Some tips for benchmarking

[Rafael Espíndola via llvm-commits]

After much pain I decided that instead of just running tests many
times I should put some effort in figuring out why perf numbers were
so hard to reproduce on my machine.

I was already disabling address space randomization and setting the
scaling_governor to performance. Other things I found

* Use https://github.com/lpechacek/cpuset to reserve a cpu for just
the program you are benchmarking.
* Disable the SMP pair (echo 0 > /sys/devices/system/cpu/cpuX/online)
of the cpu you will use for the benchmark.
* disable turbo mode (echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo).

The results are pretty awesome:

3.012355647 seconds time elapsed         ( +-  0.01% )

:-)

Cheers,
Rafael