Messages in this thread Patch in this message | | | From | Martin Steigerwald <> | Subject | [PATCH 1/1] Mention that I/O priorities also work on direct writes. | Date | Mon, 28 Nov 2011 16:19:12 +0100 |
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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@teamix.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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