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SubjectRe: 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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