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    SubjectRe: [PATCH] Reduce vm_stat cacheline contention in __vm_enough_memory
    On Thu, Oct 13, 2011 at 02:24:34PM -0700, Andrew Morton wrote:
    > On Thu, 13 Oct 2011 16:02:58 -0500 (CDT)
    > Christoph Lameter <cl@gentwo.org> wrote:
    >
    > > On Thu, 13 Oct 2011, Andrew Morton wrote:
    > >
    > > > > If there are no updates occurring for a while (due to increased deltas
    > > > > and/or vmstat updates) then the vm_stat cacheline should be able to stay
    > > > > in shared mode in multiple processors and the performance should increase.
    > > > >
    > > >
    > > > We could cacheline align vm_stat[]. But the thing is pretty small - we
    > > > couild put each entry in its own cacheline.
    > >
    > > Which in turn would increase the cache footprint of some key kernel
    > > functions (because they need multiple vm_stat entries) and cause eviction
    > > of other cachelines that then reduce overall system performance again.
    >
    > Sure, but we gain performance by not having different CPUs treading on
    > each other when they update different vmstat fields. Sometimes one
    > effect will win and other times the other effect will win. Some
    > engineering is needed..

    I think the first step is to determine the role (if any) that false sharing may be playing in this, since that's a simpler fix (cacheline align and pad the array).

    Then, if necessary, will look at contention issues within the array.


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