[Benchmarking] Periodic processes invalidating benchmarks

[Andrea Aime]

Hi,
today making a benchmark run I've found another
process competing for the CPU:

  9161 root      35  19  409m  88m 8552 R  6.0  1.1   0:03.85
yum-updatesd-he

It was not using much, but some of these fellas (like prelink,
which I spotted in another round) hit the disk enough to
flip a run from "non disk bound" to "disk bound" with
obvious consequences in the results.

We need to look into how to avoid them.

Cheers
Andrea