Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 10 Feb 2003 05:46:09 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 10 Feb 2003 05:46:09 -0500 Received: from dial-ctb04112.webone.com.au ([210.9.244.112]:56326 "EHLO chimp.local.net") by vger.kernel.org with ESMTP id ; Mon, 10 Feb 2003 05:46:08 -0500 Message-ID: <3E478528.6030009@cyberone.com.au> Date: Mon, 10 Feb 2003 21:55:36 +1100 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.1) Gecko/20020913 Debian/1.1-1 MIME-Version: 1.0 To: Andrew Morton CC: Hans Reiser , andrea@suse.de, jakob@unthought.net, david.lang@digitalinsight.com, riel@conectiva.com.br, ckolivas@yahoo.com.au, linux-kernel@vger.kernel.org, axboe@suse.de Subject: Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest] References:

<20030209203343.06608eb3.akpm@digeo.com> <20030210045107.GD1109@unthought.net> <3E473172.3060407@cyberone.com.au> <20030210073614.GJ31401@dualathlon.random> <3E47579A.4000700@cyberone.com.au> <20030210080858.GM31401@dualathlon.random> <20030210001921.3a0a5247.akpm@digeo.com> <20030210085649.GO31401@dualathlon.random> <20030210010937.57607249.akpm@digeo.com> <3E4779DD.7080402@namesys.com> <20030210024810.43a57910.akpm@digeo.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1945 Lines: 51 Andrew Morton wrote: >Hans Reiser wrote: > >>readahead seems to be less effective for non-sequential objects. Or at >>least, you don't get the order of magnitude benefit from doing only one >>seek, you only get the better elevator scheduling from having more >>things in the elevator, which isn't such a big gain. >> >>For the spectators of the thread, one of the things most of us learn >>from experience about readahead is that there has to be something else >>trying to interject seeks into your workload for readahead to make a big >>difference, otherwise a modern disk drive (meaning, not 30 or so years >>old) is going to optimize it for you anyway using its track cache. >> >> > >The biggest place where readahead helps is when there are several files being >read at the same time. Without readahead the disk will seek from one file >to the next after having performed a 4k I/O. With readahead, after performing >a (say) 128k I/O. It really can make a 32x difference. > >Readahead is brute force. The anticipatory scheduler solves this in a >smarter way. > >Yes, device readahead helps. But I believe the caches are only 4-way >segmented or thereabouts, so it is easy to thrash them. Let's test: > [snip results] > > >Still only 300%. I'd expect to see much larger differences on some older >SCSI disks I have here. > It would be interesting to see numbers with and without anticipatory scheduling as well. I have tried two streaming readers. Speed with default readahead 13MB/s Speed with no readahead, no anticipation 7MB/s Speed with no readahead, anticipation 11.8MB/s I suspect more readers will simply favour the anticipatory scheduler more. Nick - 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/