Techie question: Centrino vs P4-M

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OK. I can't find anything that I fully understand that explains how a
Centrino with a 1.73 GHz clock allegedly out-performs a P4-M running at
2.4GHz or higher.
Can anyone put it in relatively simple terms?

Re: Techie question: Centrino vs P4-M

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Modern cpu's use "pipelines" to break the execution of an instruction
into small steps called "stages".  The stages let the cpu work on
multiple partially completed instructions in parallel.  

The P4 runs at high clock speeds by having very simple stages.  That
means it needs more stages to completely execute an instruction, so to
keep fully busy, it needs to be working on more instructions
simultaneously.  That's fine as long as the instructions are in
sequence and later ones don't depend too heavily on earlier ones
already being completed.  But real programs have jumps between
instructions, data dependencies between instructions, etc.  Sometimes
the pipeline has to "stall", or pause execution until some needed
result is ready.  

A pipeline with with lots of stages will have more stalls than a
pipeline with fewer stages, which can slow it down.  On the other
hand, using lots of stages makes the stages simpler, letting you crank
up the clock speed, which speeds it up.  How many stages is best is a
complex trade-off involving the hardware technology, the cpu
instruction set architecture, and the definition of "best": "best"
might mean maximizing raw speed without regard to power consumption or
chip cost, it might mean maximizing mips per watt, it might mean
minimizing the chip cost while meeting a specified performance target,
etc.  The P4 still seems to be the raw speed champ (they make them up
to around 4 ghz while the PM currently poops out around 2.2 ghz or
something) while the PM is preferrred for notebooks because of its
power efficiency.

Re: Techie question: Centrino vs P4-M

Paul Rubin wrote:
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OK - thanks for taking the time to post that explanation.  I assume from
what you are saying that the basic principals between the desktop P4 is
very similar to the Centrino but the Centrino is adapted to process
larger stages through its pipelines and as such runs as lower speeds and
the result is that some applications may actually run more efficiently
through this?
This said, overall can you directly compare performance between a P4 and
a Centrino of the same clock speed? I ask because you can buy a P4-M
3.0GHz at less than a 2.2 GHz Centrino which prompted my question. Is it
a lower performer with a great power consumption return?

Re: Techie question: Centrino vs P4-M


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The pipelining hypothesis isn't relevant.  All the processors are
heavily pipelined.

"Centrino" isn't even a cpu...LOL.  It's a -package-, a -marketing-
gimmick; which builds -intel's- wireless lan junk into your laptop; so
you're stuck with it.  (see the whitepaper at Atheros site for full

The CPU used in "Centrino" is a normal Mobile Pentium; which is the
optimum cpu for a laptop.

If you care at all about battery run-time, you DO want a pentium-M
instead of a P4M.   CPU-speed ain't everything...

RAM quantity can make much more difference in -perceptible- speed.

In the vast majority of cases, you will have better overall
performance with, for example, a 1.5ghz cpu and 1024mb ram, than you
would with a 2ghz cpu and 512mb of ram.  This is especially valid for
win2k/xp, which eats ram.

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