Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933850Ab1DNSQR (ORCPT ); Thu, 14 Apr 2011 14:16:17 -0400 Received: from cantor2.suse.de ([195.135.220.15]:54249 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933701Ab1DNSQN (ORCPT ); Thu, 14 Apr 2011 14:16:13 -0400 Date: Thu, 14 Apr 2011 20:16:09 +0200 From: Jan Kara To: Wu Fengguang Cc: Dave Chinner , Jan Kara , Andrew Morton , Peter Zijlstra , Richard Kennedy , Hugh Dickins , Rik van Riel , LKML , Linux Memory Management List , "linux-fsdevel@vger.kernel.org" Subject: Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up time Message-ID: <20110414181609.GH5054@quack.suse.cz> References: <20110413085937.981293444@intel.com> <20110413090415.763161169@intel.com> <20110413220444.GF4648@quack.suse.cz> <20110413233122.GA6097@localhost> <20110413235211.GN31057@dastard> <20110414002301.GA9826@localhost> <20110414151424.GA367@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110414151424.GA367@localhost> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3110 Lines: 59 On Thu 14-04-11 23:14:25, Wu Fengguang wrote: > On Thu, Apr 14, 2011 at 08:23:02AM +0800, Wu Fengguang wrote: > > On Thu, Apr 14, 2011 at 07:52:11AM +0800, Dave Chinner wrote: > > > On Thu, Apr 14, 2011 at 07:31:22AM +0800, Wu Fengguang wrote: > > > > On Thu, Apr 14, 2011 at 06:04:44AM +0800, Jan Kara wrote: > > > > > On Wed 13-04-11 16:59:41, Wu Fengguang wrote: > > > > > > Reduce the dampening for the control system, yielding faster > > > > > > convergence. The change is a bit conservative, as smaller values may > > > > > > lead to noticeable bdi threshold fluctuates in low memory JBOD setup. > > > > > > > > > > > > CC: Peter Zijlstra > > > > > > CC: Richard Kennedy > > > > > > Signed-off-by: Wu Fengguang > > > > > Well, I have nothing against this change as such but what I don't like is > > > > > that it just changes magical +2 for similarly magical +0. It's clear that > > > > > > > > The patch tends to make the rampup time a bit more reasonable for > > > > common desktops. From 100s to 25s (see below). > > > > > > > > > this will lead to more rapid updates of proportions of bdi's share of > > > > > writeback and thread's share of dirtying but why +0? Why not +1 or -1? So > > > > > > > > Yes, it will especially be a problem on _small memory_ JBOD setups. > > > > Richard actually has requested for a much radical change (decrease by > > > > 6) but that looks too much. > > > > > > > > My team has a 12-disk JBOD with only 6G memory. The memory is pretty > > > > small as a server, but it's a real setup and serves well as the > > > > reference minimal setup that Linux should be able to run well on. > > > > > > FWIW, linux runs on a lot of low power NAS boxes with jbod and/or > > > raid setups that have <= 1GB of RAM (many of them run XFS), so even > > > your setup could be considered large by a significant fraction of > > > the storage world. Hence you need to be careful of optimising for > > > what you think is a "normal" server, because there simply isn't such > > > a thing.... > > > > Good point! This patch is likely to hurt a loaded 1GB 4-disk NAS box... > > I'll test the setup. > > Just did a comparison of the IO-less patches' performance with and > without this patch. I hardly notice any differences besides some more > bdi goal fluctuations in the attached graphs. The write throughput is > a bit large with this patch (80MB/s vs 76MB/s), however the delta is > within the even larger stddev range (20MB/s). Thanks for the test but I cannot find out from the numbers you provided how much did the per-bdi thresholds fluctuate in this low memory NAS case? You can gather current bdi threshold from /sys/kernel/debug/bdi//stats so it shouldn't be hard to get the numbers... Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/