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

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Message-Id: <200801030216.44446.maximlevitsky@gmail.com>
Date:	Thu, 3 Jan 2008 02:16:44 +0200
From:	Maxim Levitsky <maximlevitsky@...il.com>
To:	Matthias Schniedermeyer <ms@...d.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Why is deleting (or reading) files not counted as IO-Wait in top?

On Wednesday, 2 January 2008 21:35:03 Matthias Schniedermeyer 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.
> 
> 
> 
> 
> Bis denn
> 

The IOWAIT time is the IDLE time that was spent waiting
for I/O. (meaning that there were no tasks running, but some were waiting on I/O)

Thus if you have another task that is not I/O bound, it can run in that time,
and ideally, you shouldn't notice any I/O slowdown, but the iowait time will decrease.

It wasn't the case before CFS introduction. I did few tests that showed almost 50% slowdown 
when running another task in that iowait time.
It is not longer a problem with CFS.

Regards,
	Maxim Levitsky
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