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

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Message-ID: <465C2A11.5070208@tmr.com>
Date:	Tue, 29 May 2007 09:26:41 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Satyam Sharma <satyam.sharma@...il.com>
CC:	Linux Kernel mailing List <linux-kernel@...r.kernel.org>
Subject: Re: What causes iowait other than waiting for i/o?

Satyam Sharma wrote:
> 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 ...

I wouldn't expect /proc/stat and similar to be broken in that way, but 
If no one has a better idea I guess I will assume there's a check needed 
of where time is added to iowait. I was hoping to avoid a full kernel 
search. Never thought of /proc data as a user space tool, but I guess.

-- 
Bill Davidsen <davidsen@....com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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