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performancecounterperf

Perf: what do [<n percent>] records mean in perf stat output?

perf stat -e <events> <command> with many different events usually returns an output like this

   127.352.815.472 r53003c                                                      [23,76%]
    65.712.112.871 r53019c                                                      [23,81%]
   178.027.463.861 r53010e                                                      [23,88%]
   162.854.142.303 r5302c2                                                      [24,05%]
   ...

What do the percentage records mean?

Solution

  • The percentages show the percentage of time that the specific event was being measured in the case where perf has to multiplex events. Event multiplexing is explained in more detail on the perf wiki, and I've included a brief quote below:

    If there are more events than counters, the kernel uses time multiplexing (switch frequency = HZ, generally 100 or 1000) to give each event a chance to access the monitoring hardware. Multiplexing only applies to PMU events. With multiplexing, an event is not measured all the time. At the end of the run, the tool scales the count based on total time enabled vs time running.