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.
20.1. The purpose of perf stat
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
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.
Count the events.
perf statcommand 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 statruns 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 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
perf statoperates in per-thread mode. To change to CPU-wide event counting, pass the
perf stat. To count CPU-wide events, you need root access:
# perf stat -a ls
20.3. Interpretation of perf stat output
perf stat executes a specified command and counts event occurrences during the commands execution and displays statistics of these counts in three columns:
- 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, when running in default mode,
perf statcounts 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
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.
perf statto a running process:
$ perf stat -p ID1,ID2 sleep seconds
The previous example counts events in the processes with the IDs of
ID2for a time period of
secondsseconds as dictated by using the