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Old 2012-08-30, 21:50   #1
ixfd64
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Default TDP vs. actual power consumption with Prime95 running?

How does the power consumption of a CPU at full load (e.g., running Prime95) compare to its TDP in general? I know the TDP is the maximum power the computer is required to disperse, and that it is rarely exceeded under normal conditions. However, I'd expect a computationally intensive program like Prime95 could push the CPU power usage to near the TDP. Does anyone here have any figures for comparison?

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Old 2012-08-30, 22:12   #2
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I don't know if this helps or not. The attached are CPUID Hardware Monitor captures. These numbers are for a Phenom II x6 1090T running at 3.2GHz, its stock speed. P95 (Win7-64) is running 2x P-1. mfaktc is running in 4 instances on a GTX 570 with dedicated CPU cores. CUDALucas is running on a GTX 460.

I think the rated dissipation is 125W. The whole system (box only, no monitor) is drawing ~640W from the line at full load. Of course the majority of that is feeding GPUs.
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Old 2012-08-31, 01:57   #3
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TDP is not the same as maximum power draw.

I have not measured it, but I would expect that P95 {w|c}ould easily exceed the TDP in most systems.

When running P95 it is probably a good idea to augment a TDP-rated cooling system capability by at least 50% (figure pulled from thin air so adjust as needed for your particular system).
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Old 2012-08-31, 02:26   #4
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Hmm, I was under the impression that it would be hard to exceed the TDP just by running Prime95. For example, a Tom's Hardware team ran four instances of Prime95 on a QX9650 and measured a power usage of only 74 watts (compared to its 130 W TDP). I wonder how they did it?

I have another (dumb) question. I've noticed that some server chips have similar TDP ratings to that of consumer chips despite having more cores, and I guess a reason for this is that server chips often have a lower clock speed compared to their mainstream counterparts. However, it also seems that many server CPUs (at least the high-end Xeons) have no integrated GPU. Would the absence of an on-die GPU allow more CPU cores in its place, or are the power savings negligible in this case?
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Old 2012-08-31, 02:36   #5
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According to this article AMD and Intel define TDP differently.

There are many things that can affect power dissipation levels. Voltage, frequency and workload are the main culprits. Generally power dissipated is proportional to frequency cubed. This is because current levels are proportional to voltage squared and linearly to frequency. But as the frequency increases so does the voltage to compensate. So in summary we have:

Watts = a + V2f

And: V = b + cf

Where a, b and c are constants specific to each chip.

Therefore: Watts is proportional to f3
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Old 2012-08-31, 02:45   #6
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Quote:
Originally Posted by retina View Post
According to this article AMD and Intel define TDP differently.
Umm... that's from 2004. It's likely that at least one of them uses a different definition now.
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Old 2012-08-31, 02:58   #7
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It is probably also worth mentioning the thermal runaway effect. As a chip gets warmer the transistor leakage increases. So even if all others things are kept constant the power levels can increase simply because the chip is getting hotter. Usually this is not particularly noticeable at lower temps but as the chip starts to get over 80C-90C (approximate values here, the actual knee point depends upon the process used) thermal runaway effects become very important. Because of this, even a small change in cooling effectiveness can have a larger effect on chip temperatures and power consumption.

tl;dr - Keep your chip cool and save power

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