On Thu, Mar 08, 2012 at 05:29:47PM -0500, Phillip Susi wrote: > You also need to remember that 3.4x the clock speed does not mean you > actually end up finishing your work 3.4x faster. Intel recommends > using the ondemand governor, so if you are claiming they are wrong, > and you save more power using performance, that's going to take a > little convincing. I checked on my desktop sandybridge core i5 > system, and I found that running stress -c 4 with powersave draws 150 > watts, and with ondemand, is nearly 200 watts. Idle, the system draws > 125 watts. Factoring out that baseline gives a cpu load of 25 vs 75 > watts depending on whether you run at 1600 MHz vs 3300 MHz, or 3x the > power for twice the speed.
Do a SpecPOWER run with performance and with ondemand and you'll see that there's very little difference on modern hardware. > > But this is a very strange workload to be optimising for. First, it's > > entirely CPU-bound. If it involves IO then you're going to be keeping IO > > devices in a higher power state for longer, which wipes out the > > advantage. Second, it makes the assumption that the user doesn't care > > how much time it takes. That's basically never true. > > We're not talking about a 100% cpu bound workload of course; we're > talking about typical loads where the cpu is mostly idle, and the > question is whether it is better to spend a bit more time idle, and be > less efficient when actually executing instructions, or be more > efficient at the cost of a little less idle time. If you ignore the > time it takes to enter/exit the deep C states, you can model the > different execution speeds and how much energy they consume as a > simple 100% busy period followed by a fully idle period, with a total > duration being the same in both tests. You're ignoring far too many factors for this to be terribly relevant. -- Matthew Garrett | mj...@srcf.ucam.org -- 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
