quiet-fan controls a CPU fan using a state machine that ensures the lowest fan speed, while keeping your CPU below a desired temperature.
Other fan control algorithms (e.g. PID, linear), cause the fan speed to vary frequently, which is acoustically annoying. Often they will also allow the CPU temperature to oscillate (which causes thermal stress). This happens even when hysteresis is incorporated.
For example, when the CPU is at the desired temperature, a PID algorithm will have an error term of 0, meaning no fan speed adjustments will be made. But it could be the result of the ambient air temperature. That means you're experiencing more fan noise than is necessary to keep the CPU cool.
Quiet-fan uses a state machine to occasionally test lower fan speeds to see if the CPU temperature is negatively affected. This ensures quiet-fan will "hunt" for the lowest possible fan RPM that still keeps your CPU below a threshold.
If your system load is high, it will not test lower RPMs, further reducing fan RPM and CPU temperature oscillations.
There's probably a proper systemd way of doing this, but this line in
/etc/rc.local works:
/root/sbin/fan_adj > /var/log/fan_adj.log &
Saving stdout to a log file is unnecessary.
This is a system utility, thus its configuration isn't friendly or error-proof. Edit the program variables in quiet-fan in the DEVICES and CONFIGURATION SETTINGS sections.
System load is not the same as CPU load, so if the disks are busy, the fan RPMs may be kept high unnecessarily.