According to a friend, my CPU model "Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz" has a "Turbo Mode" which is enabled by default. The CPU tries to use the Turbo Mode whener possible, but disables it when the CPU is too hot. The change should be visible with the exact CPU frequency (the change can be a single MHz: 3400 => 3401). I didn't notice such minor CPU frequency change, but I didn't check carefully.
Anyway, I disabled the Turbo Mode and Hyperthreading in the EFI. It should avoid the strange performance "drop". Victor 2016-05-17 16:44 GMT+02:00 Victor Stinner <victor.stin...@gmail.com>: > 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 _______________________________________________ Speed mailing list Speed@python.org https://mail.python.org/mailman/listinfo/speed
