Confused on how TDP works AMD

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elec999
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Confused on how TDP works AMD

Post by elec999 » Sat Jul 29, 2006 1:46 pm

Could someone quickly explain to me, what does it mean when Amd says their EE x2 3800+ TDP is 35watts, does this mean it uses 35watts at full load for both cores, or each core uses 35watts x 2, 70watts.
Thanks

MikeC
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Post by MikeC » Sat Jul 29, 2006 2:10 pm

35watts at full load for both cores

elec999
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Post by elec999 » Sat Jul 29, 2006 2:16 pm

MikeC wrote:35watts at full load for both cores
So if you disabled one core, would the TDP remain at 35watts or become 17.5

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Post by MikeC » Sat Jul 29, 2006 2:26 pm

In theory, 17.5W, if it could be done.

Bicster
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Post by Bicster » Sat Jul 29, 2006 2:30 pm

The TDP would not change. It's just a number assigned by AMD. Keep in mind the TDP is not the power consumption of the processor.

I do not think it is safe to assume that the max power consumption would be cut precisely in half by only utilizing one core.

geforce1
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Post by geforce1 » Sat Jul 29, 2006 3:32 pm

A little more info on TDPs:

For Intel, TDP seems to be more of a typical value rather than an absolute maximum. In other words, it's possible to 100% the CPU with certain instruction mixes and not reach the TDP, and on other mixes, exceed it. This is especially true with the highest end model of a CPU line that has the same TDP across the line.

AMD tends to be a little high with their TDP ratings these days. There might be some exceptions (Sempron 2800+ and 3000+ for Socket A), but across a line, the TDP would probably be reached by the highest-end model of a CPU line.

So, Intel = TYPICAL maximum power, and
AMD = ABSOLUTE maximum power

Bicster
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Post by Bicster » Sat Jul 29, 2006 4:44 pm

Unfortunately AMD TDP's are generally stated for a range of processors.

For example - AMD's 65W TDP line of AM2 dual-core chips includes six models from 3800+ to 4800+.

All the chips are spec'd at the same voltages and made on the same process. The 4800+ has more cache (many more transistors) and a 20% higher clock speed than the 3800+ ... so it is easy to conclude that the 3800+ consumes far less than 65 watts at full load. Yet AMD has given it the same TDP.

I would be really nice to see some real measurements.

Bicster
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Post by Bicster » Tue Aug 01, 2006 7:40 am

I did some calculations to satisfy my curiosity.

I believe the 65W TDP X2 3800+ (ADO3800CUBOX) has an actual max power of around 47W. If you can undervolt it 0.1V it is 40W. This is based on calculating Tcap from the ADD3800CUBOX specs and adjusting the voltage for the other version. This chip seems to be available for ~ $200.

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Post by MikeC » Tue Aug 01, 2006 8:13 am

Several articles for more info on the topic of AMD X2 power demands:

Desktop CPU Power Survey, April 2006
Athlon 64 for Quiet Power -- includes details on the different ways that TDP is specifed by AMD and by Intel.
AMD A64 X2 TDP Poll

Bicster
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Post by Bicster » Tue Aug 01, 2006 8:26 am

Those are good resources, but no substitue for actual measurements from AM2 EE processors :D

I wish I didn't need AM2 - but I am building servers for the sole purpose of running Xen, and it would be nice to be able to throw a few instances of Windows on there.

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Post by MikeC » Tue Aug 01, 2006 8:29 am

Bicster wrote:Those are good resources, but no substitue for actual measurements from AM2 EE processors :D
They're coming. 3800x2 35W & 4600x2 65W. First round of testing on four different AM2 boards done thus far. Next step is to check undervoltability.

Peek preview: Thus far, the 3800x2 pulls less than 40W from the 2x12V plug on all the boards. That means if VRM efficiency is 80%, it's really pulling <32W. Will see if undervolting can pull it down to <30W.
Last edited by MikeC on Tue Aug 01, 2006 8:39 am, edited 1 time in total.

Bicster
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Post by Bicster » Tue Aug 01, 2006 8:32 am

Thanks MikeC. Have your PR contacts at AMD given any idication as to when these products will actually be on the market?

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Post by MikeC » Tue Aug 01, 2006 8:40 am

Bicster wrote:Thanks MikeC. Have your PR contacts at AMD given any idication as to when these products will actually be on the market?
AFAIK, they are already on the market. Also see edit of last post for peek preview.

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Post by Bicster » Tue Aug 01, 2006 8:43 am

Nobody has the EE SFF chips in stock. A few places list it as pre-order for $400 ... as they have for months.

Thanks for that preview :)

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