linux-kernel - Re: cpufreq on demand governor sampling rate restricted to HZ even on NO

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Date:	Fri, 30 Jan 2009 09:28:16 -0800
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	Thomas Renninger <trenn@...e.de>
Cc:	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: cpufreq on demand governor sampling rate restricted to HZ even
	on NO_HZ kernels

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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