linux-kernel - Re: Why is deleting (or reading) files not counted as IO-Wait in top?

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Message-Id: <20080105013115.217a5ab1.akpm@linux-foundation.org>
Date:	Sat, 5 Jan 2008 01:31:15 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matthias Schniedermeyer <ms@...d.de>
Cc:	linux-kernel@...r.kernel.org, David Chinner <dgc@....com>,
	Timothy Shimmin <tes@....com>
Subject: Re: Why is deleting (or reading) files not counted as IO-Wait in
 top?

On Wed, 2 Jan 2008 20:35:03 +0100 Matthias Schniedermeyer <ms@...d.de> wrote:

> Hi
> 
> 
> Currently i'm deleting about 500.000 files on a XFS-filesystem which 
> takes a few minutes, as i had a top open i saw that 'wa' is shown as 
> 0.0% (Nothing else running currently) and everything except 'id' is near 
> the bottom too. Kernel is 2.6.23.11.
> 
> So, as 'rm -rf' is essentially a IO (or seek, to be more correct)-bound 
> task, shouldn't that count as "Waiting for IO"?
> 
> The man-page of top says:
> 'Amount of time the CPU has been waiting for I/O to complete.'
> 
> But AFAICT wa only seams to be (ac)counted for writing and not for 
> reading. I come to that conclusion because, when i fire 'sync' i can see 
> some percent wa for a few seconds.
> 

Yes, you would absolutely expect `rm' to be stuck in D state and
contributing to both load average and io-wait in this situation.

I'd think that either XFS is playing games (and it'd take some pretty
inventive games to do this) or your observations are in error.
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