Using bechamel-perf requires running the benchmark with root privilege

[OlivierNicole]

On my system, unless using sudo the perf_event_open syscall fails with “Permission denied”. A quick search suggests this might be due to the exclude_kernel = 0 and exclude_hv = 0 attributes in the C stub.

[dinosaure]

We should probably update the documentation and explain the environment for using some of the features offered by `perf' indeed.

[dinosaure]

Fixed by #39 which explains required privileges when we use Bechamel_perf.