Re: Should Core 2 Duo CPU not be theoretically slower than single core with double MHz?



shawn wrote:
If we previously had Pentium 3.8Ghz processors, then why can't they just make 3.8Ghz dual or quad core processors? Why are they always at such low speeds?

Is size really a factor? Can't they just make the chips an inch or two larger to accomodate or will heat be the issue?

This is a question best asked in a computer architecture group, as there
are multiple factors at work.

*******

Much has changed over the years. The Prescott generation was the last
generation with what appeared to be a serious leakage problem. 25% of
the power used, was just wasted, and did no useful work. This could
not continue. The switch to 65 or 45 or 32nm could not fix the leakage
issue, which would only get worse. With each generation, the transistors
had to be redesigned, the material science changed, to make the
scaling of the transistor worthwhile. If a Prescott transistor was
just made smaller, there'd be a smoking hole in the ground.

As I understand it, more complicated structures are used now for the
gates. Some transistor structures have stuff added, to reduce or eliminate
leakage. When absolute speed is needed, perhaps a small percentage of the
structures still leak, in the interest of getting the most speed. So they
pick and choose, in the interest of reducing the leakage of the processor
to manageable levels. I think they've done an admirable job, based on
my own power measurements (my 65W processor uses 36W max, on a 65nm
Core2 Duo).

The pipeline length on the new processors is shorter. They really
couldn't continue on the "Prescott" path, because AMD was kicking
their ass. The result is

lower frequency
shorter pipeline
able to retire more instructions per clock cycle (parallelism inside a core)
multiple cores (only really impressive, for software that uses it well)

The thing is, technology exists to do really silly things, but for the
retail price point, you'd be saying "no thanks, I'd rather buy a car".
The current delivered technology is cheap and powerful, so stop complaining :-)

Technology exists that can run at 40GHz. You could build a processor with it
(if say, you had a bar bet with a buddy). No memory subsystem could reasonably
supply it with information. But it would make a great room heater.

(In the following article, some of the references to frequency, are for
Ft of the transistor. It takes multiple transistors to make a flip-flop
storage element. And the operating frequency of one of those, will be
lower than the numbers seen in this article. This is just to show that
there is stuff other than CMOS available to build circuits. The fiber
optics criss-crossing the nation, rely on some of the following
technology, to work at incredible speeds.)

http://en.wikipedia.org/wiki/Heterojunction_Bipolar_Transistor

http://www.ntt.co.jp/news/news06e/0609/060929a.html

"InP ICs, which can be operated at over 50 GHz were used in
multiplex and demultiplex circuits..."

HTH,
Paul


"Jason Stacy" <jjstacy@xxxxxxxxx> wrote in message news:4a14fb76$0$32678$9b4e6d93@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I wonder why a two-core CPU with lets say 2 * 1.7 Mhz is theoretically faster
than a single core with 3.4 MHz. I am NOT talking about additional features like
Pipeling and Hyperthreading but the core fact that the power is split over two cores.

Assume the following situation: A NON-THREADED application needs as much CPU power as possible
for some computations. On a 3.4 Mhz machine it can occupied almost 99% of the CPU power
(remaining 1 % are for system services).

When I run the same application on a two core system then it can occupy only ONE of the two cores
with 1.7 MHz. Because it is non-threaded it cannot request the other core as well.
So it must be slower (given all other side-conditions are equal).

Am I wrong?

J.



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