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

STABILITY

       Unstable - in development.

AUTHOR

       Brendan Gregg

SEE ALSO

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