Chapter 12. Counting events during process execution with perf stat

Prerequisites

• You have the perf user space tool installed as described in Installing perf.
• You have root access to count both user-space and kernel-space events.

Procedure

• Count the events.

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

    $ perf stat ls

    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

    In the previous example, 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, enter:

    # perf stat ls

    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

• The number of occurrences counted for a given event.
• The name of the event that was counted.
• When related metrics are available, a ratio or percentage is displayed after the hash sign (#) in the right-most column. For example, in default mode, perf stat counts cycles and instructions to calculate and display instructions per cycle in the right-most column. Branch misses as a percentage of total branches show similar behavior in default counts.

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.