Messages in this thread Patch in this message | | | Date | Thu, 28 Oct 2010 10:49:03 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low |
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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@csn.ul.ie> 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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