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.TH offcputime 8 "2016-01-14" "USER COMMANDS"
.SH NAME
offcputime \- Summarize off-CPU time by kernel stack trace. Uses Linux eBPF/bcc.
.SH SYNOPSIS
.B offcputime [\-h] [\-u] [\-p PID] [\-v] [\-f] [duration]
.SH DESCRIPTION
This program shows kernel stack traces and task names that were blocked and
"off-CPU", and the total duration they were not running: their "off-CPU time".
It works by tracing when threads block and when they return to CPU, measuring
both the time they were off-CPU and the blocked kernel stack trace and the
task name. This data is summarized in the kernel using an eBPF map, and by
summing the off-CPU time by unique stack trace and task name.
The output summary will help you identify reasons why threads
were blocking, and quantify the time they were off-CPU. This spans all types
of blocking activity: disk I/O, network I/O, locks, page faults, 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 off-CPU, and the output,
especially the -f format, can be used to generate an "off-CPU time flame graph".
See http://www.brendangregg.com/FlameGraphs/offcpuflamegraphs.html
The stack depth is currently limited to 20, and the stack traces are kernel
mode only. Check for newer versions where either may 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
Print output in folded stack format.
.TP
\-u
Only trace user threads (not kernel threads).
.TP
\-v
Show raw addresses (for non-folded output).
.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 stack trace and total off-CPU time:
#
.B offcputime
.TP
Trace for 5 seconds only:
#
.B offcputime 5
.TP
Trace for 5 seconds, and emit output in folded stack format (suitable for flame graphs):
#
.B offcputime -f 5
.TP
Trace PID 185 only:
#
.B offcputime -p 185
.SH OVERHEAD
This summarizes unique stack traces 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
stackcount(8)