Periodically and very suddenly the fans can spin up to maximum speed even if the PowerStation is idle and will usually return to normal speed within about 30 seconds.
To completely switch off unused cores on YDL Powerstation quad G5 (maple motherboard equipped with dual PPC970MP):
The sysctl setting is
kernel.powersave-nap which defaults to 0. When set to 1 (enabled), the kernel actually tells the cores to power down. With 0 (disabled) it just loops.
Code:
# to disable powersave
echo 0 > /proc/sys/kernel/powersave-nap
# to enable powersave
echo 1 > /proc/sys/kernel/powersave-nap