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    SubjectRe: [patch] x86, perf_counter, bts: add bts to perf_counter

    * Ingo Molnar <mingo@elte.hu> wrote:

    > btw., the number of samples seems to be varying too widely:
    >
    > titan:~> for ((i=0;i<10;i++)); do perf record -f -e branches:u -c 1 true 2>/dev/null; perf report | head -1; done
    > # Samples: 28784
    > # Samples: 24063
    > # Samples: 22788
    > # Samples: 30449
    > # Samples: 15335
    > # Samples: 30557
    > # Samples: 24010
    > # Samples: 23866
    > # Samples: 24877
    > # Samples: 24330
    >
    > compared to the branch-stat itself:
    >
    > titan:~> perf stat -v --repeat 10 -e branches:u true
    > [ perf stat: executing run #1 ... ]
    > [ perf stat: executing run #2 ... ]
    > [ perf stat: executing run #3 ... ]
    > [ perf stat: executing run #4 ... ]
    > [ perf stat: executing run #5 ... ]
    > [ perf stat: executing run #6 ... ]
    > [ perf stat: executing run #7 ... ]
    > [ perf stat: executing run #8 ... ]
    > [ perf stat: executing run #9 ... ]
    > [ perf stat: executing run #10 ... ]
    >
    > Performance counter stats for 'true' (10 runs):
    >
    > 23851 branches ( +- 0.000% )
    >
    > 0.000639653 seconds time elapsed ( +- 2.474% )
    >
    > do we lose records in the recording?

    i doubt it's lost records. Even with SCHED_FIFO sampling and with a
    huge, 512 MB mmap ring-buffer we get a BTS sample count variation in
    the +- 10% range:

    titan:/home/mingo> for ((i=0;i<10;i++)); do perf record -r 1 -m
    131072 -f -e branches:u -c 1 true 2>/dev/null; perf report | head
    -1; done
    # Samples: 24860
    # Samples: 24177
    # Samples: 26165
    # Samples: 25682
    # Samples: 29175
    # Samples: 23136
    # Samples: 27102
    # Samples: 29888
    # Samples: 25524
    # Samples: 24266

    (and these are all just user-mode executions)

    Ingo


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