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offwaketime.8
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offwaketime.8
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.TH offwaketime 8 "2016-01-30" "USER COMMANDS"
.SH NAME
offwaketime \- Summarize blocked time by kernel off-CPU stack + waker stack. Uses Linux eBPF/bcc.
.SH SYNOPSIS
.B offwaketime [\-h] [\-u] [\-p PID] [\-v] [\-f] [duration]
.SH DESCRIPTION
This program shows kernel stack traces and task names that were blocked and
"off-CPU", along with the stack traces and task names for the threads that woke
them, and the total elapsed time from when they blocked to when they were woken
up. This combines the summaries from both the offcputime and wakeuptime tools.
The time measurement will be very similar to off-CPU time, however, off-CPU time
may include a little extra time spent waiting on a run queue to be scheduled.
The combined stacks, task names, and total time is summarized in kernel context
for efficiency, using an eBPF map.
The output summary will further help you identify reasons why threads
were blocking, and quantify the time from when they were blocked to woken up.
This spans all types of blocking activity: disk I/O, network I/O, locks, page
faults, swapping, sleeping, involuntary context switches, etc.
This is complementary to CPU profiling (e.g., CPU flame graphs) which shows
the time spent on-CPU. This shows the time spent blocked off-CPU, and the
output, especially the -f format, can be used to generate an "off-wake time
flame graph".
See http://www.brendangregg.com/FlameGraphs/offcpuflamegraphs.html
The stack depth is currently limited to 20 for the off-CPU stack, and 10 for
the waker stack, and the stack traces are kernel mode only. Check for newer
versions where this should be improved.
This currently only works on x86_64. Check for future versions.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
.TP
\-h
Print usage message.
.TP
\-f
Output stacks in folded format.
.TP
\-u
Only trace user threads (not kernel threads).
.TP
\-v
Show raw addresses for non-folded mode.
.TP
\-p PID
Trace this process ID only (filtered in-kernel).
.TP
duration
Duration to trace, in seconds.
.SH EXAMPLES
.TP
Trace all thread blocking events, and summarize (in-kernel) by kernel off-CPU stack trace, waker stack traces, task names, and total blocked time:
#
.B offwaketime
.TP
Trace for 5 seconds only:
#
.B offwaketime 5
.TP
Trace for 5 seconds, and emit output in folded stack format (suitable for flame graphs), user-mode threads only:
#
.B offwaketime -fu 5
.TP
Trace PID 185 only:
#
.B offwaketime -p 185
.SH OVERHEAD
This summarizes unique stack trace pairs in-kernel for efficiency, allowing it
to trace a higher rate of events than methods that post-process in user space.
The stack trace and time data is only copied to user space once, when the output
is printed. While these techniques greatly lower overhead, scheduler events are
still a high frequency event, as they can exceed 1 million events per second,
and so caution should still be used. Test before production use.
If the overhead is still a problem, take a look at the MINBLOCK_US tunable in
the code. If your aim is to chase down longer blocking events, then this could
be increased to filter shorter blocking events, further lowering overhead.
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion _examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
offcputime(8), wakeuptime(8)