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

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Date:	Tue, 2 Feb 2010 11:14:00 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Olivier Galibert <galibert@...ox.com>
cc:	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, Olivier Galibert wrote:
>
> On Tue, Feb 02, 2010 at 10:40:41AM -0800, Linus Torvalds wrote:
> > IOW, if you start off with a SEEK_END, I think it's reasonable to expect 
> > it to _not_ read the whole thing.
> 
> I've seen a lot of:
>   int fd = open(...);
>   size = lseek(fd, 0, SEEK_END);
>   lseek(fd, 0, SEEK_SET);
> 
>   data = malloc(size);
>   read(fd, data, size);
>   close(fd);
> 
> Why not fstat?  I don't know.

Well, the above will work perfectly with or without the patch, since it 
does the read of the full size. There is no read-ahead hint necessary for 
that kind of single read behavior.

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.

			Linus
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