Chapter 20. Investigating busy CPUs with perf When investigating performance issues on a system, you can use the perf tool to identify and monitor the busiest CPUs in order to focus your efforts. 20.1. Displaying which CPU events were counted on with perf stat You can use perf stat to display which CPU events were counted on by disabling CPU count aggregation. You must count events in system-wide mode by using the -a flag in order to use this functionality. Prerequisites You have the perf user space tool installed as described in Installing perf . Procedure Count the events with CPU count aggregation disabled: The example displays counts of a default set of common hardware and software events recorded over a time period of seconds seconds, as dictated by using the sleep command, over each individual CPU in ascending order, starting with CPU0 . As such, it may be useful to specify an event such as cycles: 20.2. Displaying which CPU samples were taken on with perf report The perf record command samples performance data and stores this data in a perf.data file which can be read with the perf report command. The perf record command always records which CPU samples were taken on. You can configure perf report to display this information. Prerequisites You have the perf user space tool installed as described in Installing perf . There is a perf.data file created with perf record in the current directory. If the perf.data file was created with root access, you need to run perf report with root access too. Procedure Display the contents of the perf.data file for further analysis while sorting by CPU: You can sort by CPU and command to display more detailed information about where CPU time is being spent: This example will list commands from all monitored CPUs by total overhead in descending order of overhead usage and identify the CPU the command was executed on. Additional resources Recording and analyzing performance profiles with perf 20.3. Displaying specific CPUs during profiling with perf top You can configure perf top to display specific CPUs and their relative usage while profiling your system in real time. Prerequisites You have the perf user space tool installed as described in Installing perf . Procedure Start the perf top interface while sorting by CPU: This example will list CPUs and their respective overhead in descending order of overhead usage in real time. You can sort by CPU and command for more detailed information of where CPU time is being spent: This example will list commands by total overhead in descending order of overhead usage and identify the CPU the command was executed on in real time. 20.4. Monitoring specific CPUs with perf record and perf report You can configure perf record to only sample specific CPUs of interest and analyze the generated perf.data file with perf report for further analysis. Prerequisites You have the perf user space tool installed as described in Installing perf . Procedure Sample and record the performance data in the specific CPU's, generating a perf.data file: Using a comma separated list of CPUs: The example samples and records data in CPUs 0 and 1 for a period of seconds seconds as dictated by the use of the sleep command. Using a range of CPUs: The example samples and records data in all CPUs from CPU 0 to 2 for a period of seconds seconds as dictated by the use of the sleep command. Display the contents of the perf.data file for further analysis: This example will display the contents of perf.data . If you are monitoring several CPUs and want to know which CPU data was sampled on, see Displaying which CPU samples were taken on with perf report .

Commands:
perf stat -a -A sleep seconds
perf stat -a -A -e cycles sleep seconds
perf report --sort cpu
perf report --sort cpu,comm
perf top --sort cpu
perf top --sort cpu,comm
perf record -C 0,1 sleep seconds
perf record -C 0-2 sleep seconds
perf report