On Thu, Oct 28, 2010 at 10:09:20AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 27 Oct 2010 09:47:35 +0100
> Mel Gorman <mel@xxxxxxxxx> wrote:
>
> > Commit [aa45484: calculate a better estimate of NR_FREE_PAGES when
> > memory is low] noted that watermarks were based on the vmstat
> > NR_FREE_PAGES. To avoid synchronization overhead, these counters are
> > maintained on a per-cpu basis and drained both periodically and when a
> > threshold is above a threshold. On large CPU systems, the difference
> > between the estimate and real value of NR_FREE_PAGES can be very high.
> > The system can get into a case where pages are allocated far below the
> > min watermark potentially causing livelock issues. The commit solved the
> > problem by taking a better reading of NR_FREE_PAGES when memory was low.
> >
> > <SNIP>
> >
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 355a9e6..cafcc2d 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -81,6 +81,12 @@ EXPORT_SYMBOL(vm_stat);
> >
> > #ifdef CONFIG_SMP
> >
> > +static int calculate_pressure_threshold(struct zone *zone)
> > +{
> > + return max(1, (int)((high_wmark_pages(zone) - low_wmark_pages(zone) /
> > + num_online_cpus())));
> > +}
> > +
>
> Could you add background theory of this calculation as a comment to
> show the difference with calculate_threshold() ?
>
Sure. When writing it, I realised that the calculations here differ from
what percpu_drift_mark does. This is what I currently have
int calculate_pressure_threshold(struct zone *zone)
{
int threshold;
int watermark_distance;
/*
* As vmstats are not up to date, there is drift between the estimated
* and real values. For high thresholds and a high number of CPUs, it
* is possible for the min watermark to be breached while the estimated
* value looks fine. The pressure threshold is a reduced value such
* that even the maximum amount of drift will not accidentally breach
* the min watermark
*/
watermark_distance = low_wmark_pages(zone) - min_wmark_pages(zone);
threshold = max(1, watermark_distance / num_online_cpus());
/*
* Maximum threshold is 125
*/
threshold = min(125, threshold);
return threshold;
}
Is this better?
> And don't we need to have "max=125" thresh here ?
>
Yes.
>
> > static int calculate_threshold(struct zone *zone)
> > {
> > int threshold;
> > @@ -159,6 +165,44 @@ static void refresh_zone_stat_thresholds(void)
> > }
> > }
> >
> > +void reduce_pgdat_percpu_threshold(pg_data_t *pgdat)
> > +{
> > + struct zone *zone;
> > + int cpu;
> > + int threshold;
> > + int i;
> > +
>
> get_online_cpus();
>
Also correct.
Thanks very much. I'm revising the series.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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