[Ham-Computers] RE: Processor comparison

Wed May 30 22:11:01 EDT 2007

Oooh, this might be a long post.  I'll summarize on top and go more into detail below. 

>>> Dave wrote:
Gateway is replacing my defective PC with a new one. On the surface, the new processor appears to be slower than the old one, but I seem to recall that a P-IV would be faster than a P-III if they were both rated at the same speed.

and

Would someone be able to tell me what, if any difference I might see in going from 3.4 Ghz Pentium-4 to a 2.0 Ghz Core 2 Duo

>>> My quick reply:
In most respects (all other things being equal), the 2GHz Core2Duo will be as fast if not faster than the 3.4GHz P4, but not always.

>>> My longer reply:
First, a bit of history.  This description is not exact, but I think it will get the point across...please don't quote me on specifics!

The Pentium-III (3) processor was a very efficient design - more efficient than even Intel realized.  Intel already had the P4 in the works when the P-III was released - the P-III was based on the "old" P6 architecture (Pentium Pro & Pentium-II).  The P4 architecture was quite different from the P-III architecture in that it was designed to specifically take advantage of higher clock speeds.  It had "deeper" pipelines that was able cache more information so the "core" would always be fed.  The pipelines were so deep that Intel was able to eventually have the core handle more than one thread (Hyper-threading) as long as the pipes weren't saturated.

Ahhh, but the P4 eventually hit a snag...4GHz became a physical barrier due to technology (and the extremely deep pipes were partially to blame).  The fastest commercial P4 is about 3.8GHz - and at that speed, it is a huge power hog...power consumption is the biggest drawback to high clock speed.  The P4 processors were power hungry from the start and this wasn't a good thing for laptops.  So Intel went back to the P-III architecture, did some tweaking, and found that an optimized P-III running at 2.0GHz was "faster" than a P4 running at 2.8GHz in all but a few situations.

HUH???  How can a 2GHz processor be *faster* than a 2.8GHz processor???  Well, the Pentium-M, as it would be called, was able to process more instructions in fewer clock cycles.  Some of the optimizations that allowed this were a deep pipeline (borrowed from the P4, but not as deep), a better branch prediction unit, and, very importantly, a LARGE L2 cache memory with extremely low latency.  The L2 cache latency of the P4E processor is 23 cycles - the Pentium-M has an L2 cache latency of 10 cycles.  As you can see, if the app fit in the L2 cache, the "run speed" will be more than twice as fast on the Pentium-M just due latency (it takes less than 1/2 as long to fetch each instruction).  For reference, there are several other optimizations from the original P-III CPU not mentioned here.

OK, are we all following so far?  The Pentium-M is essentially faster than an equivalent clock speed Pentium-4...on average 50% faster.  The "Core" series processors were next and they further optimized on the Pentium-M design.  The "Core 2" is the next evolution of the same design with even more optimizations (mostly power savings, but a bit faster too).

Ahhh, but I did say faster in *most* situations...not all.  There are some things where clock speed is king - such as memory bandwidth.  Memory bandwidth is dictated by clock speed and RAM latency, so higher clock speed equals higher memory bandwidth.  So, the higher speed P4 will be faster than a lower speed CPU.  Of course, if you clock a Pentium-M/Core/Core2 CPU to match a P4, then the memory bandwidth will be the same (given the same type of RAM) - but the P4 will then be completely blown away by the Pentium-M/Core/Core2's processing power.

So, the long answer is still the same...the 2.0GHz Core 2 Duo will, for most intents and purposes, be faster than the 3.4GHz Pentium-4 in everyday computing.  On top of that, you'll have a second "core" to use if you run multi-core aware applications.

NOTE:  The Pentium-M is not to be confused with the Pentium IIIm, Pentium 4M, or MP-4M processors.  These are all lower-power variants (designed for laptops) of the original Pentium-III and Pentium-4 processors.  The Pentium-M is technically an "evolution" above the Pentium-III and Pentium-4 processors.

There are lots of articles on the 'net that can give you more details on the evolution of the "P6" family of processors (Pentium Pro, II, III, M, Core, Core 2).  The Pentium-4 is technically the "P7", but it's architecture is now dead due to is need for absolute clock speed.  Some of it's legacy (such as the fairly deep pipes) will be carried on by the "Core/Core 2" series processors, but it's essentially RIP.

That's my story and I'm sticking to it...unless someone has a correction to make <g>.

73,

  - Aaron Hsu, NN6O