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Date:	Tue, 6 Sep 2011 12:48:57 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, tytso@....edu,
	adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Put a reasonable upper bound on
 percpu_counter_batch

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 <anton@...ba.org>

Applied to percpu/for-3.2.

Thanks.

-- 
tejun
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