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    SubjectRe: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low
    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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