From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754568Ab0CYLUK (ORCPT ); Thu, 25 Mar 2010 07:20:10 -0400 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:40815 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754329Ab0CYLUH (ORCPT ); Thu, 25 Mar 2010 07:20:07 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 From: KOSAKI Motohiro To: Mel Gorman Subject: Re: [PATCH 06/11] Export fragmentation index via /proc/extfrag_index Cc: kosaki.motohiro@jp.fujitsu.com, Andrew Morton , Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org In-Reply-To: <20100325084730.GG2024@csn.ul.ie> References: <20100325102342.945A.A69D9226@jp.fujitsu.com> <20100325084730.GG2024@csn.ul.ie> Message-Id: <20100325200919.6C8F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Mailer: Becky! ver. 2.50.07 [ja] Date: Thu, 25 Mar 2010 20:20:04 +0900 (JST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Thu, Mar 25, 2010 at 11:47:17AM +0900, KOSAKI Motohiro wrote: > > > On Tue, Mar 23, 2010 at 09:22:04AM +0900, KOSAKI Motohiro wrote: > > > > > > > + /* > > > > > > > + * Index is between 0 and 1 so return within 3 decimal places > > > > > > > + * > > > > > > > + * 0 => allocation would fail due to lack of memory > > > > > > > + * 1 => allocation would fail due to fragmentation > > > > > > > + */ > > > > > > > + return 1000 - ( (1000+(info->free_pages * 1000 / requested)) / info->free_blocks_total); > > > > > > > +} > > > > > > > > > > > > Dumb question. > > > > > > your paper (http://portal.acm.org/citation.cfm?id=1375634.1375641) says > > > > > > fragmentation_index = 1 - (TotalFree/SizeRequested)/BlocksFree > > > > > > but your code have extra '1000+'. Why? > > > > > > > > > > To get an approximation to three decimal places. > > > > > > > > Do you mean this is poor man's round up logic? > > > > > > Not exactly. > > > > > > The intention is to have a value of 968 instead of 0.968231. i.e. > > > instead of a value between 0 and 1, it'll be a value between 0 and 1000 > > > that matches the first three digits after the decimal place. > > > > Let's consider extream case. > > > > free_pages: 1 > > requested: 1 > > free_blocks_total: 1 > > > > frag_index = 1000 - ((1000 + 1*1000/1))/1 = -1000 > > > > This is not your intension, I guess. > > Why not? > > See this comment > > /* Fragmentation index only makes sense when a request would fail */ > > In your example, there is a free page of the requested size so the allocation > would succeed. In this case, fragmentation index does indeed go negative > but the value is not useful. > > > Probably we don't need any round_up/round_down logic. because fragmentation_index > > is only used "if (fragindex >= 0 && fragindex <= 500)" check in try_to_compact_pages(). > > +1 or -1 inaccurate can be ignored. iow, I think we can remove '1000+' expression. > > > > This isn't about rounding, it's about having a value that normally is > between 0 and 1 expressed as a number between 0 and 1000 because we > can't use double in the kernel. Sorry, My example was wrong. new example is here. free_pages: 4 requested: 2 free_blocks_total: 4 theory: 1 - (TotalFree/SizeRequested)/BlocksFree = 1 - (4/2)/4 = 0.5 code : 1000 - ((1000 + 4*1000/2))/4 = 1000 - (1000 + 2000)/4 = 1000/4 = 250 I don't think this is three decimal picking up code. This seems might makes lots compaction invocation rather than theory.