Return-Path: <linux-kernel-owner+willy=40w.ods.org@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id <S267203AbTBJLgl>; Mon, 10 Feb 2003 06:36:41 -0500
Received: (majordomo@vger.kernel.org) by vger.kernel.org
	id <S264853AbTBJLgl>; Mon, 10 Feb 2003 06:36:41 -0500
Received: from dial-ctb04112.webone.com.au ([210.9.244.112]:17415 "EHLO
	chimp.local.net") by vger.kernel.org with ESMTP id <S267203AbTBJLgc>;
	Mon, 10 Feb 2003 06:36:32 -0500
Message-ID: <3E4790F7.2010208@cyberone.com.au>
Date: Mon, 10 Feb 2003 22:45:59 +1100
From: Nick Piggin <piggin@cyberone.com.au>
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: Andrea Arcangeli <andrea@suse.de>
CC: Hans Reiser <reiser@namesys.com>, Andrew Morton <akpm@digeo.com>,
       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: <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> <20030210101539.GS31401@dualathlon.random> <3E4781A2.8070608@cyberone.com.au> <20030210111017.GV31401@dualathlon.random> <3E478C09.6060508@cyberone.com.au> <20030210113923.GY31401@dualathlon.random>
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: 2721
Lines: 72

Andrea Arcangeli wrote:

>On Mon, Feb 10, 2003 at 10:24:57PM +1100, Nick Piggin wrote:
>
>>Andrea Arcangeli wrote:
>>
>>
>>>On Mon, Feb 10, 2003 at 09:40:34PM +1100, Nick Piggin wrote:
>>>
>>>
>>>>I don't know too much about SCSI stuff, but if driver / wire / device
>>>>overheads were that much higher at 128K compared to 512K I would
>>>>think something is broken or maybe optimised badly.
>>>>
>>>>
>>>I guess it's also a matter of the way the harddisk can serve the I/O if
>>>it sees it all at the same time, not only the cpu/bus protocol after all
>>>minor overhead.  Most certainly it's not a software mistake in linux
>>>that the big commands runs that much faster. Again go check the numbers
>>>in bigbox.html between my tree, 2.4 and 2.5 in bonnie read sequential,
>>>to see the difference between 128k commands and 512k commands with
>>>reads, these are facts.  (and no writes and no seeks here)
>>>
>>>
>>Yes it is very clear from the numbers that your tree is more than
>>150% the speed for reads. As I said I don't know too much about
>>
>
>correct, that's the huge improvement I get in the read sequential case
>(i.e. bonnie), which is a crucial common workload.
>
Yep

>
>
>>SCSI, but it is very interesting that writes don't get a noticable
>>improvement although they would be using the bigger request sizes
>>too, right? Something is causing this but the cpu, bus, wire
>>
>
>It's the readahead in my tree that allows the reads to use the max scsi
>command size. It has nothing to do with the max scsi command size
>itself.
>
>writes don't need readahead to use the max command size, they always
>used it since the first place, so they can't go even faster, they never
>had a problem.
>
Yes I am an idiot, I don't know why I said that :P

>
>
>It's by fixing readahead that reads gets boosted. this has nothing to do
>with writes or the max_sectors itself.
>
>You can wait 10 minutes and still such command can't grow.  This is why
>claiming anticipatory scheduling can decrease the need for readahead
>doesn't make much sense to me, there are important things you just can't
>achieve by only waiting.
>
Look at the few simple tests I have been posting - it clearly
indicates the need is decreased. I did say however that it would
be subject to CPU/bus/device overheads. I did not realiase
SCSI setups would behave like this. From a purely disk head
perspective it does nullify the need for readahead (though
it is obivously still needed for other reasons).

-
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/