Re: OT Dual core CPUs versus faster single core CPUs?
- From: rickman <gnuarm@xxxxxxxxx>
- Date: Fri, 2 May 2008 19:01:45 -0700 (PDT)
On May 1, 5:18 pm, dpl...@xxxxxxxxxxxx (Dave Platt) wrote:
Not trying to decide which is better for everybody. Just interested
in the difference between multiple core CPUs and faster single core
CPUs.
My understanding is that these days, it's possible to get more
computing power per watt using a multicore approach. Going to higher
and higher speeds (per core) requires the use of a smaller and smaller
feature size on the chip, and this can increase static power losses.
Lower operating voltages are required to keep from popping through the
thinner insulating layers on the chip, and this generally means that
higher currents are required, which means that I^2*R losses go up and
overall power efficiency goes down.
I'm not sure why you say the currents have to increase as feature
sizes decrease. The big speed improvement from smaller feature sizes
is the reduced capacitance. So the RC times (assuming the R is
constant) go down and the clock can run faster. Or the current can
decrease yielding much lower power consumption. This is what has been
fueling improvements in IC fabrication for the last several decades.
Where this breaks is when the voltages get so low that the transistors
don't actually shut off entirely resulting in much higher quiescent
currents... AND... the R in the RC starts to increase limiting speed
improvements.
Over the last four or five years processors have gained *nothing* in
clock speed. When I bought my last PC in 2002, the fastest chips from
Intel were clocking at 3 GHz+. The fastest clock speed today??? about
3 GHz! This is a little bit apples and oranges because the
architectures have changed a bit. The P4 in use 6 years ago was
optimized for raw clock speed at the expense of longer pipelines
causing more delays on branches. But the point is still that in 6
years the speed of CPUs due to clock increases is nearly zip.
With ever increasing density from the smaller process geometries, the
question becomes, if you can't speed up the clock, what can you do to
make the CPU faster? They have already added every optimization
possible to speed up a CPU so that just adding transistors won't
achieve much. So the only other way to get more speed is to add more
CPUs!
So that is why we have dual, triple and quad core CPUs now instead of
just making the CPUs run faster.
Using a somewhat lower-technology silicon process with lower losses,
and replicating it several times, can yield the same amount of
computing power at a lower level of energy consumption.
The goal is speed vs. cost. If we are talking about PC type CPUs, it
is never an advantage to use older technology as long as it is not so
bleeding edge that you can't get decent yields. Adding multiple CPUs
is a significant step because of the increase in die area increasing
the cost. But as the process improvements provide more transistors on
the same size die, the only useful way to take advantage of the gates
is to add more CPUs.
For desktop consumers this may not be all that significant an issue.
For server farms, where the electric bill can be a major portion of
the total expense over time, it can make a big difference. For laptop
owners, it may extend battery run-time or reduce battery weight
significantly.
Actually, they have turned the power curve around by using different
structures for the transistors. They are harder to make, but the
added cost is justified by the reduced power consumption. In 2002
CPUs were reaching the 100 W level. None were below about 60 W. Now
you can find CPUs that are only 35 or even 25 W without turning
performance back to the stone age like the Via chips.
.
- References:
- OT Dual core CPUs versus faster single core CPUs?
- From: John Doe
- Re: OT Dual core CPUs versus faster single core CPUs?
- From: Dave Platt
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