On Fri, 2009-01-30 at 06:59 -0800, Thomas Renninger wrote: > Hi, > > depending on HZ set to: > > 100 > 250 > 1000 > > the ondemand governor is currently limited to poll the CPU load > and adjust the frequency (sampling rate sysfs variable) every: > > 200ms > 80ms > 20ms > > This limitation does not consider NO_HZ which looks wrong? > If this is correct, can someone give me a pointer, I'd like > to understand why. > That is wrong. ondemand sampling_rate should not limit the sampling rate based on HZ when NO_HZ is configured. The idle statistics is not limited by HZ rate with NO_HZ, as we will have idle microaccounting. > If NO_HZ can/should go down to 20ms polling and more (current > CPUs are able to switch fast enough, so that the ondemand governor > would calculate the default polling interval below 80ms for them), > this would hurt in respect of C-states at some point. > > For performance reasons, one wants to poll as much as possible, for > powersaving reasons (C-states), one wants to poll as seldom as > possible. > > I wonder whether it makes sense to dynamically adjust the polling > interval (e.g. by a hint (and initial wakeup) from the scheduler or > taking C-states into account) to: > - increase the sampling rate, e.g. based on context switching > activity > - lower sampling rate when the system is idle (to gain > full C-state efficiency) > Or in what other way deep C-states could be taken into account > in respect of ondemand polling? > ondemand polling uses deferrable timer and hence will not be called frequently on a totally idle CPU. The main reason we did not do the dynamic sampling_rate is because it increases the ondemand response time with a sudden increase of load, which is not liked by most workloads. Thanks, Venki -- 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
