Chapter 22. Investigating busy CPUs with perf

Prerequisites

• You have the perf user space tool installed as described in Installing perf.

Procedure

• Count the events with CPU count aggregation disabled:

  # perf stat -a -A sleep seconds

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  The previous 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:

  # perf stat -a -A -e cycles sleep seconds

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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:

  # perf report --sort cpu

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  • You can sort by CPU and command to display more detailed information about where CPU time is being spent:

    # perf report --sort cpu,comm

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    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.

Prerequisites

• You have the perf user space tool installed as described in Installing perf.

Procedure

• Start the perf top interface while sorting by CPU:

  # perf top --sort cpu

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  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:

    # perf top --sort cpu,comm

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    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.

Prerequisites

• You have the perf user space tool installed as described in Installing perf.