From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx139.postini.com [74.125.245.139]) by kanga.kvack.org (Postfix) with SMTP id 0E24C6B0002 for ; Thu, 23 May 2013 05:52:29 -0400 (EDT) Date: Thu, 23 May 2013 10:52:15 +0100 From: Mel Gorman Subject: Re: [PATCHv11 2/4] zbud: add to mm/ Message-ID: <20130523095215.GV11497@suse.de> References: <1368448803-2089-1-git-send-email-sjenning@linux.vnet.ibm.com> <1368448803-2089-3-git-send-email-sjenning@linux.vnet.ibm.com> <20130517154837.GN11497@suse.de> <20130519205219.GA3252@cerebellum> <20130520135439.GR11497@suse.de> <20130520154225.GA25536@cerebellum> <20130521081020.GT11497@suse.de> <519D7827.80607@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <519D7827.80607@oracle.com> Sender: owner-linux-mm@kvack.org List-ID: To: Bob Liu Cc: Seth Jennings , Andrew Morton , Greg Kroah-Hartman , Nitin Gupta , Minchan Kim , Konrad Rzeszutek Wilk , Dan Magenheimer , Robert Jennings , Jenifer Hopper , Johannes Weiner , Rik van Riel , Larry Woodman , Benjamin Herrenschmidt , Dave Hansen , Joe Perches , Joonsoo Kim , Cody P Schafer , Hugh Dickens , Paul Mackerras , linux-mm@kvack.org, linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org On Thu, May 23, 2013 at 10:00:07AM +0800, Bob Liu wrote: > Hi Mel & Seth, > > On 05/21/2013 04:10 PM, Mel Gorman wrote: > > On Mon, May 20, 2013 at 10:42:25AM -0500, Seth Jennings wrote: > >> On Mon, May 20, 2013 at 02:54:39PM +0100, Mel Gorman wrote: > >>> On Sun, May 19, 2013 at 03:52:19PM -0500, Seth Jennings wrote: > >>>> My first guess is that the external fragmentation situation you are referring to > >>>> is a workload in which all pages compress to greater than half a page. If so, > >>>> then it doesn't matter what NCHUCNKS_ORDER is, there won't be any pages the > >>>> compress enough to fit in the < PAGE_SIZE/2 free space that remains in the > >>>> unbuddied zbud pages. > >>>> > >>> > >>> There are numerous aspects to this, too many to write them all down. > >>> Modelling the external fragmentation one and how it affects swap IO > >>> would be a complete pain in the ass so lets consider the following > >>> example instead as it's a bit clearer. > >>> > >>> Three processes. Process A compresses by 75%, Process B compresses to 15%, > >>> Process C pages compress to 15%. They are all adding to zswap in lockstep. > >>> Lets say that zswap can hold 100 physical pages. > >>> > >>> NCHUNKS == 2 > >>> All Process A pages get rejected. > >> > >> Ah, I think this is our disconnect. Process A pages will not be rejected. > >> They will be stored in a zbud page, and that zbud page will be added > >> to the 0th unbuddied list. This list maintains a list of zbud pages > >> that will never be buddied because there are no free chunks. > >> > > > > D'oh, good point. Unfortunately, the problem then still exists at the > > writeback end which I didn't bring up in the previous mail. > > What's your opinion if we write back the whole compressed page to swap disk? > I'm not sure how to answer that sensibly. If the compressed page is written to swap, in my opinion then there will be IO :/ . It will be a maximum of two pages of IO with zbud (or zpair or whatever) as currently implemented. With zsmalloc it potentially was more. -- Mel Gorman SUSE Labs -- 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