linux-kernel - Re: CFQ I/O priorities only for reads?

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Message-ID: <4ED39EA5.4000706@kernel.dk>
Date:	Mon, 28 Nov 2011 15:45:57 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Martin Steigerwald <ms@...mix.de>
CC:	linux-kernel@...r.kernel.org, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: CFQ I/O priorities only for reads?

On 2011-11-28 15:42, Martin Steigerwald wrote:
> Hi jens und Vivek,
> 
> Vivek, I cc'd you, cause you wrote the new cfq-iosched.txt.
> 
> 
> In trying to understand how I/O priorities actually really work, I tried to dd 
> with
> 
> rm nullen-id ; sync ; /usr/bin/time ionice -c3 dd if=/dev/zero of=nullen-id 
> count=500 bs=1M conv=fsync
> 
> versus
> 
> rm nullen-rl; sync ; /usr/bin/time ionice -c1 -n0 dd if=/dev/zero of=nullen-rl 
> count=500 bs=1M conv=fsync
> 
> concurrently. No differences. At first I was puzzled, then I thought maybe 
> direct I/O makes a difference. So I tried with oflag=direct.
> 
> And it does.
> 
> Then I actually read the documentation block/ioprio.txt (3.1 here):
> 
>> With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
>> priorities are supported for reads on files.  This enables users to io nice
>> processes or process groups, similar to what has been possible with cpu
>> scheduling for ages.  This document mainly details the current
>> possibilities with cfq; other io schedulers do not support io priorities
>> thus far.
> 
> According to it I/O priorities will even only work on reads. Is that correct? 
> I mean they do work on reads, I tested it, but *only* on reads?
> 
> From what I see here, it also works for direct I/O write requests
> 
> So from what I conclude is that CFQ I/O priorities work for all requests that 
> are issued via synchronous system calls, but not for those issued via 
> asynchronous calls, i. e. everything that goes through the pagecache.
> 
> Is that correct?

Priorities work for reads AND direct writes. In other words, it does not
work for buffered writes.

> Vivek, one thing on cfq-iosched.txt: Could slice_idle=0 make sense on
> SSDs?  Later on you write that there are some SSD optimizations in
> place that cut down idling already.

It will have a functional difference even on SSDs, depending on your
workload, even if the scope of idling is smaller on an SSD.

-- 
Jens Axboe

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