linux-kernel - Re: What causes iowait other than waiting for i/o?

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Message-ID: <a781481a0705282328u54a14143jc60194fcf7b5aa23@mail.gmail.com>
Date:	Tue, 29 May 2007 11:58:54 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Bill Davidsen" <davidsen@....com>
Cc:	"Linux Kernel mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: What causes iowait other than waiting for i/o?

Hi Bill,

On 5/29/07, Bill Davidsen <davidsen@....com> wrote:
> I recently noted that my system was spending a lot of time in i/o wait
> when doing some tasks which I thought didn't involve i/o, as noted by
> the lack of disk light activity most of the time. I thought of network,
> certainly the NIC had no activity for this job. So I set up a little
> loop to capture all disk i/o and network activity (including loopback).
> That was no obvious help, and the program doesn't use pipes.
>
> At this point I'm really curious, does someone have a good clue?
>
> Note: I don't think this is a bug or performance issue, unless the
> kernel is doing something and charging time to iowait instead of system
> I don't see anything to fix, but I would like to understand.

What tool / kernel instrumentation / mechanism are you using to
determine that some task(s) are indeed blocked waiting for i/o? Perhaps
some userspace process accounting tools could be "broken" in the sense
that they generalize all uninterruptible sleep as waiting for i/o ...

Satyam
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