On Mon, 2012-03-05 at 21:39 -0500, Phillip Susi wrote: > On 03/05/2012 09:19 PM, Christopher James Halse Rogers wrote: > > Less power per instruction, or less power per instruction amortized over > > the run-time? My understanding was that hitting the low C-states was > > such a huge power win that the increased power per instruction was > > offset by the longer C-state residency¹. > > > > ¹: http://www.codon.org.uk/~mjg59/power/good_practices.html > > That's the article that I was thinking of that is mostly about the > p4clockmod driver. Using it means you have the same power per > instruction, but also are spending less time in the deeper C states, > and so is bad. With correct frequency management, the lower power per > instruction of the lower frequencies outweighs the reduced time in the > lower C states. There probably still is an optimal point somewhere > between the min and max frequency where you get the benefit of both > lower power per instruction when executing instructions, without > giving up too much time in the lower C states, but finding that > balance is tricky and highly hardware and load specific. My current > CPU operates from 1600 to 3301 MHz ( where the last 1 MHz just enables > turbo boost ). Disabling that turbo boost state would probably > provide the most savings in power power per instruction with minimal > loss of deep C6 time. At 1600 MHz with a load that keeps that > frequency mostly busy very well may be more effi > cient at > one of the intermediate frequencies, but figuring out which is tricky. > One thing is almost certain: it isn't 3301 MHz. Of course, a load > that keeps 1600 MHz rather busy would trigger the ondemand governor to > shift to one of the intermediate frequencies anyway, so the default > situation is probably quite near optimal. Load spikes that would > cause the ondemand governor to shift to 3301 ( turbo boost ) would be > best disabled when on battery power, but I believe that many laptops > have proper ACPI bios that does disable turbo boost when on battery > power, so we're good there too. >
I think we might have wildly different interpretations of that article. The third paragraph is: “C states offer significant power savings, but cannot be entered when the CPU is executing instructions. The best power savings can be obtained by running the CPU as fast as possible until any outstanding work is completed and then allowing the CPU to go completely idle. The powersave governor will extend the time taken to complete the work and reduce the amount of time spent idle. ************ On any modern CPU the benefit of carrying out the work at a lower clock and voltage will be outweighed by the loss of the idle time. ************ In almost any workload, powersave will consume more energy than any other option.” And has: “Summary: Use ondemand. Conservative is a valid option for processors that take a sufficiently long time to switch performance states that ondemand will not work.” There's a *separate* dot-point about not using p4clockmod, but I read that as entirely separate. Of course, that was written in 2008; IIRC this is before the age of "turbo mode", so that might change the conclusions.
signature.asc
Description: This is a digitally signed message part
-- Ubuntu-devel-discuss mailing list Ubuntu-devel-discuss@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-devel-discuss
