[Speed] CPU speed of one core changes for unknown reason

[Victor Stinner]

FYI I'm running the CPython Benchmark Suite with:

taskset -c 1,3 python3 -u perf.py --rigorous ../ref_python/pgo/python
../fastcall/pgo/python -b all

I was asked to use --rigorous and -b all when I worked on other patches, like:
https://bugs.python.org/issue21955#msg259431

2016-05-19 0:04 GMT+02:00 R. David Murray <rdmurray at bitdance.com>:
> On Wed, 18 May 2016 21:05:11 -0000, Paul Graydon <paul at paulgraydon.co.uk> wrote:
>> I do have some concerns that you're increasingly creating a synthetic
>> environment to benchmark against, and that you're at risk of
>> optimising towards an environment the code won't actually run in, and
>> might even end up pursuing the wrong
>> optimisations.
>
> My understanding is that Victor isn't using this to guide optimization,
> but rather to have a quick-as-possible way to find out that he screwed up
> when he made a code change.  I'm sure he's using much longer benchmarks
> runs for actually looking at the performance impact of the complete
> changeset.

Right, I don't use the benchmark suite to choose which parts of the
code should be optimized, but only to ensure that my optimizations
make Python faster, as expected :-)

But I understood what Paul wrote. He says that modifying a random
parameter to make it constant (like random hash function) can lead to
wrong conclusion on the patch. Depending on the chosen fixed value,
the benchmark can say that the patch makes Pyhon faster or slower.
Well, at least in corner cases, especially microbenchmarks like
call_simple.

Victor