linux-kernel - [PATCH 1/1] Mention that I/O priorities also work on direct writes.

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Message-Id: <201111281619.12924.ms@teamix.de>
Date:	Mon, 28 Nov 2011 16:19:12 +0100
From:	Martin Steigerwald <ms@...mix.de>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, Vivek Goyal <vgoyal@...hat.com>
Subject: [PATCH 1/1] Mention that I/O priorities also work on direct writes.

Am Montag, 28. November 2011 schrieb Jens Axboe:
> 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.

>From 5414ce9fd8c384a3a25a478490a022539694e4e0 Mon Sep 17 00:00:00 2001
From: Martin Steigerwald <ms@...mix.de>
Date: Mon, 28 Nov 2011 16:10:32 +0100
Subject: [PATCH 1/3] Mention that I/O priorities also work on direct writes.

---
 Documentation/block/ioprio.txt |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/Documentation/block/ioprio.txt b/Documentation/block/ioprio.txt
index 8ed8c59..a555c59 100644
--- a/Documentation/block/ioprio.txt
+++ b/Documentation/block/ioprio.txt
@@ -6,10 +6,11 @@ Intro
 -----
 
 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.
+priorities are supported for reads and direct, not buffered, writes 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.
 
 Scheduling classes
 ------------------
-- 
1.7.7.3

Thanks,
-- 
Martin Steigerwald - teamix GmbH - http://www.teamix.de
gpg: 19E3 8D42 896F D004 08AC A0CA 1E10 C593 0399 AE90
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