linux-kernel - Re: [PATCH] fadvise: perform WILLNEED readahead in a workqueue

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Date:	Fri, 22 Feb 2013 21:13:08 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Phillip Susi <psusi@...ntu.com>
Cc:	Dave Chinner <david@...morbit.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fadvise: perform WILLNEED readahead in a workqueue

Phillip Susi <psusi@...ntu.com> wrote:
> > On Sat, Dec 15, 2012 at 12:54:48AM +0000, Eric Wong wrote:
> >> "strace -T" timing on an uncached, one gigabyte file:
> >> 
> >> Before: fadvise64(3, 0, 0, POSIX_FADV_WILLNEED) = 0 <2.484832> 
> >> After: fadvise64(3, 0, 0, POSIX_FADV_WILLNEED) = 0 <0.000061>
> 
> It shouldn't take 2 seconds to queue up some async reads.  Are you
> using ext3?  The blocks have to be mapped in order to queue the reads,
> and without ext4 extents, this means the indirect blocks have to be
> read and can cause fadvise to block.

You're right, I originally tested on ext3.

I just tested an unpatched 3.7.9 kernel with ext4 and is much faster
(~250ms).  I consider ~250ms acceptable for my needs.  Will migrate
the rest of my setup to ext4 soon, thanks for the tip!
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