NAME¶

runqlat - Run queue (scheduler) latency as a histogram.

SYNOPSIS¶

runqlat [-h] [-T] [-m] [-P] [--pidnss] [-L] [-p PID] [interval] [count]

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 ttwu_do_wakeup(), wake_up_new_task() -> finish_task_switch() with either raw tracepoints (if supported) or kprobes 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.

REQUIREMENTS¶

CONFIG_BPF and bcc.

OPTIONS¶

-h

Print usage message.

-T

Include timestamps on output.

-m

Output histogram in milliseconds.

-P

Print a histogram for each PID.

--pidnss

Print a histogram for each PID namespace (short for PID namespaces). For container analysis.

-L

Print a histogram for each thread ID.

-p PID

Only show this PID (filtered in kernel for efficiency).

interval

Output interval, in seconds.

count

Number of outputs.

EXAMPLES¶

Summarize run queue latency as a histogram:

# runqlat

Print 1 second summaries, 10 times:

# runqlat 1 10

Print 1 second summaries, using milliseconds as units for the histogram, and include timestamps on output:

# runqlat -mT 1

Trace PID 186 only, 1 second summaries:

# runqlat -P 185 1

FIELDS¶

usecs

Microsecond range

msecs

Millisecond range

count

How many times a task event fell into this range

distribution

An ASCII bar chart to visualize the distribution (count column)

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.

SOURCE¶

This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS¶

Linux

AUTHOR¶

Brendan Gregg

SEE ALSO¶

runqlen(8), runqslower(8), pidstat(1)