linux-kernel - Re: [RFC/PATCH 2/2] block: Adding ROW scheduling algorithm

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Date:	Wed, 19 Sep 2012 07:29:29 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...i.de>
To:	Jeff Moyer <jmoyer@...hat.com>
cc:	Tatyana Brokhman <tlinder@...eaurora.org>, axboe@...nel.dk,
	linux-mmc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/PATCH 2/2] block: Adding ROW scheduling algorithm

On Monday 2012-08-06 18:35, Jeff Moyer wrote:
>Tatyana Brokhman writes:
>
>> This patch adds the implementation of a new scheduling algorithm - ROW.
>> The policy of this algorithm is to prioritize READ requests over WRITE
>> as much as possible without starving the WRITE requests.
>
>Perhaps you could start off by describing the workload, and describing
>why the existing I/O schedulers do not perform well.

My setup is a 1 GB RAM Atom N450 netbook, combined with the use of
dm-crypt for the 5400 rpm disk, which limits the effective write
throughput to somewhere around 20–26 MB/s. Now try this:

  ddrescue /dev/zero ~/bluntfile (or)

  (remote)# >nullfile
  (remote)# truncate -s 10G nullfile
  (local)# rsync -HPavz remote:nullfile .

The transfer rates of ddrescue/rsync first show 50–60 MB/s. Wait
until writeout is forced - which is after ~700–800 MB in my case when
all buffers are full. The transfer rates then drop to the
aforementioned 20–26 MB/s.

In the so-loaded system, try to start an xterm (preferably by the use
of a shortcut, or starting one from another xterm). The wait time for
the shell prompt to appear is then the measure for interactivity,
with lower being better.

I have casually observed that ROW has 1/4th of the wait time in this
heavy disk write scenario (around 5 s) for the shell to start up than
with CFQ (20–21 s). "Casual" meaning I had a clock with 1.0 s
granularity beside me and ran this for like 3 times for each
scheduler, averaging it out.
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