wqlat(8)

NAME
wqlat - Summarize kernel workqueue latency as a histogram.

SYNOPSIS
wqlat [-h] [-T] [-N] [-W] [-w WQNAME] [interval [count]]

DESCRIPTION
wqlat traces work's waiting on workqueue, and records the distribution of work's queuing latency (time). This is printed as a histogram either on Ctrl-C, or after a given interval in seconds.

This tool uses in-kernel eBPF maps for storing timestamps and the histogram, for efficiency.

This tool uses the workqueue:workqueue_queue_work and workqueue:workqueue_execute_start kernel tracepoints, which is a stable tracing mechanism. Please note BPF programs can attach to tracepoints from Linux 4.7 only, so this tools can only support kernel 4.7 or later version.

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.

-N  Output histogram in nanoseconds.

-W  Print a histogram per workqueue.

-w WQNAME
    Trace this workqueue only

interval
    Output interval, in seconds.

count
    Number of outputs.

EXAMPLES

Summarize kernel workqueue latency as a histogram:
    # wqlat

Print 1 second summaries, 10 times:
    # wqlat 1 10

Print 1 second summaries, using nanoseconds as units for the histogram, and
    include timestamps on output: # wqlat -NT 1

Print 1 second summaries, 10 times per workqueue:
    # wqlat -W 1 10

Print 1 second summaries for workqueue nvmet_tcp_wq:
    # wqlat -w nvmet_tcp_wq 1

FIELDS

usecs
    Microsecond range

nsecs
    Nanosecond range

count
    How many works into this range

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

OVERHEAD
This traces kernel functions and maintains in-kernel timestamps and a histogram, which are asynchronously copied to user-space. This method is very efficient, and the overhead for most workqueue scheduling rates (< 100k) should be negligible.If you have a higher workqueue scheduling, please test and 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

STABILITY
Unstable - in development.

AUTHOR
Ping Gan

SEE ALSO
biolatency