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

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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 <[email protected]>
 "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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