From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 10 Nov 2008 14:12:56 -0800 From: Andrew Morton Subject: Re: [PATCH -mm] mm: fine-grained dirty_ratio_pcm and dirty_background_ratio_pcm (v2) Message-Id: <20081110141256.05214dbe.akpm@linux-foundation.org> In-Reply-To: <4918AFA1.4000102@gmail.com> References: <1221232192-13553-1-git-send-email-righi.andrea@gmail.com> <20080912131816.e0cfac7a.akpm@linux-foundation.org> <532480950809221641y3471267esff82a14be8056586@mail.gmail.com> <48EB4236.1060100@linux.vnet.ibm.com> <48EB851D.2030300@gmail.com> <20081008101642.fcfb9186.kamezawa.hiroyu@jp.fujitsu.com> <48ECB215.4040409@linux.vnet.ibm.com> <48EE236A.90007@gmail.com> <4918A074.1050003@gmail.com> <20081110131255.ce71ce60.akpm@linux-foundation.org> <4918AFA1.4000102@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: righi.andrea@gmail.com Cc: kamezawa.hiroyu@jp.fujitsu.com, rientjes@google.com, balbir@linux.vnet.ibm.com, mrubin@google.com, menage@google.com, dave@linux.vnet.ibm.com, chlunde@ping.uio.no, dpshah@google.com, eric.rannaud@gmail.com, fernando@oss.ntt.co.jp, agk@sourceware.org, m.innocenti@cineca.it, s-uchida@ap.jp.nec.com, ryov@valinux.co.jp, matt@bluehost.com, dradford@bluehost.com, kosaki.motohiro@jp.fujitsu.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, containers@lists.osdl.org List-ID: On Mon, 10 Nov 2008 23:03:13 +0100 Andrea Righi wrote: > On 2008-11-10 22:12, Andrew Morton wrote: > > On Mon, 10 Nov 2008 21:58:28 +0100 > > Andrea Righi wrote: > > > >> The current granularity of 5% of dirtyable memory for dirty pages writeback is > >> too coarse for large memory machines and this will get worse as > >> memory-size/disk-speed ratio continues to increase. > >> > >> These large writebacks can be unpleasant for desktop or latency-sensitive > >> environments, where the time to complete each writeback can be perceived as a > >> lack of responsiveness by the whole system. > >> > >> Following there's a similar solution as discussed in [1], but a little > >> bit simplified in order to provide the same functionality (in particular > >> to avoid backward compatibility problems) and reduce the amount of code > >> needed to implement an in-kernel parser to handle percentages with > >> decimals digits. > >> > >> The kernel provides the following parameters: > >> - dirty_ratio, dirty_background_ratio in percentage (1 ... 100) > >> - dirty_ratio_pcm, dirty_background_ratio_pcm in units of percent mille (1 ... 100,000) > > > > hm, so how long until dirty_ratio_pcm becomes too coarse... > > > > What happened to the idea of specifying these in units of kilobytes? > > The conclusion was that with units in KB requires much more complexity > to keep in sync the old dirty_ratio (and dirty_background_ratio) > interface with the new one. > > The KB limit is a static value, the other depends on the dirtyable > memory. If we want to preserve the same behaviour we should do the > following: > > - when dirty_ratio changes to x: > dirty_amount_in_bytes = x * dirtyable_memory / 100. > > - when dirty_amount_in_bytes changes to x: > dirty_ratio = x / dirtyable_memory * 100 > > But anytime the dirtyable memory changes (as well as the total memory in > the system) we should update both values accordingly to preserve the > coherency between them. OK. > I wonder if setting also PERCENT_PCM (that is 1% expressed in > fine-grained units) as a parameter could be a better long-term solution. > And also use another name for it, because in this case this would be not > a milli-percent value anymore. How about we forget the percentage thing and create /proc/sys/vm/dirty_ratio_millionths? That will give us a few more years of moores_law(memory size)/mores_law(disk speed) too.. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org