Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756397Ab3ETPmi (ORCPT ); Mon, 20 May 2013 11:42:38 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:50303 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752024Ab3ETPmg (ORCPT ); Mon, 20 May 2013 11:42:36 -0400 Date: Mon, 20 May 2013 10:42:25 -0500 From: Seth Jennings To: Mel Gorman Cc: 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 Subject: Re: [PATCHv11 2/4] zbud: add to mm/ Message-ID: <20130520154225.GA25536@cerebellum> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130520135439.GR11497@suse.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13052015-5806-0000-0000-00002139C946 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1614 Lines: 36 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. In other words, changing NCHUNKS has no effect on the acceptable size of allocations. Seth -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/