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    SubjectRe: [PATCH 2/3] mm: page allocator: Calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake
    On Fri, 3 Sep 2010 18:17:46 -0500 (CDT)
    Christoph Lameter <cl@linux.com> wrote:

    > On Fri, 3 Sep 2010, Andrew Morton wrote:
    >
    > > Can someone remind me why per_cpu_pageset went and reimplemented
    > > percpu_counters rather than just using them?
    >
    > The vm counters are per zone and per cpu and have a flow from per cpu /
    > zone deltas to zone counters and then also into global counters.

    hm. percpu counters would require overflow-time hooks to do that.
    Might be worth looking at.

    > > Is this really the best way of doing it? The way we usually solve
    > > this problem (and boy, was this bug a newbie mistake!) is:
    > >
    > > foo = percpu_counter_read(x);
    > >
    > > if (foo says something bad) {
    > > /* Bad stuff: let's get a more accurate foo */
    > > foo = percpu_counter_sum(x);
    > > }
    > >
    > > if (foo still says something bad)
    > > do_bad_thing();
    > >
    > > In other words, don't do all this stuff with percpu_drift_mark and the
    > > kswapd heuristic. Just change zone_watermark_ok() to use the more
    > > accurate read if it's about to return "no".
    >
    > percpu counters must always be added up when their value is determined.

    Nope. That's the difference between percpu_counter_read() and
    percpu_counter_sum().

    > This seems to be a special case here where Mel does not want to have to
    > cost to bring the counters up to date nor reduce the delta/time limits to
    > get some more accuracy but wants take some sort of snapshot of the whole
    > situation for this particular case.

    My suggestion didn't actually have anything to do with percpu_counters.




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