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

[Posted February 2, 2010 by corbet]

From:		Linus Torvalds <torvalds-AT-linux-foundation.org>
To:		david-AT-lang.hm
Subject:		Re: [PATCH 10/11] readahead: dont do start-of-file readahead after lseek()
Date:		Tue, 2 Feb 2010 12:22:09 -0800 (PST)
Cc:		Olivier Galibert <galibert-AT-pobox.com>, Wu Fengguang <fengguang.wu-AT-intel.com>, Andrew Morton <akpm-AT-linux-foundation.org>, Jens Axboe <jens.axboe-AT-oracle.com>, Peter Zijlstra <a.p.zijlstra-AT-chello.nl>, Linux Memory Management List <linux-mm-AT-kvack.org>, linux-fsdevel-AT-vger.kernel.org, LKML <linux-kernel-AT-vger.kernel.org>

On Tue, 2 Feb 2010, david@lang.hm wrote:

> On Tue, 2 Feb 2010, Linus Torvalds wrote:
> > 
> > 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.

Yeah. And the reason Wu did this patch is similar: something that _should_ 
have taken just quarter of a second took about 7 seconds, because 
read-ahead triggered on this really slow device that only feeds about 
15kB/s (yes, _kilo_byte, not megabyte).

You can always use POSIX_FADVISE_RANDOM to disable it, but it's seldom 
something that people do. And there are real loads that have random 
components to them without being _entirely_ random, so in an optimal world 
we should just have heuristics that work well.

The problem is, it's often easier to test/debug the "good" cases, ie the 
cases where we _want_ read-ahead to trigger. So that probably means that 
we have a tendency to read-ahead too aggressively, because those cases are 
the ones where people can most easily look at it and say "yeah, this 
improves throughput of a 'dd bs=8192'". 

So then when we find loads where read-ahead hurts, I think we need to take 
_that_ case very seriously. Because otherwise our selection bias for 
testing read-ahead will fail.

		Linus

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