Subject: Re: IO scheduler based IO controller V10
From: Mike Galbraith <efault@gmx.de>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <jens.axboe@oracle.com>,
       Ulrich Lukas <stellplatz-nr.13a@datenparkplatz.de>,
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       righi.andrea@gmail.com, m-ikeda@ds.jp.nec.com, agk@redhat.com,
       akpm@linux-foundation.org, peterz@infradead.org, jmarchan@redhat.com,
       torvalds@linux-foundation.org, mingo@elte.hu, riel@redhat.com
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Date: Mon, 28 Sep 2009 20:24:57 +0200
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On Mon, 2009-09-28 at 13:48 -0400, Vivek Goyal wrote:

> Hmm.., so close to 25% reduction on average in completion time of konsole.
> But this is in presece of writer. Does this help even in presence of 1 or
> more sequential readers going?

Dunno, I've only tested sequential writer.

> So here latency seems to be coming from three sources.
> 
> - Wait in CFQ before request is dispatched (only in case of competing seq readers).
> - seek latencies
> - latencies because of bigger requests are already dispatched to disk.
> 
> So limiting the size of request will help with third factor but not with first  
> two factors and here seek latencies seem to be the biggest contributor.

Yeah, seek latency seems to dominate.

	-Mike

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