General System Tweaks

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General System Tweaks

1. CPU mitigations

A public speculative execution attack exploiting return instructions (retbleed) was revealed in July 2022. The kernel has fixed this, but it results in a significant performance regression (14-39%).

The following CPU’s are affected:

AMD: Zen 1, Zen 1+, Zen 2
Intel: 6th to 8th Generation, Skylake, Kaby Lake, Coffee Lake

Check which mitigation’s your CPU is affected by using:

grep . /sys/devices/system/cpu/vulnerabilities/*

Disabling mitigations

While disabling the mitigation’s increases performance, it also introduces security risks.

Add the following to your kernel command line: retbleed=off or to disable all mitigation’s: mitigations=off

Edit the appropriate file to make the changes persistent:

GRUB: /etc/default/grub
systemd boot: /etc/sdboot-manage.conf
rEFInd: /boot/refind_linux.conf

For more information:

Downfall

Downfall is characterized as a vulnerability due to a memory optimization feature that unintentionally reveals internal hardware registers to software. With Downfall, untrusted software can access data stored by other programs that typically should be off-limits: the AVX GATHER instruction can leak the contents of the internal vector register file during speculative execution. Downfall was discovered by security researcher Daniel Moghimi of Google. Moghimi has written demo code for Downfall to show 128-bit and 256-bit AES keys being stolen from other users on the local system as well as the ability to steal arbitrary data from the Linux kernel.

This affects the following CPU generations:

Skylake
Tiger Lake
Ice Lake

Disabling Downfall

Add gather_data_sampling=off to your kernel cmdline options. mitigations=off will also disable downfall.

2. AMD P-State Driver

amd-pstate is the AMD CPU performance scaling driver that introduces a new CPU frequency control mechanism on modern AMD APU and CPU series in Linux kernel. The new mechanism is based on Collaborative Processor Performance Control (CPPC) which provides finer grain frequency management than the acpi-cpufreq driver. CPPC allows a flexible, low-latency interface for the Linux kernel to directly communicate the performance hints to hardware.

Below are 3 operation modes of the amd-pstate driver and kernel cmdline entries to use them on boot:

AMD P-State (Non-Autonomous Mode): amd-pstate=passive
AMD P-State Guided (Guided Autonomous Mode): amd-pstate=guided
AMD P-State EPP (Autonomous Mode): amd-pstate=active

You can also switch between operation modes at runtime to test the options:

Autonomous mode: platform considers only the values set for Minimum performance, Maximum performance, and Energy Performance Preference.
Terminal window
```
echo active | sudo tee /sys/devices/system/cpu/amd_pstate/status
```
Guided-autonomous mode: platform sets operating performance level according to the current workload and within limits set by the OS through minimum and maximum performance registers.
Terminal window
```
echo guided | sudo tee /sys/devices/system/cpu/amd_pstate/status
```
Non-autonomous mode: platform gets desired performance level from OS directly through Desired Performance Register.
Terminal window
```
echo passive | sudo tee /sys/devices/system/cpu/amd_pstate/status
```

For more information:

3. Using AMD P-State EPP

To use the P-State EPP, there are two CPU frequency scaling governors available: powersave and performance. It is recommended to use the powersave governor and set a preference.

Set powersave governor: sudo cpupower frequency-set -g powersave
Set performance governor: sudo cpupower frequency-set -g performance

To set a preference, run the following command with the desired preference:

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

Available preferences: performance, power, balance_power, balance_performance

Benchmarks for each preference can be found here: https://lore.kernel.org/lkml/[email protected]/

4. AMD P-State Preferred Core Handling

AMD Pstate driver will provide an initial core ordering at boot time. It relies on the CPPC interface to communicate the core ranking to the operating system and scheduler to make sure that OS is choosing the cores with highest performance firstly for scheduling the process. When AMD Pstate driver receives a message with the highest performance change, it will update the core ranking.

This can result into a better performance and process handling. More information here: https://lore.kernel.org/linux-pm/[email protected]/

You can use the following command to check if your CPU supports it:

cat /sys/devices/system/cpu/amd_pstate/prefcore

cat /sys/devices/system/cpu/amd_pstate/status

to see if it is enabled

5. Disabling Split Lock Mitigate

In some cases, split lock mitigate can slow down performance in some applications and games. A patch is available to disable it via sysctl.

Disable split lock mitigate: sudo sysctl kernel.split_lock_mitigate=0
Enable split lock mitigate: sudo sysctl kernel.split_lock_mitigate=1

To make the change persistent, add the following line to /etc/sysctl.d/99-splitlock.conf:

kernel.split_lock_mitigate=0

For more information on split lock, see:

6. Enabling Kernel Samepage Merging

CachyOS has used earlier as default uksmd (userspace kernel samepage merging) and then replaced this my the MemoryKSM function by systemd. Since there is a general cpu overhead and mainly a benefit for Hosts, which are running VM’s we decided to disable this feature for now.

User can enable again by following command:

sudo ksmctl --enable

After that Kernel Samepage Merging is enabled and it will merge memory pages into one, if possible.

to disable KSM again, run following

sudo ksmctl --disable