From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1422721AbWJXWUI (ORCPT ); Tue, 24 Oct 2006 18:20:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1422719AbWJXWUG (ORCPT ); Tue, 24 Oct 2006 18:20:06 -0400 Received: from gprs189-60.eurotel.cz ([160.218.189.60]:20697 "EHLO amd.ucw.cz") by vger.kernel.org with ESMTP id S1422644AbWJXWUD (ORCPT ); Tue, 24 Oct 2006 18:20:03 -0400 Date: Wed, 25 Oct 2006 00:19:50 +0200 From: Pavel Machek To: Nigel Cunningham Cc: "Rafael J. Wysocki" , Andrew Morton , LKML Subject: Re: [PATCH] Use extents for recording what swap is allocated. Message-ID: <20061024221950.GB5851@elf.ucw.cz> References: <1161576857.3466.9.camel@nigel.suspend2.net> <200610242208.34426.rjw@sisk.pl> <20061024213402.GC5662@elf.ucw.cz> <1161728153.22729.22.camel@nigel.suspend2.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1161728153.22729.22.camel@nigel.suspend2.net> X-Warning: Reading this can be dangerous to your mental health. User-Agent: Mutt/1.5.11+cvs20060126 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi! > > > > Switch from bitmaps to using extents to record what swap is allocated; > > > > they make more efficient use of memory, particularly where the allocated > > > > storage is small and the swap space is large. > > > > > > As I said before, I like the overall idea, but I have a bunch of > > > comments. > > > > Okay, if Rafael likes it... lets take a look. > > > > First... what is the _worst case_ overhead? AFAICT extents are very > > good at the best case, but tend to suck for the worst case...? > > That's right. In using this, we're relying on the fact that the swap > allocator tries to act sensibly. I've only seen worse case performance > when a user had two swap devices with the same priority (striped), but > that was a bug. :) Ok, but if the allocator somehow manages to stripe between two swap devices, what happens? IIRC original code was something like .1% overhead (8bytes per 4K, or something?), bitmaps should be even better. If it is 1% in worst case, that's probably okay, but it would be bad if it had overhead bigger than 10times original code (worst case). Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html