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    SubjectRe: [PATCH] Reduce vm_stat cacheline contention in __vm_enough_memory
    On Wed, 12 Oct 2011 11:02:02 -0500
    Dimitri Sivanich <sivanich@sgi.com> wrote:

    > Tmpfs I/O throughput testing on UV systems has shown writeback contention
    > between multiple writer threads (even when each thread writes to a separate
    > tmpfs mount point).
    >
    > A large part of this is caused by cacheline contention reading the vm_stat
    > array in the __vm_enough_memory check.
    >
    > The attached test patch illustrates a possible avenue for improvement in this
    > area. By locally caching the values read from vm_stat (and refreshing the
    > values after 2 seconds), I was able to improve tmpfs writeback performance from
    > ~300 MB/sec to ~700 MB/sec with 120 threads writing data simultaneously to
    > files on separate tmpfs mount points (tested on 3.1.0-rc9).
    >
    > Note that this patch is simply to illustrate the gains that can be made here.
    > What I'm looking for is some guidance on an acceptable way to accomplish the
    > task of reducing contention in this area, either by caching these values in a
    > way similar to the attached patch, or by some other mechanism if this is
    > unacceptable.

    Yes, the global vm_stat[] array is a problem - I'm surprised it's hung
    around for this long. Altering the sysctl_overcommit_memory mode will
    hide the problem, but that's no good.

    I think we've discussed switching vm_stat[] to a contention-avoiding
    counter scheme. Simply using <percpu_counter.h> would be the simplest
    approach. They'll introduce inaccuracies but hopefully any problems
    from that will be minor for the global page counters.

    otoh, I think we've been round this loop before and I don't recall why
    nothing happened.


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