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

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Message-ID: <465C8684.2030600@tmr.com>
Date:	Tue, 29 May 2007 16:01:08 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Rik van Riel <riel@...hat.com>
CC:	Linux Kernel mailing List <linux-kernel@...r.kernel.org>
Subject: Re: What causes iowait other than waiting for i/o?

Rik van Riel wrote:
> Bill Davidsen 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.
>
> All filesystem IO and direct disk IO can cause iowait.
>
> This includes NFS activity.
>
If I didn't note it before, I'm read the the data from /proc, cpustats, 
net/dev, and diskstats. I assume that all i/o would show up in one of 
those places. NFS isn't involved, although this machine is a fileserver 
as a side job the modules weren't even loaded during testing.

A puzzlement for future consideration. If I get a chance later this week 
I'll make a pretty graphic of all the stuff going on when the iowait 
spiked, ctx rate, inq rate, hell the last time I even grabbed the CPU 
temp to see if it told me anything (didn't, thermal throttling NOT).

Thanks for the feedback, I think that lets out the obvious stuff.

-- 
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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