Re: [RFC PATCH 06/16] arm: topology: Define TC2 sched energy and provide it to scheduler

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > Voltage is combined with frequency, roughly, voltage is 
> > > proportional to freuquecy, so roughly, power is proportionaly to 
> > > voltage^3. You
> > 
> > P ~ V^2, last time I checked.
> 
> Yes, that's a good approximation for CMOS gates:
> 
>   The switching power dissipated by a chip using static CMOS gates is 
>   C·V^2·f, where C is the capacitance being switched per clock cycle, 
>   V is the supply voltage, and f is the switching frequency,[1] so 
>   this part of the power consumption decreases quadratically with 
>   voltage. The formula is not exact however, as many modern chips are 
>   not implemented using 100% CMOS, but also use special memory 
>   circuits, dynamic logic such as domino logic, etc. Moreover, there 
>   is also a static leakage current, which has become more and more 
>   accentuated as feature sizes have become smaller (below 90 
>   nanometres) and threshold levels lower.
> 
>   Accordingly, dynamic voltage scaling is widely used as part of 
>   strategies to manage switching power consumption in battery powered 
>   devices such as cell phones and laptop computers. Low voltage modes 
>   are used in conjunction with lowered clock frequencies to minimize 
>   power consumption associated with components such as CPUs and DSPs; 
>   only when significant computational power is needed will the voltage 
>   and frequency be raised.
> 
>   Some peripherals also support low voltage operational modes. For 
>   example, low power MMC and SD cards can run at 1.8 V as well as at 
>   3.3 V, and driver stacks may conserve power by switching to the 
>   lower voltage after detecting a card which supports it.
> 
>   When leakage current is a significant factor in terms of power 
>   consumption, chips are often designed so that portions of them can 
>   be powered completely off. This is not usually viewed as being 
>   dynamic voltage scaling, because it is not transparent to software. 
>   When sections of chips can be turned off, as for example on TI OMAP3 
>   processors, drivers and other support software need to support that.
> 
>   http://en.wikipedia.org/wiki/Dynamic_voltage_scaling
> 
> Leakage current typically gets higher with higher frequencies, but 
> it's also highly process dependent AFAIK.
> 
> If switching power dissipation is the main factor in power use, then 
> we can essentially assume that P ~ V^2, at the same frequency - and 
> scales linearly with frequency - but real work performed also scales 
> semi-linearly with frequency for many workloads, so that's an 
> invariant for everything except highly memory bound workloads.

So in practice this probably means that Turbo probably has a somewhat 
super-linear power use factor.

At lower frequencies the leakage current difference is probably 
negligible.

In any case, even with turbo frequencies, switching power use is 
probably an order of magnitude higher than leakage current power use, 
on any marketable chip, so we should concentrate on being able to 
cover this first order effect (P/work ~ V^2), before considering any 
second order effects (leakage current).

Thanks,

	Ingo
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