full-disclosure - Re: "IO wait chains" in Linux??

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Message-ID: <27336.1297096094@localhost>
Date: Mon, 07 Feb 2011 11:28:14 -0500
From: Valdis.Kletnieks@...edu
To: "Cal Leeming [Simplicity Media Ltd]" <cal.leeming@...plicitymedialtd.co.uk>
Cc: full-disclosure@...ts.grok.org.uk
Subject: Re: "IO wait chains" in Linux??

On Mon, 07 Feb 2011 06:41:53 GMT, "Cal Leeming [Simplicity Media Ltd]" said:

> Is anyone aware of a Linux based CLI equivalent, which will show the
> processes stuck in IO wait, in a tree format?

ps ax | grep ' [D] '   gives a pretty good approximation of "currently in I/O wait".
But remember that each process (or actually, each thread within a process)
can individually be stuck in I/O wait, so it's unclear what the "tree format"
would consist of, exactly.  If you have a process that has parent, siblings,
and children, what else would show up in the tree if it's in an I/O wait?

There's the slightly more difficult issue that if you're trying to do
system-level analysis, you're looking at really bad race conditions.  Processes
often go into and leave I/O wait status in literally milliseconds.  At best,
you can run through the process list several times and get a statistical view
of "these 4 processes are in I/O wait most of the time".  'pstree' mostly
avoids that issue because if the system is small enough that the pstree output
is still useful, the fork/exec rate is low enough that pstree can mostly ignore
it.  That's not true for I/O.

If you're trying to identify processes that are truly and literally *stuck* in
I/O wait due to a hardware or kernel error, you're probably better off enabling
the watchdog timer in the kernel and watching dmesg for it triggering.

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