From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754629AbXG1QdY (ORCPT ); Sat, 28 Jul 2007 12:33:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752186AbXG1QdQ (ORCPT ); Sat, 28 Jul 2007 12:33:16 -0400 Received: from shawidc-mo1.cg.shawcable.net ([24.71.223.10]:56834 "EHLO pd3mo2so.prod.shaw.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752480AbXG1QdP (ORCPT ); Sat, 28 Jul 2007 12:33:15 -0400 Date: Sat, 28 Jul 2007 10:32:47 -0600 From: Robert Hancock Subject: Re: How can we make page replacement smarter (was: swap-prefetch) In-reply-to: To: Al Boldi Cc: Chris Snook , linux-kernel@vger.kernel.org, linux-mm@kvack.org Message-id: <46AB6FAF.5030306@shaw.ca> MIME-version: 1.0 Content-type: text/plain; charset=ISO-8859-1; format=flowed Content-transfer-encoding: 7bit References: User-Agent: Thunderbird 2.0.0.5 (Windows/20070716) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Al Boldi wrote: > Chris Snook wrote: >> Al Boldi wrote: >>> Because it is hard to quantify the expected swap-in speed for random >>> pages, let's first tackle the swap-in of consecutive pages, which should >>> be at least as fast as swap-out. So again, why is swap-in so slow? >> If I'm writing 20 pages to swap, I can find a suitable chunk of swap and >> write them all in one place. If I'm reading 20 pages from swap, they >> could be anywhere. Also, writes get buffered at one or more layers of >> hardware. > > Ok, this explains swap-in of random pages. Makes sense, but it doesn't > explain the awful tmpfs performance degradation of consecutive read-in runs > from swap, which should have at least stayed constant > >> At best, reads can be read-ahead and cached, which is why >> sequential swap-in sucks less. On-demand reads are as expensive as I/O >> can get. > > Which means that it should be at least as fast as swap-out, even faster > because write to disk is usually slower than read on modern disks. But > linux currently shows a distinct 2x slowdown for sequential swap-in wrt > swap-out. And to prove this point, just try suspend to disk where you can > see sequential swap-out being reported at about twice the speed of > sequential swap-in on resume. Why is that? Depends if swap-in is doing any read-ahead. If it's reading one page at a time in from the disk then the performance will definitely suck because of all the overhead from the tiny I/O's. With random swap-in you then pay the horrible seek penalty for all the reads as well. -- Robert Hancock Saskatoon, SK, Canada To email, remove "nospam" from hancockr@nospamshaw.ca Home Page: http://www.roberthancock.com/