linux-ext4 - Re: [PATCH 2/2] percpu_counter: Put a reasonable upper bound on percpu_counter

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Message-ID: <20110826090016.GD2632@htj.dyndns.org>
Date:	Fri, 26 Aug 2011 11:00:16 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Anton Blanchard <anton@...ba.org>
Cc:	tytso@....edu, adilger.kernel@...ger.ca, eric.dumazet@...il.com,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: 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 <anton@...ba.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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