From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752443Ab1K1OqB (ORCPT ); Mon, 28 Nov 2011 09:46:01 -0500 Received: from casper.infradead.org ([85.118.1.10]:55430 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750878Ab1K1OqA (ORCPT ); Mon, 28 Nov 2011 09:46:00 -0500 Message-ID: <4ED39EA5.4000706@kernel.dk> Date: Mon, 28 Nov 2011 15:45:57 +0100 From: Jens Axboe MIME-Version: 1.0 To: Martin Steigerwald CC: linux-kernel@vger.kernel.org, Vivek Goyal Subject: Re: CFQ I/O priorities only for reads? References: <201111281542.22258.ms@teamix.de> In-Reply-To: <201111281542.22258.ms@teamix.de> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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