From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764724AbXGVJSL (ORCPT ); Sun, 22 Jul 2007 05:18:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1764549AbXGVJRj (ORCPT ); Sun, 22 Jul 2007 05:17:39 -0400 Received: from viefep18-int.chello.at ([213.46.255.22]:40493 "EHLO viefep20-int.chello.at" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1764512AbXGVJRh (ORCPT ); Sun, 22 Jul 2007 05:17:37 -0400 Subject: Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size From: Peter Zijlstra To: Jens Axboe Cc: linux-kernel , Fengguang Wu , riel , Andrew Morton , Rusty Russell , Tim Pepper , Chris Snook In-Reply-To: <20070722085041.GS11657@kernel.dk> References: <20070721210005.000228000@chello.nl> <20070721210052.497469000@chello.nl> <20070722082434.GR11657@kernel.dk> <1185093405.20032.205.camel@twins> <20070722085041.GS11657@kernel.dk> Content-Type: text/plain Date: Sun, 22 Jul 2007 11:17:32 +0200 Message-Id: <1185095852.20032.229.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2007-07-22 at 10:50 +0200, Jens Axboe wrote: > On Sun, Jul 22 2007, Peter Zijlstra wrote: > > On Sun, 2007-07-22 at 10:24 +0200, Jens Axboe wrote: > > > On Sat, Jul 21 2007, Peter Zijlstra wrote: > > > > > > +static __init int readahead_init(void) > > > > +{ > > > > + /* > > > > + * Scale the max readahead window with system memory > > > > + * > > > > + * 64M: 128K > > > > + * 128M: 180K > > > > + * 256M: 256K > > > > + * 512M: 360K > > > > + * 1G: 512K > > > > + * 2G: 724K > > > > + * 4G: 1024K > > > > + * 8G: 1448K > > > > + * 16G: 2048K > > > > + */ > > > > + ra_pages = int_sqrt(totalram_pages/16); > > > > + if (ra_pages > (2 << (20 - PAGE_SHIFT))) > > > > + ra_pages = 2 << (20 - PAGE_SHIFT); > > > > + > > > > + return 0; > > > > +} > > > > > > How did you come up with these numbers? > > > > Well, most other places in the kernel where we scale by memory size we > > use the a sqrt curve, and the specific scale was the result of some > > fiddling, these numbers looked sane to me, nothing special. > > > > Would you suggest a different set, and if so, do you have any rationale > > for them? > > I just wish you had a rationale behind them, I don't think it's that > great of a series. Well, I was quite ignorant of the issues you just pointed out. Thanks those do indeed provide basis for a more solid set. > I agree with the low point of 128k. Perhaps that should be enforced then, because currently a system with <64M will get less. > Then it'd be sane > to try and determine what the upper limit of ra window size goodness is, > which is probably impossible since it depends on the hardware a lot. But > lets just say the upper value is 2mb, then I think it's pretty silly > _not_ to use 2mb on a 1g machine for instance. So more aggressive > scaling. Right, I was being a little conservative here. > Then there's the relationship between nr of requests and ra size. When > you leave everything up to a simple sqrt of total_ram type thing, then > you are sure to hit stupid values that cause a queue size of a number of > full requests, plus a small one at the end. Clearly not optimal! And this is where Wu's point of power of two series comes into play, right? So something like: roundup_pow_of_two(int_sqrt((totalram_pages << (PAGE_SHIFT-10)))) memory in MB RA window in KB 64 128 128 256 256 256 512 512 1024 512 2048 1024 4096 1024 8192 2048 16384 2048 32768 4096 65536 4096