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From: HeadCase on 1 Apr 2008 04:38
Hi all

Can someone please explain the quad core bit?
My understanding of the dual core was that there were effectively 2
"thinking-bits" run side by side to give the equivalent processing power of
the previous single core pentium chip. Therefore, as the clock speed
approached 4GHz these suddenly became 2 x 2GHz cores. Presumably the
sharing of the workload between the dual cores is controlled via software
somewhere.
Having bought a couple of dual core machines since their introduction, the
performance seems in tune with the above argument.

Now I am looking at the Dell Precision box and see that quad core Xeon
processors have been thrown into the mix.
The processing speed stated with these is also 2GHz. Does this mean that
the quad core will be approximately twice the speed of the dual core fitted
with 2GHz processor?
Don't want to go mad and over specify, but I am looking for a capable CAD
machine nonetheless.

Thanks in anticipation

Phil


From: Calab on 1 Apr 2008 05:35

"HeadCase" <phil(a)dropthespam.com> wrote in message
news:W5CdneqtidThbmzanZ2dnUVZ8q2dnZ2d(a)eclipse.net.uk...
> Hi all
>
> Can someone please explain the quad core bit?
> My understanding of the dual core was that there were effectively 2
> "thinking-bits" run side by side to give the equivalent processing power
> of the previous single core pentium chip. Therefore, as the clock speed
> approached 4GHz these suddenly became 2 x 2GHz cores. Presumably the
> sharing of the workload between the dual cores is controlled via software
> somewhere.

No...

A 4Ghz single core CPU runs twice as fast as a 2Ghz dualcore CPU.

A 2Ghz dual core CPU can only do a single task as a 2Ghz single core. Where
a dual core shines is having multiple tasks going on at the same time, which
makes the computer more responsive to the user.

A 2Ghz quad core CPU can do four tasks at the same time, but won't do single
tasks faster than 2Ghz.

Choosing which is better all depends on what you use your PC for.


From: Paul on 1 Apr 2008 06:05
HeadCase wrote:
> Hi all
>
> Can someone please explain the quad core bit?
> My understanding of the dual core was that there were effectively 2
> "thinking-bits" run side by side to give the equivalent processing power of
> the previous single core pentium chip. Therefore, as the clock speed
> approached 4GHz these suddenly became 2 x 2GHz cores. Presumably the
> sharing of the workload between the dual cores is controlled via software
> somewhere.
> Having bought a couple of dual core machines since their introduction, the
> performance seems in tune with the above argument.
>
> Now I am looking at the Dell Precision box and see that quad core Xeon
> processors have been thrown into the mix.
> The processing speed stated with these is also 2GHz. Does this mean that
> the quad core will be approximately twice the speed of the dual core fitted
> with 2GHz processor?
> Don't want to go mad and over specify, but I am looking for a capable CAD
> machine nonetheless.
>
> Thanks in anticipation
>
> Phil
>

There is no real magic involved here. There are a couple levels
of parallelism.

A user can do four things at once. That would be program parallelism.

If four programs run to completion on a single core, it looks like this.

<-----A-----> <---B----> <-----C-----> <----D----->

If I run them on a quad core, then the run time looks like this. This
represents an unusual level of program parallelism, in the sense that
most people cannot keep this up all the time. (I've read some comments
from Mac users, and the comment "I hardly ever see the fourth core
busy" comes to mind.)

<-----A----->
<---B---->
<-----C----->
<----D----->

Now, instead of program level parallelism, there is thread level
parallelism. Say, for example, I'm running Photoshop. For some of
the Photoshop filters, the picture is cut into four pieces, and
a thread is forked onto each core. On the quad core, my Photoshop
filter operation looks like this. There would be a bit of
"mop up" when the threads terminate, in order to finish
reassembling the picture.

<-----A1---->
<-----A2---->
<-----A3---->
<-----A4---->

If Photoshop was running on a single core, doing the same
work, then only one thread is running, and it processes the
whole picture. It looks like this.

<------------------------A------------------------->

So my quad core sped up my Photoshop operation rather
nicely.

Now, how does CAD work ? Well, it is your software, so
you have a better idea than I do. On the CAD software
I used to use, all of it was single threaded. And the
only opportunity I had, was program level parallelism.
I could have an autolayout running in the background,
while doing schematic capture or modification in the
foreground. At best, I might make use of a dual core,
with that crappy old software. More cores would have
meant, more idle cores.

If your budget is not constrained, this discussion is
moot, because you could buy a dual core 3GHz or a quad
core 3GHz, and the only difference would be the price.
So you don't really have to make much of a choice, if
you have the dollars. (Just get the expensive one.) If,
on the other hand, your budget is $250-$300, and you
are comparing the dual core 3GHz to the quad core 2.4GHz,
then the decision is a tougher one.

For CAD, a dual core 3GHz might handle most of what you
do. It really depends on how many compute jobs that
you batch, and run on multiple machines, as to whether the
quad makes more sense.

Another thing - the clock rate. Performance is proportional
to clock rate times IPC (instructions per clock). The
current processors have a 1.5X IPC of the older ones.
That is why a 2GHz current processor, equals a 3GHz
older processor. It is because of the (unspoken) IPC.
That is what makes the difference.

So your quad doesn't "run at 8GHz". It is four units of
2GHz each. And depending on whether there is program level
parallelism or thread level parallelism, that decides to
what extent it behaves "like an 8GHz machine".

If you want a good chuckle, there is this. Gateway FX540XT
with factory overclocked QX6850 running at 3.66GHz. Too bad
you're stuck with dual 8800GTX cards (a gamer configuration).
Must be pretty hard on the power bill, and the air conditioning.

http://www.gateway.com/systems/product/529668003.php

To give you some idea how expensive a desktop processor can
get, this is the latest (a 3.2GHz Quad core 45nm processor).
Draws 136W of power at full load. Only $1410. Quite a bit
more than a run of the mill 2.4GHz Quad core at $250.

http://www.newegg.com/Product/Product.aspx?Item=N82E16819115050
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115017

Have fun,
Paul
From: HeadCase on 1 Apr 2008 06:51
Thanks to Calab and particularly Paul for such a comprehensive response.
Guess I need to consult Autocad about the operation of the software now to
understand how much real gain will be seen.

Phil


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