From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757689Ab1DMXwS (ORCPT ); Wed, 13 Apr 2011 19:52:18 -0400 Received: from ipmail07.adl2.internode.on.net ([150.101.137.131]:63132 "EHLO ipmail07.adl2.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932269Ab1DMXwQ (ORCPT ); Wed, 13 Apr 2011 19:52:16 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AukDALM1pk15LHHJgWdsb2JhbACmIxUBARYmJcMCDoVgBA Date: Thu, 14 Apr 2011 09:52:11 +1000 From: Dave Chinner To: Wu Fengguang Cc: 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: <20110413235211.GN31057@dastard> References: <20110413085937.981293444@intel.com> <20110413090415.763161169@intel.com> <20110413220444.GF4648@quack.suse.cz> <20110413233122.GA6097@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110413233122.GA6097@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 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.... Cheers, Dave. -- Dave Chinner david@fromorbit.com