linux-kernel - Re: [PATCH 10/11] readahead: dont do start-of-file readahead after lseek()

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Message-ID: <alpine.DEB.2.00.1002021157280.3707@asgard.lang.hm>
Date:	Tue, 2 Feb 2010 11:59:03 -0800 (PST)
From:	david@...g.hm
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Olivier Galibert <galibert@...ox.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/11] readahead: dont do start-of-file readahead after
 lseek()

On Tue, 2 Feb 2010, Linus Torvalds wrote:

> Rememebr: read-ahead is about filling the empty IO spaces _between_ reads,
> and turning many smaller reads into one bigger one. If you only have a
> single big read, read-ahead cannot help.
>
> Also, keep in mind that read-ahead is not always a win. It can be a huge
> loss too. Which is why we have _heuristics_. They fundamentally cannot
> catch every case, but what they aim for is to do a good job on average.

as a note from the field, I just had an application that needed to be 
changed because it did excessive read-ahead. it turned a 2 min reporting 
run into a 20 min reporting run because for this report the access was 
really random and the app forced large read-ahead.

David Lang
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