Messages in this thread | | | Date | Mon, 28 Nov 2011 15:45:57 +0100 | From | Jens Axboe <> | 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
| |