Power efficiency, UNDERclocking a Tbred 1700+, motherboards?

[cmetz]

One odd project I'm working on is to build a lower-power server PC. It doesn't need to be the fastest box in the world, but I want to get as much as I can per unit electricity (since it'll be on all the time, and electricity is cheap but not free).

I was originally looking into the Via C3 line, but a lot of things about those chips make me nervous (compatibility, lack of a lot of information). In my research, it sure seems to me that if I could take an Athlon XP .13u chip and underclock it down to similar speeds as the C3, I should be able to get similar power consumption and better performance (that is to say, a Tbred underclocked to 1GHz should be able to use a lot less than normal power, and should be in the ballpark of a C3 1GHz). In order to do this, it is very important to be able to undervolt as far as possible (that whole voltage-squared thing), as well as down the multiplier. Presumably this would require a chip that's already unlocked, or unlockable, which is primarily why I'm on the AMD page (my impression is that the Intel chips are harder if not impossible to unlock, while the AMD chips appear to be pretty darn easy to unlock, and it sounds like many Tbreds are now unlocked to begin with).

What motherboards would folks recommend for doing this? I currently have a Palomino(? - .18 XP) 1900+ and a MSI KT333 (I think KT3?) motherboard. The MSI BIOS defaults to 1.750 volts and won't let me lower it beyond 1.725, though it gives me plenty of options for overvolting. I expect this to be a common problem -- boards with the tweaks have been designed for OVERclocking, not UNDERclocking. The TBreds' default Vcores seem to be around 1.6V, lower to start, but I'm going to need to lower it more along with the clock speed to realize an interesting power savings.

Other suggestions are welcomed.

 

[cmetz]

Because the question is bound to come up... a Tbred 1700+'s typical power consumption is 44.9W says the chart I have, and I'm hoping to end up at around half of that, or circa 22W. Okay, that's a lot more than the 5.7W typical on a C3 1GHz. But it's still a much better power consumption per unit speed than the full 45W.

 

[Adul]

have u look at the movile athlon XP? maybe u can really undervolt those dunno just guessing

 

[cmetz]

I have looked at the mobile Athlon XP - it has a much lower voltage rating to begin with, and thus should work a lot better. AMD does not make power consumption and compatibility information on the mobile chips available without NDA, and the AMD support people I asked about this were not particularly polite about it either (that's okay, AMD's making huge profits and doesn't need the business of enthusiasts like me to buy their more expensive chips, right? Oh, wait... )

Has anyone here actually used a mobile Athlon XP chip in a desktop system? Does it work ok? In particular, do desktop socket-A motherboards provide the right lower voltages? Of course, cost and availability are still issues with that approach.

I know that if I really wanted lowest power, just buying a laptop would probably be the best solution. What I'm actually looking for is low power and desktop flexibility on a budget. I want it all, of course

 

[KF]

You can undervolt Athlons easily by cutting the bridges which set the voltage. Its simple compared to joining. If you have a ceramic mounted AMD, neither is hard. Lots of info about this on the web, although they assume you want to overvolt of course. I really don't think you can undervolt a lot, because the voltage also sets the operating point of the chip circuits and the designers try to get this as low as they can.

I'm told file servers don't require much of a CPU, and not floating point. It is mostly data movement. If this is just for your home using one 100 megabit NIC, low CPU speed shouldn't matter. Cyrix power consumption is low enough that it can be used without a fan (supposedly.)

Power consumption is proportional to speed. Running the CPU at 500MHz consumes 5/14 =.36 as much power as 1400MHz. Athlons can set a 5x multiplier and a 100MHz bus.