Thursday, June 12, 2014

FreeBSD and CPU scaling

Recently built a new FreeBSD machine with AMD Athlon 5350. Found that if powerd is enabled, the machine will randomly reboot itself. Forcing the lowest CPU frequency to max by setting debug.cpufreq.lowest=2050 in /boot/loader.conf can prevent that. So I suspect it is related to the CPU scaling or C-state.

This problem can be avoided if disabling the C6 state in BIOS.

For reference, the motherboard is ASRock AM1H-ITX.

Here are the CPU freq under normal powerd management.

$ sysctl dev.cpu.0.freq_levels
dev.cpu.0.freq_levels: 2050/5021 1850/4590 1650/3675 1443/3215 1400/2937 1225/2569 1200/2266 1050/1982 1000/1880 875/1645 800/1527 700/1336 600/1145 500/954 400/763 300/572 200/381 100/190

Update: 14 Jun

Found a matching bug report and reported my findings too. Basically, either C6 in BIOS or powerd in FreeBSD need to be disabled to get around the issue. Or, with C6 and powerd enable, the issue can be avoided if CPU throttling is disabled. Add this to /boot/loader.conf:


But this can affect the reported lowest CPU freq.
Throttling disabled:
root@bart:~ # sysctl dev.cpu | grep freq
dev.cpu.0.freq: 800
dev.cpu.0.freq_levels: 2050/5021 1850/4590 1650/3675 1400/2937 1200/2266 1000/1880 800/1527