[Speed] intel_pstate C0 bug on isolated CPUs with the performance governor

[Armin Rigo]

Hi Victor,

On 24 September 2016 at 01:49, Victor Stinner <victor.stinner at gmail.com> wrote:
> When running benchmarks, raw timings and CPU performance don't matter.
> Only comparisons between benchmark results and stable performances
> matter.

IMHO this is not a very good solution.  With the CPU running at, say,
a fifth of its nominal performance, you can't expect that it will
behave in a remotely similar way.  For example, it makes the RAM
appear five times faster.  I would guess (but I don't know) that even
the on-core L2/L3 caches are not slowed down by nearly as much as five
times.  As a result, it is easy to introduce changes to the CPython
core that appear beneficial, but are actually detrimental, or
vice-versa.  For example, replacing some computation by lookups in a
table may look like a good idea, when it is not.


A bientôt,

Armin.