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systemd package

Comment 18 for bug 1885730

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Colin Ian King (colin-king) wrote on 2020-08-18: Re: Bring back ondemand.service or switch kernel default governor for pstate - pstate now defaults to performance governor

#18

LibreOffice |Calc spread sheet of boot timings and power measurements Edit (21.8 KiB, application/vnd.oasis.opendocument.spreadsheet)

@xnox,

one can detect the machine type from the DMI data (iff it is available and reliable).

e.g. on my laptop:

sudo dmidecode -s "chassis-type"
Notebook

on my desktop server:
sudo dmidecode -s "chassis-type"

There are quite a few chassis-type, see https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_2.7.1.pdf - page 36, section 7.4.1, table 16 "System Enclosure or Chassic Type".
*Note* some machines may have illegal data or omitted this information and it's DMI specific, so s390, arm64, ppc64el platforms most probably won't have this.

So some context. This performance governor was chosen for the boot default setting because it speeds up boot. I've performed some simple boot tests on an older generation Lenovo x220 (i3-2350M), timings are below for a 5.8 kernel:

Governor Kernel Userspace EndBoot
PEFORFMANCE 2.15s 10.37s 12.52s
POWERSAVE 2.87s 18.49s 21.37s
ONDEMAND 2.41s 10.46s 12.87s

So PERFORMANCE saves a few tenths of a second over ONDEMAND, hence this choice for boot. The setting thereafter is obviously more complex. I've compared the governor settings on the same laptop running idle, with 50% CPU utilization and 100% CPU utilization. I used powerstat to monitor CPU power, CPU freq and C7 (deep sleep) residency.

1. Idle.
- power utilization roughly the same in power usage, but powersave clocks the CPU lowest. Note that the CPU freq is cranked up when measurements are taken, so it's hard to get a correct freq. measurement.

2. 50% CPU busy.
- performance consumes most power (as expected) and the CPU is running marginally faster than on-demand.

3. 100% CPU busy.
- performance and on-demand are comparable in terms of power consumption and CPU frequency.

PERFORMANCE essentially runs the CPU at higher frequency all the time, whereas ONDEMAND will scale up/down depending on the utilization. I believe the default should be ONDEMAND post boot as this is the most flexible option and will provide power saving when the system is less utilized. If users want to burn power and they can opt-in to manually setting to PERFORMANCE, but I think this should be opt-in rather than the default setting for any class of machine.

Finally, if we don't want the userspace changes, we could default the kernel to ONDEMAND and take a hit on slower boot performance.

To clarify I see the options as:

1. Boot with PERFORMANCE and fix the userspace to set ONDEMAND in the post-boot stage
2. Failing this, don't let userspace do anything smart post-boot and default to ONDEMAND

Attached are some data points I gathered with the 5.8 kernel.

@xnox,

one can detect the machine type from the DMI data (iff it is available and reliable).

e.g. on my laptop:

sudo dmidecode  -s "chassis-type"
Notebook

on my desktop server:
sudo dmidecode  -s "chassis-type"

Governor     Kernel   Userspace  EndBoot
PEFORFMANCE  2.15s    10.37s     12.52s
POWERSAVE    2.87s    18.49s     21.37s
ONDEMAND     2.41s    10.46s     12.87s

So PERFORMANCE saves a few tenths of a second over ONDEMAND, hence this choice for boot. The setting thereafter is obviously more complex.  I've compared the governor settings on the same laptop running idle, with 50% CPU utilization and 100% CPU utilization.  I used powerstat to monitor CPU power, CPU freq and C7 (deep sleep) residency.

1. Idle.
  - power utilization roughly the same in power usage, but powersave clocks the CPU lowest. Note that the CPU freq is cranked up when measurements are taken, so it's hard to get a correct freq. measurement.

2. 50% CPU busy.
   - performance consumes most power (as expected) and the CPU is running marginally faster than on-demand.

3. 100% CPU busy.
   - performance and on-demand are comparable in terms of power consumption and CPU frequency.

PERFORMANCE essentially runs the CPU at higher frequency all the time, whereas ONDEMAND will scale up/down depending on the utilization.  I believe the default should be ONDEMAND post boot as this is the most flexible option and will provide power saving when the system is less utilized.  If users want to burn power and they can opt-in to manually setting to PERFORMANCE, but I think this should be opt-in rather than the default setting for any class of machine.