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    SubjectRe: [PATCH] percpu_counter: Put a reasonable upper bound on percpu_counter_batch

    On Sep 5, 2011, at 11:48 PM, Tejun Heo wrote:

    > On Mon, Aug 29, 2011 at 09:46:09PM +1000, Anton Blanchard wrote:
    >> When testing on a 1024 thread ppc64 box I noticed a large amount of
    >> CPU time in ext4 code.
    >> ext4_has_free_blocks has a fast path to avoid summing every free and
    >> dirty block per cpu counter, but only if the global count shows more
    >> free blocks than the maximum amount that could be stored in all the
    >> per cpu counters.
    >> Since percpu_counter_batch scales with num_online_cpus() and the maximum
    >> amount in all per cpu counters is percpu_counter_batch * num_online_cpus(),
    >> this breakpoint grows at O(n^2).
    >> This issue will also hit with users of percpu_counter_compare which
    >> does a similar thing for one percpu counter.
    >> I chose to cap percpu_counter_batch at 1024 as a conservative first
    >> step, but we may want to reduce it further based on further benchmarking.
    >> Signed-off-by: Anton Blanchard <>
    > Applied to percpu/for-3.2.

    Um, this was an ext4 patch and I pointed out it could cause problems. (Specifically, data loss…)

    - Ted

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