While analyzing a performance regression ( http://bugs.python.org/issue28243 ) , I had a major issue with my benchmark. Suddenly, for no reason, after 30 minutes of benchmarking, the benchmark became 2x FASTER...
A similar issue occurred to me the last week when testing if PGO compilation makes Python performance unstable.
It might be an issue related to the intel_pstate driver on Linux and CPU isolation. I reported the bug in the Fedora bug tracker in the kernel category: https://bugzilla.redhat.com/show_bug.cgi?id=1378529
I don't know much about this issue yet, I contacted Intel engineers who knows these things better than me :-)
If you have an Intel CPU, use Linux, have a CPU with multiple physical cores and have 15 minutes to run a test, I would appreciate if you can try to reproduce the bug!