Why do the CPUs on dual-processor Mac Pros...

...only go up to 2.93GHz (6-core), when the single-processor version has an option for a 3.33GHz (6-core) CPU?

Is it some form of technical limitation - where the logic board and/or OS can't handle more than 35.16GHz combined processing speed - or is it simply that there's never been enough call for 39.96GHz total, even in a server, to make it worth offering the option?

Doesn't really matter, obviously - as a poor student it'll be the better part of a decade before I could afford something as beastly as even the lowest-end single quad-core processor Mac Pro! - I'm just curious...

I think you're confused as to what a "core" is and what "processor" speed is. For example, having two 2.0GHz processors doesn't mean you have a total of 4.0GHz of processing speed. Neither does a core. I even don't really understand all of it myself, but I know the Mac Pro doesn't have a total of 39.96GHz!

MacDude121 said:

I think you're confused as to what a "core" is and what "processor" speed is. For example, having two 2.0GHz processors doesn't mean you have a total of 4.0GHz of processing speed. Neither does a core. I even don't really understand all of it myself, but I know the Mac Pro doesn't have a total of 39.96GHz!

Click to expand...

Oh, I know those cores run in parallel, not in series, meaning that the actual clock rate within the chip never exceeds the basic quoted figure.

But - to use an analogy I've seen quoted elsewhere - just as two trucks travelling at 60 mph will be able to move twice as much stuff a mile down a highway in a minute as a single truck could do in the same time, similarly a 2GHz dual-core processor will be able to crunch twice as many numbers per second as a 2GHz single-core processor.

Thus, the simple formula of "clock speed times number of cores" does give a good sense of the effective speed of multi-core processors.

And, regardless of the actual relationship between numbers of cores and/or number of processors versus overall speed; it is still obviously the case that the faster and/or greater number of processors one has, the faster one's computer will be.

So, my question remains; if you can get a dual-processor Mac Pro with two 6-core 2.93GHz processors (i.e. one capable of handling & making full use of 12 cores, regardless of their speed), why can't you get the same machine but with two 6-core 3.33GHz processors?

Granted it would "only" be around 14% faster, but that still could be worth it for some very processor-intensive tasks (in media studio render farms or grid-based supercomputing applications, etc.)...

There is more to it than just clock speed. Cache size, die size, QPI link speed, they all contribute to the overall performance of the processor.

As far as cores are concerned, the software needs to be able to take advantage of multi core. There are plenty of programs that can use 2 cores, but not 4 so in that case, the extra 2 cores don't contribute to overall performance.

Since, multi-threaded applications are still not as mainstream as you would think, it's benefitial to Apple to offer fewer cores at higher speeds for cost benefit reasons.

Also,the higher clock speed produces more heat and may not be capable of running in the config you want simply due to excess temp.

-MikeM

MikeM said:

There is more to it than just clock speed. Cache size, die size, QPI link speed, they all contribute to the overall performance of the processor.

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Well, like I said, clock-times-cores gives a "sense of the effective speed"; more specifically when comparing various versions of the same type of processor, as is the case here...

MikeM said:

Also,the higher clock speed produces more heat and may not be capable of running in the config you want simply due to excess temp.

Click to expand...

I'm thinking this is the most likely technical explanation for my original question.

Thank you

The technical reason is that the W3680 (that 3.33Ghz chip) can only be used in a single CPU configuration. It's also only $1000 per 1,000 in price.

The X5670 (the 2.93 Ghz chip) can be used in a 2 CPU configuration. It is a $1,440 per 1,000 piece of silicon. It runs almost 10 degrees hotter than the above 3.33 chip.

The W3680 is not a competitor of the X5670.


This is an example of the faiiling of most folks (just due to a lack of knowledge) only comparing speed and not comparing the part number and the specs of the chips they're purchasing in their computers. Apple typically is not using the best chips available. But they are quite often using higher end chips than other manufacturers, even when you have matching speeds.

Comparing the 2 above chips against each other would be almost like comparing a Celeron against a P4.


(The other reason, no manufacturer can afford to offer/stock every single chip Intel makes in every series. The tough choice has to be made at some point and Apple typically seems to narrow the available chices down to only 2 or 3 chips max per machine. They're not interested in being the next Dell, where you can configure the same machine in $5 increments all the way from $500 to $3000.)

So, that's the technical reason...

Fair enough, I was just asking...