When you are doing a locally-originated truly CPU-bound task, "race to idle" does make some sense. But I can think of a couple of caveats. 1) If you care about power consumption, you want to avoid super-power-hungry turbo states, as you get less done per watt-hour than in some of the middle states. 2) CPU usage that is related to I/O (network, disk, video) doesn't necessarily let you go to idle sooner if at all. In this case if you want to minimize power consumption you may want to use middle states a lot. But if you care more about responsiveness or latency than power consumption, you might want to go to a high state anyway; that is why we have tunables -- so we can configure based on the actual priorities for the machine. DCN On 06/05/13 12:17, Borislav Petkov wrote: > On Wed, Jun 05, 2013 at 07:01:25PM +0300, Stratos Karafotis wrote: >> Ondemand calculates load in terms of frequency and increases it only >> if the load_freq is greater than up_threshold multiplied by current >> or average frequency. This seems to produce oscillations of frequency >> between min and max because, for example, a relatively small load can >> easily saturate minimum frequency and lead the CPU to max. Then, the >> CPU will decrease back to min due to a small load_freq. > Right, and I think this is how we want it, no? > > The thing is, the faster you finish your work, the faster you can become > idle and save power. > > If you switch frequencies in a staircase-like manner, you're going to > take longer to finish, in certain cases, and burn more power while doing > so. > > Btw, racing to idle is also a good example for why you want boosting: > you want to go max out the core but stay within power limits so that you > can finish sooner. > >> This patch changes the calculation method of load and target frequency >> considering 2 points: >> - Load computation should be independent from current or average >> measured frequency. For example an absolute load 80% at 100MHz is not >> necessarily equivalent to 8% at 1000MHz in the next sampling interval. >> - Target frequency should be increased to any value of frequency table >> proportional to absolute load, instead to only the max. Thus: >> >> Target frequency = C * load >> >> where C = policy->cpuinfo.max_freq / 100 >> >> Tested on Intel i7-3770 CPU @ 3.40GHz and on Quad core 1500MHz Krait. >> Phoronix benchmark of Linux Kernel Compilation 3.1 test shows an >> increase ~1.5% in performance. cpufreq_stats (time_in_state) shows >> that middle frequencies are used more, with this patch. Highest >> and lowest frequencies were used less by ~9% > I read this as "the workload takes longer to complete" which means > higher power consumption and longer execution times which means less > time spent in idle. And I don't think we want that. > > Yes, no? > > Thanks. > -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
