linux-kernel - Re: [PATCH] block: per-cpu counters for in-flight IO accounting

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Message-ID: <536CE25C.5040107@kernel.dk>
Date:	Fri, 09 May 2014 08:12:44 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Matias Bjørling <m@...rling.me>,
	sbradshaw@...ron.com
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: per-cpu counters for in-flight IO accounting

On 05/09/2014 03:17 AM, Matias Bjørling wrote:
> With multi-million IOPS and multi-node workloads, the atomic_t in_flight
> tracking becomes a bottleneck. Change the in-flight accounting to per-cpu
> counters to elevate.

The part stats are a pain in the butt, I've tried to come up with a
great fix for them too. But I don't think the percpu conversion is
necessarily the right one. The summing is part of the hotpath, so percpu
counters aren't necessarily the right way to go. I don't have a better
answer right now, otherwise it would have been fixed :-)

There's some low hanging fruit (like doing part_in_flight() twice in
part_round_stats_single()), though.

So I'm not going to apply this one as-is, lets see if we can find a
better solution. Perhaps local_t would be a good solution.

-- 
Jens Axboe

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