From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761060AbXGVIKc (ORCPT ); Sun, 22 Jul 2007 04:10:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753389AbXGVIKQ (ORCPT ); Sun, 22 Jul 2007 04:10:16 -0400 Received: from smtp.ustc.edu.cn ([202.38.64.16]:39869 "HELO ustc.edu.cn" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with SMTP id S1750993AbXGVIKM (ORCPT ); Sun, 22 Jul 2007 04:10:12 -0400 Message-ID: <385091802.23116@ustc.edu.cn> X-EYOUMAIL-SMTPAUTH: wfg@mail.ustc.edu.cn Date: Sun, 22 Jul 2007 16:10:10 +0800 From: Fengguang Wu To: Dave Jones , Peter Zijlstra , linux-kernel , riel , Andrew Morton , Rusty Russell , Tim Pepper , Chris Snook Subject: Re: [PATCH 0/3] readahead drop behind and size adjustment Message-ID: <20070722081010.GA6317@mail.ustc.edu.cn> Mail-Followup-To: Dave Jones , Peter Zijlstra , linux-kernel , riel , Andrew Morton , Rusty Russell , Tim Pepper , Chris Snook References: <20070721210005.000228000@chello.nl> <20070722023923.GA6438@mail.ustc.edu.cn> <20070722024428.GA724@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070722024428.GA724@redhat.com> X-GPG-Fingerprint: 53D2 DDCE AB5C 8DC6 188B 1CB1 F766 DA34 8D8B 1C6D User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jul 21, 2007 at 10:44:28PM -0400, Dave Jones wrote: > On Sun, Jul 22, 2007 at 10:39:23AM +0800, Fengguang Wu wrote: > > > It makes sense to raise it beyond 128K. 1M default readahead > > absolutely makes sense for sequential workloads. For the desktop, > > this increases boot speed and readahead misses, both due to more > > aggressive mmap read-around. Most users will be glad to feel the > > speedup, and happily ignore the readahead misses, which may be > > "invisible" in case of large memory. > > > > In theory, the distributions can do the same tuning. So we have an > > interesting question for Dave: > > Does fedora desktop raise the default readahead size? Why or > > why not? It goes so far to do userland readahead ;) > > Fedora takes whatever defaults for readahead the kernel.org kernel has. > The only reasoning being if anyone reported VM bugs, we'd be able > to say to interested upstream developers "we're running the stock VM". > without having to get the user to try and reproduce on unpatched > kernels. Thank you. Now I'm more confident that the kernel should have a reasonable default readahead size. The current one is 5+ years old and should be updated now. > > - drop behind > > > > Sorry, I still doubt it will benefit all/most workloads. Leave it off > > by default, and leave the enabling decision to Dave? I do hope that > > it help general desktops. > > It's not a subject that I'm intimatly familiar with, and when it > comes to decisions like this, I tend to just take whatever the > upstream defaults are. - It will avoid large-file-reads-thrashing-my-desktop problem, so most desktop users should like it. But sure there will be counter cases when a user want to keep the data cached. - File servers may hurt from it. Imagine a mp3/png file server. The files are large enough to trigger drop-behind, but small (and hot) enough to be cached. Also when a new fedora DVD iso is released, it may be cached for some days. These are only the obvious cases. So I opt for it being made tunable, safe, and turned off by default.