throttle

Daemon to regulate the maximum CPU clock based on its temperature.

Compilation & Installation

After cloning the repository and cding into the main directory, execute make to compile. If you want a debug version, run make debug instead. The debug version reads the temperature from a local file temp_input and writes the frequencies to cpu#freq, where # runs through the cores.

The daemon can be installed as systemd service. Just execute (as root) make install, then systemctl daemon-reload and systemctl enable throttle.service. With start instead of enable, you can try the daemon in the current session. You might want to adjust the settings in throttle.conf before (see below). Uninstalling works similarly by make uninstall (as root, of course).

The program can also be called directly via

throttle [<config file> [<command pipe>]]

If not set, they default to throttle.conf and pipe, respectively. Don't forget to run as root, if you want to let it change frequencies.

Mechanism

CPU frequency drivers typically regulate the core frequencies based purely on demand. For notebooks however, this is not the only factor to consider, especially if you use them for lengthy computations.

We do not override the on-demand mechanisms, instead we modify their input. Whenever the temperature exceeds a certain threshold, we decrease the maximum CPU frequency. If the CPU cools down and reaches another threshold, we increase the maximum frequency again. Both thresholds can be set as temp_max and temp_min (in °C) via the configuration file (see below).

To efficiently regulate a system, one has to take into account the delay between changing the input and the effect on the output. Therefore, we wait at least 3*wait seconds after increasing/decreasing the frequency before we increase/decrease again. After that, we check every three seconds if the temperature is still in the desired interval.

Configuration

Parameters are passed to the program via a configuration file. If installed as a service, this is the file /etc/trottle.conf. A template is produced by the Makefile. Further, users can adjust some parameters at runtime by writing to a command pipe. If installed as a service, this is the file /run/throttle. Permissions should be set appropriately. (The systemd configuration file sets them such that users can write to the pipe.)

The following settings are automatically determined at installation time and do not need to be changed under most circumstances.

• cores: number of CPU cores
• temp_file: a file from which we can read the current CPU temperature, like /sys/class/hwmon/hwmon0/device/temp1_input.
• freq_list: list of available CPU frequencies: We try to read them from /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies. If this file doesn't exist, you have to provide the data yourself.
• freq_set_prefix and freq_set_suffix: describes thefiles containing the maximum frequency for each core. The cores are numbered 0, 1, 2, and so on. The file names are constructed as freq_set_prefix + core number + freq_set_suffix. Example: "/sys/devices/system/cpu/cpu" + 0 + "/cpufreq/scaling_max_freq"

The behavior of the throttling daemon is controlled by the settings temp_min, temp_max, and wait. These were explained in the last section.

Real-time control

You can control the throttling in real-time by sending commands to /run/throttle (if installed as service, or any other file you set in the command line). This can be done by

echo "<command>" >/run/throttle

Currently, the following commands are available:

• max n sets the maximum temperature to n,
• min n sets the minimum temperature to n, both in °C.
• freq n sets the current (maximum) frequency directly to n MHz. This overrides the throttling mechanism.
• reset reinstates the throttling mechanism.