Re: --mmap-pages option seemingly has no effect to help with LOST samples

[David Ahern <dsahern@gmail.com>]

On 6/22/12 9:59 AM, Maynard Johnson wrote:
> The 'perf record' tool can easily handle a sampling rate of one sample per 100,000 cycles *or* instructions (i.e., one at a time), so I would have expected it to be handle one sample per 500,000 events when profiling on both events?  Am I missing something?
>
> Another related issue is the number of samples being recorded varies wildly when profiling on multiple events.  For example, profiling on just cycles with --count=500000, 'perf report -n' reports ~87k samples.  And profiling on just instructions with the same rate, I get ~102k.  When profiling with both events, I get cycles/instruction sample counts ranging from a low of 6k/7k to a high of 88k/102k.  Usually, I get counts around 12k/15k.  The higher the count seen with 'perf report' (i.e., the closer to true values), the more likely that perf record fails with the "LOST" samples message.


It might well be a RHEL6 problem.

Host OS is 3.4.0
perf version 3.5.rc1.87.gcb9dd4.dirty

Using Peter Z's 1billion instruction, first up is instructions sampling:
$ perf record -fo /tmp/perf.data -e instructions -c 100000 
./loop_1b_instructions

$ perf report --stdio -i /tmp/perf.data -D | tail -12
Aggregated stats:
            TOTAL events:      10161
             MMAP events:        106
             COMM events:          2
             EXIT events:          2
           SAMPLE events:      10051
instructions stats:
            TOTAL events:      10161
             MMAP events:        106
             COMM events:          2
             EXIT events:          2
           SAMPLE events:      10051

1billion instructions sampled every 100,000 = ~10,000 samples which 
corresponds to the above (ish).

And now looking at cycles:

$ perf record -fo /tmp/perf.data -e cycles -c 100000 ./loop_1b_instructions
$ perf report --stdio -i /tmp/perf.data -D | tail -12
Aggregated stats:
            TOTAL events:       7961
             MMAP events:        106
             COMM events:          2
             EXIT events:          2
           SAMPLE events:       7851
cycles stats:
            TOTAL events:       7961
             MMAP events:        106
             COMM events:          2
             EXIT events:          2
           SAMPLE events:       7851


And both events combined:
$ perf record -fo /tmp/perf.data -e cycles -e instructions -c 100000 
./loop_1b_instructions

$ /tmp/pbuild/perf report --stdio -i /tmp/perf.data -D | tail -15
Aggregated stats:
            TOTAL events:      18078
             MMAP events:        106
             COMM events:          2
             EXIT events:          2
           SAMPLE events:      17968
cycles stats:
            TOTAL events:       7900
             MMAP events:          4
             COMM events:          1
             EXIT events:          2
           SAMPLE events:       7893
instructions stats:
            TOTAL events:      10075
           SAMPLE events:      10075

So we get ~10,000 samples due to the instructions event and ~7800 for 
the cycles event.

David