > > +		/*
> > > +		 * If max drift are less than 1%, reserve max drift pages
> > > +		 * instead costly runtime calculation.
> > > +		 */
> > > +		if (zone->percpu_drift_mark < (zone->present_pages/100)) {
> > > +			pages_min += zone->percpu_drift_mark;
> > > +			zone->percpu_drift_mark = 0;
> > > +		}
> > > +
> > 
> > I don't see how this solves Shaohua's problem as such. Large systems will
> > still suffer a bug performance penalty from zone_page_state_snapshot(). I
> > do see the logic of adjusting min for larger systems to limit the amount of
> > time per-cpu thresholds are lowered but that would be as a follow-on to my
> > patch rather than a replacement.
> 
> My patch rescue 256cpus or more smaller systems.

True, and it would be nice to limit how many machines any of this logic
applies to.

> and I assumed 4096cpus system don't
> run IO intensive workload such as Shaohua's case.

Also true, but they still suffer the drift problem. The reproduction
case would change but otherwise the drift must still be handled.

> they always use cpusets and run hpc
> workload.
> 
> If you know another >1024cpus system, please let me know.
> And again, my patch works on 4096cpus sysmtem although slow, but your don't.
> 
> Am I missing something?
> 

I think both are ultimately needed. For my patch, I need to make sure that
wakeup_kswapd() actually wakes up kswapd by using
zone_page_state_snapshot() when necessary. Your patch avoids the problem
differently but in a way that is nice for "smaller" machines.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>