Re: [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 <redacted>

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