Chapter 20. Counting events during process execution with perf stat

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

    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

    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

    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

    • 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

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

  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.