linux-kernel - Re: [PATCH RFC - TAKE TWO - 09/12] block, bfq: reduce latency during request-pool saturation

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Message-ID: <20140531135402.GC24557@mtj.dyndns.org>
Date:	Sat, 31 May 2014 09:54:02 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Paolo Valente <paolo.valente@...more.it>
Cc:	Jens Axboe <axboe@...nel.dk>, Li Zefan <lizefan@...wei.com>,
	Fabio Checconi <fchecconi@...il.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>,
	Paolo Valente <posta_paolo@...oo.it>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC - TAKE TWO - 09/12] block, bfq: reduce latency during
 request-pool saturation

On Thu, May 29, 2014 at 11:05:40AM +0200, Paolo Valente wrote:
> This patch introduces an heuristic that reduces latency when the
> I/O-request pool is saturated. This goal is achieved by disabling
> device idling, for non-weight-raised queues, when there are weight-
> raised queues with pending or in-flight requests. In fact, as
> explained in more detail in the comment to the function
> bfq_bfqq_must_not_expire(), this reduces the rate at which processes
> associated with non-weight-raised queues grab requests from the pool,
> thereby increasing the probability that processes associated with
> weight-raised queues get a request immediately (or at least soon) when
> they need one.

Wouldn't it be more straight-forward to simply control how many
requests each queue consume by returning ELV_MQUEUE_NO?  Seeky ones do
benefit from larger number of requests in elevator but to only certain
number given the fifo timeout anyway and controlling that explicitly
would be a lot easier to anticipate the behavior of than playing
roulette with random request allocation failures.

Thanks.

-- 
tejun
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