linux-kernel - Very high IOWait during all disk activity

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Message-ID: <74d8fc520802290616w36a245e5qa851ad2f0b068472@mail.gmail.com>
Date:	Fri, 29 Feb 2008 14:16:48 +0000
From:	"Gordon Mckeown" <ml_lkml@...fluffyone.net>
To:	linux-kernel@...r.kernel.org
Subject: Very high IOWait during all disk activity

(Please cc me on replies)

I recently noticed on a number of my Linux boxes that during disk
activity, CPU usage was consistently hitting 100%. A little digging
showed that the CPU was spending up to around 65% of its time in an
IOWait state. Checked this with kernels 2.6.22 and 2.6.25-rc3, and
also across SATA and PATA drives on three different machines, all with
the same results. I also checked back with an old Ubuntu 6.06 Live CD
and that also exhibits the problem.

Having done some digging on the net, I can't get a definitive answer
as to whether this is considered "normal". Some people suggest that
IOWait is informational and doesn't indicate a problem, but based on
my admittedly limited understanding of such things, the CPU shouldn't
need to spend much time on disk I/O these days due to the use of DMA.

Is it expected behaviour for the CPU to spend such a large amount of
time in the IOWait state during disk I/O?

(For anyone who wants to see a more detailed analysis, I have an open
bug on Ubuntu's Launchpad: https://bugs.launchpad.net/bugs/192353).

Cheers,

Gordon.
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