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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
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      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
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      • 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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