From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757095Ab0GGQNQ (ORCPT ); Wed, 7 Jul 2010 12:13:16 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:50470 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756702Ab0GGQNP (ORCPT ); Wed, 7 Jul 2010 12:13:15 -0400 Date: Wed, 7 Jul 2010 12:13:08 -0400 From: Christoph Hellwig To: Vivek Goyal Cc: Corrado Zoccolo , Jens Axboe , Linux-Kernel , Jeff Moyer , Shaohua Li , Gui Jianfeng Subject: Re: [PATCH 2/2] cfq-iosched: RQ_NOIDLE enabled for SYNC_WORKLOAD Message-ID: <20100707161308.GA12991@infradead.org> References: <1278516227-1643-1-git-send-email-czoccolo@gmail.com> <1278516227-1643-2-git-send-email-czoccolo@gmail.com> <20100707154631.GG2474@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100707154631.GG2474@redhat.com> User-Agent: Mutt/1.5.20 (2009-08-17) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 07, 2010 at 11:46:31AM -0400, Vivek Goyal wrote: > Whether to idle on WRITE_SYNC or not, I will leave it to Jens (I just > don't know the answer to that question. :-)). But in general I want to > get rid of idling as much as possible otherwise it becomes a serious > bottleneck in any kind of performance testing on higher end storage. After I've been thinking about this for a while I think the major problems is that we use WRITE_SYNC for two very different I/O patterns. One is synchronous data I/O (O_SYNC/O_DIRECT/fsync). While this is a high-level synchronous workload in the sense that someone waits for the I/O to finish, the I/O can still be batched as we're doing relatively large amounts of bios. The other one is synchronous writeout of metadata or the journal. Here we typically wait on that single I/O we're just submitting (or at most a handfull), and there is absolutely no point in idling. We already have the REQ_NOIDLE flag to distinguish between the two, so instead of second guessing we should actually make use of it.