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why don't video cards overclock as well as CPUs - Page 2

post #11 of 18
Quote:
Originally Posted by TurboHertz View Post
Graphic cards have like, hundreds of cores, CPU's have like, 12 at the most. Imagine trying to run those hundreds of cores at 3 or even 4GHz, bad idea.
Corrected.
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post #12 of 18
All good points.

There is another also, or at least an extension on these points.

In an effort to compete, nVidia has pushed the limits of engineering margins to an extent that caused overheating problems, which provoked a counter response from ATI to do something similar.

Intel's manufacturing technique is superior at producing high speed transistors, and with very high IPC on their processors, they have very comfortable margins for engineering their stock clock values while still producing highly competitive products. AMD's current 45 nm process doesn't reach the same speeds as easily (though both can produce 4+Ghz), but as AMD's IPC isn't as high, they already assume somewhat lesser margins than does Intel.

When circuits are more complex, the upper speed at which they can be clocked is limited. A crude example goes like this: Circuits which perform simple tasks may require as many as 200 to 300 transistors to work in unison to complete a task, like addition in a general CPU. Counterparts in GPU's, on the other hand, perform more complex work with multiple operands, requiring perhaps ten times as many transistors to complete the work, and as each cascade of circuit complexity completes their transitions from one logic state to another, more real time is required to complete the work, making the upper clock limit much slower (while the total work accomplished is actually much higher).

Standard CPU's are expected to perform work analogous to:

c = a + b or c = a * b

Whereas GPU's are expected to solve

x = xa * ma + xb * ma
y = ya * mb + xb * mb
z = za * mc + zb * mc

As a single operation.

The collection of circuits required to complete the former are much simpler than those required to complete the latter, and that's exactly what is meant when sLowEnd said they're different architectures.

Even the layout of a circuit can cause changes in the upper speed at which the whole circuit can operate.

If memory serves, the total transistor count in the modern CPU's hovers around 1 billion, while some of the GPU's approach 3 billion.
Edited by JVene - 6/11/10 at 7:16am
post #13 of 18
Quote:
Originally Posted by purpleannex View Post
Corrected.
Most consumer CPUs.
    
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post #14 of 18
It's not really the core count though.

It's the circuit complexity given a particular fabrication node.

Anyone remember the new Pentium 90's introduction?

There were 486 CPU's at 100Mhz which couldn't match a P90, not even close.

Or, from the P90 through the P4, the floating point unit was clocked at 1/2 the speed of the processor (AMD's K6-2 was I think 3/5th's or something).

These, too, were related to the complexity of the circuit, not the number of cores, at that particular manufacturing node.

Imagine 5 transistors in a circuit. Transistor 1 feeds Transistor 2, which THEN feeds #3, and when it's finally turned on feeds #4, then that feeds #5...the timing of the clock has to be long enough for that entire chain of cascading logic states to complete.

Now, if the circuit complexity increases so there are 20 transistors involved, the total time required is 4 times as long if the transistors all have the same architecture, so the clock speed must be slower.
post #15 of 18
So if I dumb this down a bit and put in simpler terms...

A CPU is like a nice straight multi-lane highway, to get things moving faster simply just bump up the speed limit; the road is more forgiving to higher speeds.

Whereas a GPU is like hundreds of backroads. Sure you can get a hell of a lot more cars onto those roads, but speeds are limited because of so many more bends in the roads (i.e. calculations).
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post #16 of 18
Quote:
Originally Posted by TurboHertz View Post
Most consumer CPUs.
Then it would be 6, not 4.
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post #17 of 18
Ignore this
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post #18 of 18
Ignore this
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