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    SubjectRe: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low
    On Thu, 28 Oct 2010, Andrew Morton wrote:

    > > To ensure that kswapd wakes up, a safe version of zone_watermark_ok()
    > > is introduced that takes a more accurate reading of NR_FREE_PAGES when
    > > called from wakeup_kswapd, when deciding whether it is really safe to go
    > > back to sleep in sleeping_prematurely() and when deciding if a zone is
    > > really balanced or not in balance_pgdat(). We are still using an expensive
    > > function but limiting how often it is called.
    >
    > Here I go again. I have a feeling that I already said this, but I
    > can't find versions 2 or 3 in the archives..
    >
    > Did you evaluate using plain on percpu_counters for this? They won't
    > solve the performance problem as they're basically the same thing as
    > these open-coded counters. But they'd reduce the amount of noise and
    > custom-coded boilerplate in mm/.

    The zone counters are done using the ZVCs in vmstat.c to save space and to
    be in the same cacheline as other hot data necessary for allocation and
    free.
    >
    > > + threshold = max(1, (int)(watermark_distance / num_online_cpus()));
    > > +
    > > + /*
    > > + * Maximum threshold is 125
    >
    > Reasoning?

    Differentials are stored in 8 bit signed ints.

    > > + put_online_cpus();
    > > +}
    >
    > Given that ->stat_threshold is the same for each CPU, why store it for
    > each CPU at all? Why not put it in the zone and eliminate the inner
    > loop?

    Doing that caused cache misses in the past and reduced the performance of
    the ZVCs. This way the threshold is in the same cacheline as the
    differentials.





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