From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760506AbXGVQoJ (ORCPT ); Sun, 22 Jul 2007 12:44:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755264AbXGVQn5 (ORCPT ); Sun, 22 Jul 2007 12:43:57 -0400 Received: from brick.kernel.dk ([80.160.20.94]:8161 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755232AbXGVQn4 (ORCPT ); Sun, 22 Jul 2007 12:43:56 -0400 Date: Sun, 22 Jul 2007 18:44:03 +0200 From: Jens Axboe To: Peter Zijlstra Cc: linux-kernel , Fengguang Wu , riel , Andrew Morton , Rusty Russell , Tim Pepper , Chris Snook Subject: Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size Message-ID: <20070722164403.GU11657@kernel.dk> References: <20070721210005.000228000@chello.nl> <20070721210052.497469000@chello.nl> <20070722082434.GR11657@kernel.dk> <1185093405.20032.205.camel@twins> <20070722085041.GS11657@kernel.dk> <1185095852.20032.229.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1185095852.20032.229.camel@twins> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 22 2007, Peter Zijlstra wrote: > 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. I think it should remain the low point. > > 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 Only if you assume that max request size is always a power of 2. That's usually true, but there are cases where it's 124kb for instance. And there's still an issue when max_sectors isn't the deciding factor, if we end up having to stop merging on a request because we hit other limitations. So there's definitely room for improvement! Even today, btw, it's not all because of these changes. -- Jens Axboe