linux-kernel - Re: [PATCH 0/4] block: account io: kill atomic operations

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Date:	Thu, 16 Jul 2015 09:02:56 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Ming Lei <tom.leiming@...il.com>, linux-kernel@...r.kernel.org,
	Mike Snitzer <snitzer@...hat.com>
Subject: Re: [PATCH 0/4] block: account io: kill atomic operations

On 07/16/2015 08:59 AM, Tejun Heo wrote:
> On Thu, Jul 16, 2015 at 08:48:41AM -0600, Jens Axboe wrote:
>> I've played with this before, but always ran into the hurdle of making
>> part_in_flight() too expensive ended up hurting results in the end. Making
>> the inc/dec parts of accounting percpu is a no-brainer, unfortunately the
>> summing then becomes pretty expensive. I'll run this through some testing
>> and see what kind of results I get.
>
> The only place which could be a problem is part_round_stats() and we
> can do that *way* lazier.  I don't think we're using that internally.
> Why are we even invoking it from IO issue / completion path?

Right, that's where it's called in the fast path. Right now, it's 
per-jiffy lazy. As to how/when it's called, I don't believe that has 
changed since those stats were introduced. If we can move that out of 
the fast path, then the percpu changes are straight forward.

-- 
Jens Axboe

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