Re: [PATCH 1/1] Mention that I/O priorities also work on direct writes.

[Martin Steigerwald]

Hi Jens!

Did you notice the three patches I send?

Thanks,
Martin

----- UrsprÃngliche Mail -----
> 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@xxxxxxxxx>
> 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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-- 
Martin Steigerwald
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