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.TH runqlat 8 "2016-02-07" "USER COMMANDS"
.SH NAME
runqlat \- Run queue (scheduler) latency as a histogram.
.SH SYNOPSIS
.B runqlat [\-h] [\-T] [\-m] [\-P] [\-p PID] [interval] [count]
.SH DESCRIPTION
This measures the time a task spends waiting on a run queue (or equivalent
scheduler data structure) for a turn on-CPU, and shows this time as a
histogram. This time should be small, but a task may need to wait its turn due
to CPU load. The higher the CPU load, the longer a task will generally need to
wait its turn.
This tool measures two types of run queue latency:
1. The time from a task being enqueued on a run queue to its context switch
and execution. This traces enqueue_task_*() -> finish_task_switch(),
and instruments the run queue latency after a voluntary context switch.
2. The time from when a task was involuntary context switched and still
in the runnable state, to when it next executed. This is instrumented
from finish_task_switch() alone.
This tool uses in-kernel eBPF maps for storing timestamps and the histogram,
for efficiency. Despite this, the overhead of this tool may become significant
for some workloads: see the OVERHEAD section.
This works by tracing various kernel scheduler functions using dynamic tracing,
and will need updating to match any changes to these functions.
Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
.TP
\-h
Print usage message.
.TP
\-T
Include timestamps on output.
.TP
\-m
Output histogram in milliseconds.
.TP
\-P
Print a histogram for each PID.
.TP
\-p PID
Only show this PID (filtered in kernel for efficiency).
.TP
interval
Output interval, in seconds.
.TP
count
Number of outputs.
.SH EXAMPLES
.TP
Summarize run queue latency as a histogram:
#
.B runqlat
.TP
Print 1 second summaries, 10 times:
#
.B runqlat 1 10
.TP
Print 1 second summaries, using milliseconds as units for the histogram, and include timestamps on output:
#
.B runqlat \-mT 1
.TP
Trace PID 186 only, 1 second summaries:
#
.B runqlat -P 185 1
.SH FIELDS
.TP
usecs
Microsecond range
.TP
msecs
Millisecond range
.TP
count
How many times a task event fell into this range
.TP
distribution
An ASCII bar chart to visualize the distribution (count column)
.SH OVERHEAD
This traces scheduler functions, which can become very frequent. While eBPF
has very low overhead, and this tool uses in-kernel maps for efficiency, the
frequency of scheduler events for some workloads may be high enough that the
overhead of this tool becomes significant. Measure in a lab environment
to quantify the overhead before use.
.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
runqlen(8), pidstat(1)