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SubjectRe: cpufreq on demand governor sampling rate restricted to HZ even on NO_HZ kernels
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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



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