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SubjectRe: [PATCH 2/2] percpu_counter: Put a reasonable upper bound on percpu_counter_batch
On Fri, Aug 26, 2011 at 07:29:27AM +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 <anton@samba.org>

Yeah, capping the upper bound seems reasonable but can you please add
some comment explaining why the upper bound is necessary there?

Thank you.

--
tejun


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