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Re: 10Ghz CPUs

 
 
Paul
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      11-30-2008
John wrote:
> Why don't we have 10Ghz CPUs as standard in computers today?
>
> We seem to have been stuck around the 3Ghz mark give or take 13% or so
> for some time now. Why haven't processor speeds gone upwards for
> consumers? And why do we continue to see more and more multi-cores
> coming to the market?
>
> Surely at some point you have to say enough is enough and there is
> only so many cores any individual is going to need for a system? Some
> people may only need one core and just require much better speeds than
> 3.4Ghz. We seem to be stuck in a speed bottle neck.
>
> Anyone who knows how long it's going to be before we see a jump in
> processor speeds I'd love to know.
>
> Thanks,
>
> John


Years ago, my company had in-house technology that ran at 10GHz.
I was sent on a CAD course, to design using that technology, but
no need ever arose to use it (at least for me).

What limitations did it have ? The chip could contain about 2000 gates.
Modern processors have hundreds of millions of transistors in them.
So the difference in capacities is quite striking. Even a
relatively simple design, like a PowerPC core (sometimes
embedded "for free" in some FPGA technologies), would be
too much for a single chip of the 10GHz technology.

One of our competitors had a version of that technology,
that might have run some kinds of circuits at 40GHz. But
again, it would have similar limitations. You can only
do the simplest of data transfer functions with stuff like
that. And it also ran hot. You'd be surprised how high
a voltage it needed to operate.

The transistors inside those devices, are relatively large
by comparison to Intel's current 45nm designs. I specifically
asked, while taking my CAD course, whether the company would be
able to scale the transistors down, allowing larger density
chips to be built. The answer I got, is they were as small
as they were ever going to get.

Now, assume they were wrong, and with the usual Moore's law
improvements, a smaller transistor could be made. Say there
were enough transistors for a simple RISC processor.

If the core runs at 10GHz, it has to be fed by a memory of
some kind. The ratio between core speed, and memory subsystem
speed, is already ridiculous. It is made tenable, by more
than one layer of caching. The size of each level of caching
has some threshold, below which things really start to slow
down. There is no point running at 10GHz, if you have to
wait 1000 cycles for something to come from some level of
cache. Like, if I wanted 32KB or 64KB of cache for an
L1, again, it might take multiple chips just to give me
that much cache. Then I'd need microwave wiring to connect
the chips together. (Driving a 10GHz signal through
FR-4 is not easy.)

The beauty of CMOS, is it has continued to scale for so many
years. At one time, the features were large enough, you
could view them with a child's microscope or even a
magnifying glass. It's a miracle that it made it
to 3GHz. I can remember a time, when if you said
"3 GHz", the instant thought process would be
"that'll take GaAs". But now, CMOS routinely fills
that role.

There is still a usage for the ultra-high speed technologies.
For example, to operate high speed fiber optic trunks between
cities, the signal coming from the receiver, has to be
reduced in speed and made more parallel, to be handled
by other layers of circuitry. For those applications,
some of that 100GHz Ft technology is still the right answer.

HTH,
Paul
 
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