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Chapter 20. Counting events during process execution with perf stat

You can use the perf stat command to count hardware and software events during process execution.

Prerequisites

  • You have the perf user space tool installed as described in Installing perf.

20.1. The purpose of perf stat
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The perf stat command executes a specified command, keeps a running count of hardware and software event occurrences during the commands execution, and generates statistics of these counts. If you do not specify any events, then perf stat counts a set of common hardware and software events.

20.2. Counting events with perf stat
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You can use perf stat to count hardware and software event occurrences during command execution and generate statistics of these counts. By default, perf stat operates in per-thread mode.

Prerequisites

  • You have the perf user space tool installed as described in Installing perf.

Procedure

  • Count the events.

    • Running the perf stat command without root access will only count events occurring in the user space: 

      $ perf stat ls
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      Example 20.1. Output of perf stat ran without root access

      Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

 Performance counter stats for 'ls':

              1.28 msec task-clock:u               #    0.165 CPUs utilized
                 0      context-switches:u         #    0.000 M/sec
                 0      cpu-migrations:u           #    0.000 K/sec
               104      page-faults:u              #    0.081 M/sec
         1,054,302      cycles:u                   #    0.823 GHz
         1,136,989      instructions:u             #    1.08  insn per cycle
           228,531      branches:u                 #  178.447 M/sec
            11,331      branch-misses:u            #    4.96% of all branches

       0.007754312 seconds time elapsed

       0.000000000 seconds user
       0.007717000 seconds sys
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      As you can see in the previous example, when perf stat runs without root access the event names are followed by :u, indicating that these events were counted only in the user-space.

    • To count both user-space and kernel-space events, you must have root access when running perf stat: 

      # perf stat ls
      Copy to Clipboard Toggle word wrap

      Example 20.2. Output of perf stat ran with root access

      Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

 Performance counter stats for 'ls':

              3.09 msec task-clock                #    0.119 CPUs utilized
                18      context-switches          #    0.006 M/sec
                 3      cpu-migrations            #    0.969 K/sec
               108      page-faults               #    0.035 M/sec
         6,576,004      cycles                    #    2.125 GHz
         5,694,223      instructions              #    0.87  insn per cycle
         1,092,372      branches                  #  352.960 M/sec
            31,515      branch-misses             #    2.89% of all branches

       0.026020043 seconds time elapsed

       0.000000000 seconds user
       0.014061000 seconds sys
      Copy to Clipboard Toggle word wrap

      • By default, perf stat operates in per-thread mode. To change to CPU-wide event counting, pass the -a option to perf stat. To count CPU-wide events, you need root access: 

        # perf stat -a ls
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20.3. Interpretation of perf stat output
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perf stat executes a specified command and counts event occurrences during the commands execution and displays statistics of these counts in three columns:

  1. The number of occurrences counted for a given event
  2. The name of the event that was counted

  3. When related metrics are available, a ratio or percentage is displayed after the hash sign (#) in the right-most column.

    For example, when running in default mode, perf stat counts both cycles and instructions and, therefore, calculates and displays instructions per cycle in the right-most column. You can see similar behavior with regard to branch-misses as a percent of all branches since both events are counted by default.

20.4. Attaching perf stat to a running process
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You can attach perf stat to a running process. This will instruct perf stat to count event occurrences only in the specified processes during the execution of a command.

Prerequisites

  • You have the perf user space tool installed as described in Installing perf.

Procedure

  • Attach perf stat to a running process: 

    $ perf stat -p ID1,ID2 sleep seconds
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    The previous example counts events in the processes with the IDs of ID1 and ID2 for a time period of seconds seconds as dictated by using the sleep command.

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