From: "Phil Endecott" Subject: Re: Power measurements Date: Sun, 24 Jun 2007 23:14:18 +0100 Message-ID: <1182723258692@dmwebmail.belize.chezphil.org> References: <467DB356.7050309@interia.pl> Mime-Version: 1.0 Return-path: In-Reply-To: <467DB356.7050309@interia.pl> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: cpufreq-bounces@lists.linux.org.uk Errors-To: cpufreq-bounces+glkc-cpufreq=gmane.org+glkc-cpufreq=gmane.org@lists.linux.org.uk Content-Type: text/plain; format="flowed"; charset="us-ascii" Content-Transfer-Encoding: 7bit To: cpufreq@lists.linux.org.uk Rafal Bilski wrote: >> Len Brown wrote: >>> On Saturday 23 June 2007 08:54, Phil Endecott wrote: >>>> 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: >>>> >>>> 400 1200 >>>> Idle 16W 16W >>>> Busy 17W 20W 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.] >>>> What I find interesting is that the idle power at 1.2GHz is identical >>>> to that at 400 MHz. > It isn't. >>>> This makes me wonder if the on-demand governor would actually save >>>> any power at all. > There is a difference of 0,5W~1,0W max. Agreed! >>> 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.) (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?) >>> 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. > In real life "performance" governor will use less power. Well, if that's true then I think we ought to discourage people from using 'ondemand' with the C7. 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! Regards, Phil.