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Operating Systems Linux how to trace iowait to a certain process Post 302234401 by jim mcnamara on Tuesday 9th of September 2008 03:07:08 PM
Old 09-09-2008
iowait means the process is wating to complete an I/O and another process is ahead of it, or the I/O takes a while to complete. It is symptomatic of either poor programming or insufficient memory.

If you happen have RH running a P4 processor there is a known problem with iowait under certain confgurations. search their website for iowait.

This can usually be fixed for RH by updating to the latest kernel and/or putting the following lines into your /etc/rc.d/rc.local file:

echo 100 > /proc/sys/vm/inactive_clean_percent
echo 2 10 20 > /proc/sys/vm/pagecache

DO NOT DO THIS until you are certain what is going on.
 

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TRACE-CMD-START(1)														TRACE-CMD-START(1)

NAME
trace-cmd-start - start the Ftrace Linux kernel tracer without recording SYNOPSIS
trace-cmd start [OPTIONS] DESCRIPTION
The trace-cmd(1) start enables all the Ftrace tracing the same way trace-cmd-record(1) does. The difference is that it does not run threads to create a trace.dat file. This is useful just to enable Ftrace and you are only interested in the trace after some event has occurred and the trace is stopped. Then the trace can be read straight from the Ftrace pseudo file system or can be extracted with trace-cmd-extract(1). OPTIONS
The options are the same as trace-cmd-record(1), except that it does not take options specific to recording (-s, -o, -F, -N, and -t). SEE ALSO
trace-cmd(1), trace-cmd-record(1), trace-cmd-report(1), trace-cmd-stop(1), trace-cmd-extract(1), trace-cmd-reset(1), trace-cmd-split(1), trace-cmd-list(1), trace-cmd-listen(1) AUTHOR
Written by Steven Rostedt, <rostedt@goodmis.org[1]> RESOURCES
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/trace-cmd.git COPYING
Copyright (C) 2010 Red Hat, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL). NOTES
1. rostedt@goodmis.org mailto:rostedt@goodmis.org 06/11/2014 TRACE-CMD-START(1)
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