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I'm looking for some clarification on how to use either rapl-set or powercap-set to change the long term power limit on my Intel N4200 laptop running Ubuntu 20.04.

Firstly, I know that my bios allows modification of the long-term constraint (up to a preset max) as I've had good results with ThrottleStop on Windows.

My default constraints:

powercap-info -p intel-rapl         
Zone 0
  name: package-0
  enabled: 0
  max_energy_range_uj: 262143328850
  energy_uj: 1431662228
  Constraint 0
    name: long_term
    power_limit_uw: 3999744
    time_window_us: 27983872
    max_power_uw: 5999616
  Constraint 1
    name: short_term
    power_limit_uw: 5999616
    time_window_us: 976
    max_power_uw: 0
  Zone 0:0
    name: core
    enabled: 0
    max_energy_range_uj: 262143328850
    energy_uj: 975192877
  Zone 0:1
    name: uncore
    enabled: 0
    max_energy_range_uj: 262143328850
    energy_uj: 308897463
  Zone 0:2
    name: dram
    enabled: 0
    max_energy_range_uj: 262143328850
    energy_uj: 410797898
    Constraint 0
      name: long_term
      power_limit_uw: 0
      time_window_us: 976

So I'm looking at Zone 0, Constraint 0. You can see that it's set to 4w long-term with a short-term limit of 6w. What I would like to achieve is to make the long-term constraint equal to the short term constraint, which would replicate what Ive achieved with ThrottleStop on Windows.

To that end, I have tried the following, which all seem to do the same thing:

cd /sys/class/powercap/intel-rapl/intel-rapl:0 && cat constraint_0_max_power_uw | tee constraint_0_power_limit_uw
rapl-set -p 0 -c 0 -l 5999616 -e 1
powercap-set -p intel-rapl -z 0 -c 0 -l 5999616 -e 1

Either one of these appear to set the constraint as required:

Zone 0
  name: package-0
  enabled: 1
  max_energy_range_uj: 262143328850
  energy_uj: 4600742311
  Constraint 0
    name: long_term
    power_limit_uw: 5999616
    time_window_us: 27983872
    max_power_uw: 5999616

With either method, there is no error and the relevant files are successfully updated. However, the new power limit is not adhered to. Please see the attached s-tui screenshot which illustrates the clocks & power limit dropping regardless of settings.

The CPU governor is set to 'performance' and disabled TLP is disabled.

Am I missing something here? I assume that the relevant driver/module is loaded due to the files being present and no errors being produced, but it feels as tough I'm overlooking something of that nature.

Any help/suggestions would be greatly appreciated. Has anyone even seen this work on Linux?

s-tui screenshot

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2 Answers 2

Reset to default
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I do not use rapl-set or powercap-set for power capping. What I use in general is that

    echo 5999616 | sudo tee constraint_0_power_limit_uw

It always works for me.

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  • Thanks for the reply. That's essentially the same as the 1st thing I tried though. Glad to hear that it 'should' work though...
    – ahaslam
    Apr 12, 2021 at 10:47
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The power limits are also controlled by hostbridge mmio register PACKAGE_RAPL_LIMIT_0_0_0_MCHBAR_PCU. You can use the setPL.sh script to set power limits which takes care of that mmio register.

$ sudo setPL.sh 6 6
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