Re: Power measurements

["Rafał Bilski" <rafalbilski@interia.pl>]

>>>>> Dear All,
>>>>>
>>>>> I would like to share some power consumption numbers that I have just
>>>>> measured on my VIA C7-M system.  It uses the e_powersaver module and
>>>>> can run at 400MHz or 1.2GHz.  I have measured the power consumption
>>>>> when idle and when running a "while(1){}" program at each speed:
>>>>>
>>>>> [...]
> 
> Here are some more accurate numbers:
> 
>         400      1200
> Idle    10.55    10.91
> Busy    11.47    13.48
> 
> [The first numbers were made with a mains power meter.  The second ones
> were made between the power supply and the motherboard; I used a power
> supply extension cable, cut in half, with some 0.01 and 0.1 ohm series
> resistors.]
> 
> [...]
> 
>>>> The exception is if the processor doesn't automatically
>>>> adjust it voltage when entering idle.  In this case,
>>>> idle power would depend on the frequency of the processor
>>>> when idle was entered -- not because of the frequency --
>>>> but because of the associated voltage.
>>>
>>> Ah, I had wondered about this.  According to sensors, Vcore is always
>>> 1.09 V.  (Hmm, I can't be sure that I'm looking at the right line in the
>>> output of the sensors program, or that it is configured correctly.  But
>>> none of the voltages change.)  Are there any VIA experts reading this
>>> who know what's supposed to happen?
>> % dmesg | grep eps 
> 
> Thanks, yes, I found this after I posted my last message.  Apparently it
> should use 860 mV at 1.2 GHz and 844 mV at 400 MHz.  So I was looking at
> the wrong line when I reported 1.09 V;  0.86 V is reported in another line.
> 
>> But most C7-Eden doesn't scale voltage.
> 
> That is consistent with what I'm seeing, and it's unfortunate.  (You say
> 'most'; do you know of *any* C7 boards that DO change the voltage?  And
> how much difference would it make?  (0.844/0.860)^2 = 0.96, i.e. a 4%
> saving if power is proportional to V^2, which is what how CMOS worked
> back in the days when I understood it.)
I was thinking about processors. Most C7-Eden CPU's have same voltage on 
high and low frequency. Looks like Your's has different voltages. If 
You don't see 844mV at 400MHz then probably VID pins aren't connected 
to VRM on motherboard.
Mode	MHz		Voltage (mV)	Power (W)
P0	1200		860		7
P1	400		844		3
Looks like P0 is much more power efficient then P1. I'm assuming that 
1200 will be 3 times faster then 400MHz. It is what I saw on VIA C3 
processors.
But it is easier to keep CPU cool at 400MHz.
> (And just to be certain: in e_powersaver you write to one register which
> both changes the frequency and asks for a different voltage at the same
> time; so there's no chance of a software change to correct this - right?)
Right.
>>>> back to you question -- 20 - 16W = 4W is on the table;
>>>> and the performance difference between 400 and 1200 is on the table.
>>>
>>> Well, in idle, the power difference is zero and the performance
>>> difference is zero.
>>> When active, on-demand would use the higher frequency anyway.
>>> So it looks to me as if 'on-demand' and 'performance' will behave
>>> identically in terms of both power and performance.
>> If CPU is sleeping a lot even 0,5W can make a difference. But there is
>> time needed to enter 400MHz and time needed to leave 400MHz.
> 
> My understanding of the C7 clock switching (which only comes from a VIA
> powerpoint presentation) is that because it has 2 PLLs it can change
> from one clock to the other much more quickly than other processors
> which have to wait thousands of cycles for the PLL to stabilise after
> changing.
Yes. And C7 is better from P4-M in this because C7 doesn't require 
chipset support. P4-M SpeedStep is only supported on Intel chipsets. 
Unfortunatly I have SiS. HT isn't working either.
> [...]
> Many thanks for your feedback.  My aim is to make this machine as
> power-efficient as possible, which I've achieved mainly by working on
> the PSU (the first one I had was < 40% efficient, now I'm up to about
> 70%), and by using a solid-state disk.  It's getting increasingly
> difficult to find things to improve!
Newer PSU's should have 80%-85% efficiency. But 70% isn't that bad.

You should probably poke some registers in northbridge. I don't 
know much about newer VIA chipsets, but older (CLE266) had many bits 
which could improve power savings - dynamic clock stop to different 
chipset components, ACPI C3 can put memory to sleep and so on. 
Unfortunatly these bits are disabled by default and are never touched 
by BIOS.

Btw. Can You benchmark Your's C7 for me? I would like to compare it to 
my P4-M which is eating much more power. I'm not sure how much exacly 
because I had to change FSB frequency, but something about 25W.
Maybe "nbench"?
> 
> Regards,
> 
> Phil.

Regards
Rafa³




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