Problem with fixed frequencies using CPUFreq Gnome extension

cpu gnome power-management

I installed CPUFreq Gnome extension on two different PCs with Ubuntu 18.04. One has a Haswell the other one Skylake CPU. I set minimum and maximum frequencies to be equal and turn Frequency Boost off. PCs are well cooled and CPU temperature is below 40 deg. C. Frequency on Haswell system stays very stable with occasional dips less than 1%, but it regularly dips a few percent on Skylake system, occasionally up to 10%:

enter image description here

The terminal above shows output of watch grep \"cpu MHz\" /proc/cpuinfo.

Is there anything I can do to make it more stable?


Solution 1:

The processor will back off on CPU frequency by itself, if the load is low enough, even with both min and max set to highest level. You shouldn't notice performance wise. However if it really matters the only way that i know of to lock the CPU frequencies at maximum is to disable every idle state deeper than 0, but at the cost of a tremendous amount of power.

So, for normal default settings:

doug@s18:~/c$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver
/sys/devices/system/cpu/cpu0/cpufreq/scaling_driver:intel_pstate
/sys/devices/system/cpu/cpu1/cpufreq/scaling_driver:intel_pstate
/sys/devices/system/cpu/cpu2/cpufreq/scaling_driver:intel_pstate
/sys/devices/system/cpu/cpu3/cpufreq/scaling_driver:intel_pstate
/sys/devices/system/cpu/cpu4/cpufreq/scaling_driver:intel_pstate
/sys/devices/system/cpu/cpu5/cpufreq/scaling_driver:intel_pstate
doug@s18:~/c$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor:powersave
/sys/devices/system/cpu/cpu1/cpufreq/scaling_governor:powersave
/sys/devices/system/cpu/cpu2/cpufreq/scaling_governor:powersave
/sys/devices/system/cpu/cpu3/cpufreq/scaling_governor:powersave
/sys/devices/system/cpu/cpu4/cpufreq/scaling_governor:powersave
/sys/devices/system/cpu/cpu5/cpufreq/scaling_governor:powersave

you would get something like this for light loads:

doug@s18:~/c$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:800229
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:800287
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:800174
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:800286
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:800049
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:800188

now, make the changes, as you have done:

doug@s18:~/c$ echo performance | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
performance
doug@s18:~/c$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor:performance
/sys/devices/system/cpu/cpu1/cpufreq/scaling_governor:performance
/sys/devices/system/cpu/cpu2/cpufreq/scaling_governor:performance
/sys/devices/system/cpu/cpu3/cpufreq/scaling_governor:performance
/sys/devices/system/cpu/cpu4/cpufreq/scaling_governor:performance
/sys/devices/system/cpu/cpu5/cpufreq/scaling_governor:performance
doug@s18:~/c$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:3987126
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:4600628
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:2559618
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:4600104
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:4601383
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:4600459
doug@s18:~/c$ echo 1 | sudo tee /sys/devices/system/cpu/intel_pstate/no_turbo
1
doug@s18:~/c$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:3700405
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:3700291
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:3700381
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:3700612
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:3701802
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:3700702

Actually, surprisingly stable. And power is still low:

doug@s18:~/c$ sudo ~/turbostat --Summary --quiet --show Busy%,Bzy_MHz,PkgTmp,PkgWatt,GFXWatt,IRQ --interval 6
Busy%   Bzy_MHz IRQ     PkgTmp  PkgWatt GFXWatt
0.01    3689    196     30      1.91    0.00
0.01    3700    90      30      1.90    0.00
0.01    3667    142     30      1.91    0.00
0.01    3701    93      30      1.91    0.00
0.01    3620    119     30      1.89    0.00
0.01    3648    118     30      1.88    0.00
0.01    3603    147     30      1.86    0.00
0.00    3701    85      30      1.77    0.00
0.01    3700    102     30      1.84    0.00

However, if you want it even more stable at 3.7 GHz, then disable deep idle states, such the processor never goes into idle (idle state 0, or poll, is not actually idle at all):

doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state8/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state7/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state6/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state5/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state4/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state3/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state2/disable
1
doug@s18:~/idle/perf/results/a04$ echo 1 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state1/disable

And now the CPU frequencies are locked, the power is not terrible, mainly because turbo is disabled (turbostat, with the setting I used, does not distinguish between idle state 0 and really busy):

doug@s18:~/c$ sudo ~/turbostat --Summary --quiet --show Busy%,Bzy_MHz,PkgTmp,PkgWatt,GFXWatt,IRQ --interval 6
Busy%   Bzy_MHz IRQ     PkgTmp  PkgWatt GFXWatt
100.26  3700    36069   36      21.66   0.00
100.26  3700    36072   36      21.70   0.00
100.26  3700    36065   36      21.66   0.00
100.26  3700    36079   36      21.89   0.00
100.26  3700    36070   36      21.65   0.00
100.26  3700    36101   36      21.80   0.00
100.26  3700    36072   37      21.74   0.00

As a side note, the power for real work will be higher:

doug@s18:~/idle/perf/results/a04$ sudo ~/turbostat --Summary --quiet --show Busy%,Bzy_MHz,PkgTmp,PkgWatt,GFXWatt,IRQ --interval 6
Busy%   Bzy_MHz IRQ     PkgTmp  PkgWatt GFXWatt
100.26  3700    36169   55      72.08   0.00
100.26  3700    36260   55      71.96   0.00
100.26  3700    36074   55      71.97   0.00
100.26  3700    36082   55      71.99   0.00
100.26  3700    36080   56      72.03   0.00

The examples for this answer were done with Intel(R) Core(TM) i5-9600K CPU @ 3.70GHz. Readers might have a different number of idle states, depending on their processor.

To enable all idle states again, after the test work:

doug@s18:~/idle/perf/results/a04$ echo 0 | sudo tee /sys/devices/system/cpu/cpu*/cpuidle/state*/disable

And then check:

doug@s18:~/idle/perf/results/a04$ grep . /sys/devices/system/cpu/cpu*/cpuidle/state*/disable
/sys/devices/system/cpu/cpu0/cpuidle/state0/disable:0
/sys/devices/system/cpu/cpu0/cpuidle/state1/disable:0
/sys/devices/system/cpu/cpu0/cpuidle/state2/disable:0
...
/sys/devices/system/cpu/cpu5/cpuidle/state6/disable:0
/sys/devices/system/cpu/cpu5/cpuidle/state7/disable:0
/sys/devices/system/cpu/cpu5/cpuidle/state8/disable:0

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