Linux

Hi everyone, I'm quite new to Lemmy (this is actually my fist post!) During the past weeks I've sunk into the magical world of AMD P-States, and, I ended up putting together a quick post that I thought might be useful for someone else.

I'm a Linux amateur, so this could be very much wrong, but I'm very much open to any corrections or improvements :)

1. Requirements

Currently, some of the Zen2 and Zen3 processors support amd-pstate and the new amd_pstate_epp scaling driver. You also have to have CPPC support enabled in your UEFI. In the future, it will be supported on more and more AMD processors.

2. amd-pstate vs acpi-cpufreq

There are two methods for adjusting CPU performance on AMD CPU/APUs:

• amd-pstate
• acpi-cpufreq

acpi-cpufreq is currently default for most distros, regardless of the CPU in use. on most AMD CPUs this is a limiting factor, as it offers limited performance options with only a few fixed levels for CPU speed.

On newer AMD CPUs and APUs (aka Zen2 and above), there is a more advanced method called Collaborative Processor Performance Control (CPPC mentioned in the requirements), which allows for fine-tuned and continuous adjustments of the CPU frequency, with the potential to provide better performance and energy efficiency compared to the older fixed levels.

And that's where amd-pstate comes in, as it is a new kernel module that supports the newer and more efficient AMD P-States mechanism.

There are 3 options available, listed below, in order of release:

• amd_pstate=passive (Kernel 6.1+)

• amd_pstate=active (Kernel 6.3+)

• amd_pstate=guided (kernel 6.4+)

Passive Mode

amd_pstate=passive

When you set amd_pstate=passive, the processor aims for a certain performance level relative to its maximum capacity. Below a specific point, the performance is average, while above it, the performance remains at its best.

Active Mode

amd_pstate=active

Setting amd_pstate=active gives low-level control to the processor's firmware. It can prioritize either performance or energy efficiency based on software hints AND the amd_pstate_epp driver. The amd_pstate_epp (Energy Performance Preference) driver provides the firmware with a hint. On most AMD CPUs, these hints are:

• default
• performance
• balance_performance
• balance_power
• power

Guided Mode

amd_pstate=guided

Choosing amd_pstate=guided lets the platform automatically select a suitable performance level within a given range based on the workload.

3a. Configure amd_pstate to either Passive or Guided

To enable the amd_pstate_epp scaling driver, which also includes instructions for the original amd_pstate scaling driver, you will need to add a kernel parameter. If you are using PopOS (like me) or any other distribution utilising kernelstub, this process can be easily accomplished with the following steps:

IMPORTANT: The option 'amd_pstate=guided' is only available on Kernel 6.4 or later versions.

1. Add the desired kernel parameter by running the following command:

# Add the desired Kernel Parameter
sudo kernelstub -a "amd_pstate=guided" # Change this to passive if preferred

2. To confirm that the kernel parameter has been successfully added, use the following command:

# Verify that the kernel parameter has been added
sudo kernelstub -p 

Verify amd_pstate

To verify that this is functioning correctly, reboot your machine, and run

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver

If amd_pstate was set to either passive or guided, this should now show:

amd-pstate

3b. Configure amd_pstate_epp to Active

To enable the amd_pstate_epp scaling driver, which also includes instructions for the original amd_pstate scaling driver, you will need to add a kernel parameter. If you are using PopOS (like me) or any other distribution utilising kernelstub, this process can be easily accomplished with the following steps:

IMPORTANT: The option 'amd_pstate=guided' is only available on Kernel 6.3 or later versions.

1. Add the desired kernel parameter by running the following command:

# Add the desired Kernel Parameter
sudo kernelstub -a "amd_pstate=active"

2. To confirm that the kernel parameter has been successfully added, use the following command:

# Verify that the kernel parameter has been added
sudo kernelstub -p 

Verify amd_pstate

To verify that this is functioning correctly, reboot your machine, and run

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver

If amd_pstate was set to active, this should now show:

amd-pstate-epp

Configure amd_pstate_epp Energy Performance Preference

The amd_pstate_epp scaling driver introduces a new parameter known as "Energy Performance Preference" (EPP) hint. This setting can be adjusted through sysfs, with two main files controlling it:

• /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_preference: This file displays the current EPP hint for the respective CPU core.

• /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_available_preferences: This file provides the available EPP hints for the respective CPU core.

To see your current EPP hints (note * = all CPU cores), use the following command:

cat /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_preference

To view the available EPP hints (which should be the same for all cores), use this command:

cat /sys/devices/system/cpu/cpu0/cpufreq/energy_performance_available_preferences

# What you see below, is my results on my Ryzen 7 7735HS
default performance balance_performance balance_power power 

If you'd like to set the same EPP hint across all cores, for instance, setting EPP to "power" (like in my case), you can use this command:

echo "power" | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_preference
power

NOTE: This is not permament, and will be reverted upon reboot. To make this permanent, you can use multiple tools, or, create a cron job

4. Scaling Driver vs CPU Governor

The Scaling Driver is different than the CPU governor (e.g. powersave, performance, ondemand, schedulutil, etc.), and the two can be mixed and matched to create your perfect combo.

To check what's the current cpu governor, use the command below:

cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

In my case, that's what I'm seeing:

user@machine ~> cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave
powersave

If you've configured amd_pstate=active, you can mix and match governors with EPP hints. For me the default after I enabled amd_pstate_epp was governor = powersave and EPP hint = performance. Phoronix has an excellent breakdown of all the combinations of governors + EPP hints (referenced in the resources section at the end of this post).

Resources:

• an amazing Redditor (whose post I cannot find anymore) that served as a basis for this very post (if anyone finds it, please do let me know, and I'll reference them right away)

• ChatGPT who helped me phrase some sentences a bit better

• Benchmarks for server using AMD P-State EPP: https://www.phoronix.com/review/linux-63-amd-epyc-epp

• Benchmarks for Ryzen mobile system using AMD P-State EPP: https://www.phoronix.com/review/amd-pstate-epp-ryzen-mobile

• Arch Wiki page on CPU Scaling: https://wiki.archlinux.org/title/CPU_frequency_scaling

• Kernel.org documentation on new AMD P-State driver: https://www.kernel.org/doc/html/latest/admin-guide/pm/amd-pstate.html