You will have to work that out for yourself. Different cards would be affected in different ways (in terms of performance). Low to mid range cards may be less affected. Lowering the GPU memory will slow the task down a bit, and the more you lower it the more it will slow, but how much will vary by task type and app (3.1/4.2). You may also reach a point were the GPU becomes unstable, or you may find that performance suddenly drops off (bad balance &/or recoverable errors). Up to ~20% doesn't usually result in a big performance drop, but after that further power savings would probably become insignificant.
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ok, i understand there will be suddenly some surprices i wont expect. ok, i will try.

This will help, but you'Ve got to measure performance as well. Average over at least 10 WUs (calculate something like credits/day). Then return to stock, measure performance again and report absolute power draw. For example if you save 5% electricity but crunch 10% slower, then it's not worth it.

BTW: increasing GPU voltage increases power consumption of the main consumer.. I wouldn't do this just to clock the memory lower :p

MrS
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calulated with official power consumption data at 160 watts.

Taking a look at your PCs.. you might want to try to merge similar ones, which appear multiple times. Makes it easier to look at the right tasks.

And then all we really need is the following data: WU run time + credits granted. We can work out the following numbers for you, no problem. However, if you can't measure the real power consumption, it will be difficult to judge whether downclocking the VRAM was worth it or not. If you could borrow a power consumption measurement device (~15€, became quite popular in the recent years) we could get hard numbers.

MrS
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It's normal that the DRAM frequency doesn't impact overall power consumption much. Otherwise we couldn't get away without actively cooling them :)

At Milkyway one can go to really low frequencies, about 1/3 the stock frequency. There it really starts to matter, saving you about 15 - 30 W per card.

MrS
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On a GTX470 I can save 20W by reducing the GDDR from 1700 to 1300MHz.
I don't know what performance difference it will make, but I'm 25% into a Nathan task so I will work it out later.

- It turns out that it's 6% slower.
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How are you measuring these 25W/GPU, Ratanplan?

BTW: if GPU performance suffers from the reduced memory clock, the GPU will also consume less power ;)

MrS
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Just for reference:
My 20W power saving (from underclocking my GTX470's memory) came at a cost of 6.05% task performance loss. The power usage was measured at the wall socket for the Main System Unit (MSU), running the same tasks (CPU and GPU) under the same settings, only a few seconds after reading it at normal frequency.

Without underclocking the GDDR:
3 NATHAN_RPS tasks at 22,046.59, 22,020.22, 22,031.58

After underclocking the GDDR:
1 NATHAN_RPS task took 23,066 seconds (but had already completed 25% of the task). So basically the remaining 75% took an extra 1000sec, which would mean a full task would take 1333seconds. 23333/22000=1.06 or 6% longer.

My MSU use to use 330W and now uses 310W. So it's now 310/330 more efficient. It just so happens that this is 6.45% which basically means the system isn't any more efficient in terms of tasks completed/Watt; it's just about the same.

If you really accurately measured this and tuned the GPU you might become about 1% more efficient per Watt, but that's about it.

So 'generally speaking' don't underclock here expecting to be more efficient; your just doing less for less electric. If you have multiple GPU's then that 0.4% improvement would increase slightly, but even then your talking about 2 or 3% more efficient and all this ignores the GPU purchase costs!

I completely expected these results, and I know that overclocking results in the same task performance per Watt; you use more energy but crunch faster (when overclocking without increasing the voltage, and up to ~15% OC). Even with a slight V increase and going up to ~20% the performance per watt is fairly similar, so long as you measure at the wall and use one GPU.

BTW my MSU just uses ~78W idle.
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Thanks SK, I think that's as much analysis as we need :)

@Ratanplan: power consumption obviously depends on load. Some variation in backgroud tasks might cause noticeable differences if average GPU utilization was changed. Different GPUGrid WUs achieve different GPU utilization levels, so this will affect power draw as well. And there's always temperature.. if the ambient temperature varied by 10°C during measurements, leakage currents will change measureably.

MrS
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At linear scaling of GPU-Grid performance with core clock one should have expected a runtime reduction from 18k to 12.2k seconds by increasing the GPU core clock from 850 to 1256 MHz. This seems to be a good approximation of what you're actually seeing, although your quoted value is slightly worse than this. I postulate you'll see this reduction to 14k seconds at 2900 MHz memory as well as at 2500 MHz.

MrS
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