Ubuntu – How to control CPU frequency

For compatible processors, by default Ubuntu now uses the intel_pstate CPU frequency governor, whereas it used to use the acpi_cpufreq CPU frequncy governor.

The intel_pstate drive does not have ondemand mode, but its powersave mode should be the equivalent of the acpi_cpufreq ondemand mode. Your system should default to powersave mode about 1 minute after boot, and via the /etc/init.d/ondemand script. In the recent past, that script was not properly dealing with the intel_pstate case, but it should have been fixed for all use cases by now. Reference.

To answer your actual questions:

A.) The master min and max frequencies are stored as percentage numbers.
cat /sys/devices/system/cpu/intel_pstate/min_perf_pct
cat /sys/devices/system/cpu/intel_pstate/max_perf_pct
The interpretation of those numbers is a function of the turbo enabled or disabled flag, and in my opinion there is an inconsistency in the definitions.
cat /sys/devices/system/cpu/intel_pstate/no_turbo

Example from my i7-2600K: min freq 1.6GHz; max non-turbo 3.4GHz; max turbo 3.8GHz.
Therefore as percentages:
Turbo off: max = 100%, min = 47.1%
Turbo on: max = 100%, min = 42.1%

$ cat /sys/devices/system/cpu/intel_pstate/no_turbo
0
$ cat /sys/devices/system/cpu/intel_pstate/min_perf_pct
42
$ cat /sys/devices/system/cpu/intel_pstate/max_perf_pct
100

and

$ cat /sys/devices/system/cpu/intel_pstate/no_turbo
1
$ cat /sys/devices/system/cpu/intel_pstate/max_perf_pct
100
$ cat /sys/devices/system/cpu/intel_pstate/min_perf_pct
42

B.) The intel-pstate powersave mode should be the equivalent of the acpi-cpufreq ondemand mode.

C.) There is something wrong, resulting in your grief. There have been other reports similar to yours. Myself, I do not know the root issue, but there have also been reports of incompatibilities with cpufrequtils. I do not know if they are true or not, as I don't use any such things. I only use the most primitive level of controls with the intel-pstate driver.

You can set maximum and minimum frequencies by setting /sys/devices/system/cpu/cpu**N**/cpufreq/scaling_min_freq and /sys/devices/system/cpu/cpu**N**/cpufreq/scaling_max_freq

Details are described in this document: https://software.intel.com/sites/default/files/comment/1716807/how-to-change-frequency-on-linux-pub.txt

Edit: You can reach the same using cpufreq-set in cpufrequtils package: https://www.kernel.org/pub/linux/utils/kernel/cpufreq/cpufreq-set.html