Fighting for the power.

Alexander Motin mav at
Mon May 4 12:24:33 UTC 2009

Alexandre "Sunny" Kovalenko wrote:
> On Mon, 2009-05-04 at 01:18 +0300, Alexander Motin wrote:
>>   - C3 state allows CPU completely stop all internal clocks, reduce
>> voltage and disconnect from system bus. This state gives additional
>> power saving effect, but it is not cheap and require trade-offs.
>> As soon as CPU is completely stopped in C3 state, local APIC timers in
>> each CPU core, used by FreeBSD as event sources on SMP, are not
>> functioning. It stops system time, breaks scheduling that makes system
>> close to dead. 
> Did you try to see whether putting one of the cores in C3 state by doing
> something like
> dev.cpu.1.cx_lowest=C3
> makes any difference?
> # sysctl dev.cpu | grep cx_usage
> dev.cpu.0.cx_usage: 0.00% 100.00% 0.00%
> dev.cpu.1.cx_usage: 0.00% 5.18% 94.81%

I did. As soon as first CPU core is not in C3 state, second core unable 
to enter C3 completely and disconnect from the bus, as cores are sharing 
common resources. Such technique allows to avoid LAPIC timer problems, 
but I haven't noticed any effect from this on CPU idle power. The only 
difference I have noticed was in the case, when first core is busy. C3 
on second idle core then somehow reduces summary consumption a bit.

In other words, C3 state should be active on both cores simultaneously 
to give real effect.

Alexander Motin

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