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

[Victor Stinner]

Hi,

I'm still having fun with microbenchmarks. I disabled Power States
(pstate) of my Intel CPU and forced the frequency for 3.4 GHz. I
isolated 2 physical cores on a total of 4. Timings are very stable
*but* sometimes, I get impressive slowdown: like 60% or 80% slower,
but only for a short time.

Do you know which CPU feature can explain such temporary slowdown?

I tried cpupower & powertop tools to try to learn more about internal
CPU states, but I don't see anything obvious. I also noticed that
powertop has a major side effect: it changes the speed of my CPU
cores! Since the CPU cores used to run benchmarks are isolated,
powertop uses a low speed (like 1.6 GHz, half speed) while benchmarks
are running, probably because the kernel doesn't "see" the benchmark
processes.

My CPU model is: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz

I'm using "userspace" scaling governor for isolated CPU cores, but
"ondemand" for other CPU cores.

I disabled pstate (kernel parameter: intel_pstate=disable), the CPU
scaling driver is "acpi-cpufreq".

CPUs 2,3,6,7 are isolated.

In the following examples, the same microbenchmark takes ~196 ms on
all cores, except of the core 3 on the first example.

Example 1:
---
$ for cpu in $(seq 0 7); do echo "=== CPU $cpu ==="; PYTHONHASHSEED=0
taskset -c $cpu ../fastcall/pgo/python performance/bm_call_simple.py
-n 1 --timer perf_counter; done
=== CPU 0 ===
0.19619656700160704
=== CPU 1 ===
0.19547197800056892
=== CPU 2 ===
0.19512042699716403
=== CPU 3 ===
0.35738898099953076
=== CPU 4 ===
0.19744606299718725
=== CPU 5 ===
0.195480646998476
=== CPU 6 ===
0.19495172200186062
=== CPU 7 ===
0.19495161599843414
---

Example 2:
---
$ for cpu in $(seq 0 7); do echo "=== CPU $cpu ==="; PYTHONHASHSEED=0
taskset -c $cpu ../fastcall/pgo/python performance/bm_call_simple.py
-n 1 --timer perf_counter; done
=== CPU 0 ===
0.19725238799946965
=== CPU 1 ===
0.19552089699936914
=== CPU 2 ===
0.19495758999983082
=== CPU 3 ===
0.19517506799820694
=== CPU 4 ===
0.1963375539999106
=== CPU 5 ===
0.19575440099652042
=== CPU 6 ===
0.19582506000006106
=== CPU 7 ===
0.19503543600148987
---

If I repeat the same test, timings are always ~196 ms on all cores.

It looks like some cores decide to sleep.

Victor