Prime95 and Dual Processors

[AntonVrba]

I now run a 2 x Dual XEON 3,6GHZ system and have noted that Prime95 does not take advantage of the two processors

1) Best time for 4096K FFT length: 125.782 ms. while running PFGW in parallel

and

2) Best time for 4096K FFT length: 124.827 ms. when PFGW is stopped.

In case 1) the windows task manager shows 50% CPU utilisation

and in case 2) 25% CPU utilisation.

Similar to Prime95, PFGW did not show any slow down either.

regards - Anton

[dsouza123]

Possible scenario:

Hyperthreading on, Windows sees 4 CPUs,
using 100 % of 1/2 of total CPU utilization is 50%,
using 100 % of 1/4 of total CPU utilization is 25%.

Really using 100% of a single physical HT CPU when Prime95 is running.
The hyperthreading only gives a small percent of true extra CPU capacity
if one application is using FP and another integer.

For one core, if only one program is using FP then it uses 100% of the CPU
and the second virtual HT 0%.

[AntonVrba]

Originally Posted by dsouza123

Possible scenario:

For one core, if only one program is using FP then it uses 100% of the CPU
and the second virtual HT 0%.

Your explanation is correct and logical.

FFT math is very suitable for parallel processing, lots of parallel small multiplications that are combined at the end.

So the point is that Prime95 takes no advantage of the fact that parallel processors do exist.

regards

[rgiltrap]

Originally Posted by AntonVrba

So the point is that Prime95 takes no advantage of the fact that parallel processors do exist.

Or you could simply have two instances of Prime95 running to ensure 100% utlization, which is probably a great deal less effort than trying to parallelize the code

[fetofs]

Originally Posted by AntonVrba

Your explanation is correct and logical.

FFT math is very suitable for parallel processing, lots of parallel small multiplications that are combined at the end.

So the point is that Prime95 takes no advantage of the fact that parallel processors do exist.

regards

Originally Posted by George Woltman

I own a dual-CPU computer or several computers networked together. Can these CPUs work together on the same exponent to get results faster?

No. It is not uncommon for the program to read and write nearly 100MB of data every second. This is far too much data to transfer around the network in a timely manner.

Although, a program could be written for dual-CPU systems (it would be quite time-consuming), the machine will still get more throughput by working on separate exponents.

.

[AntonVrba]

I now disabled hyperthreading on my 2 x dual xeon 3.6 GHz system

Best time for 4096K FFT length: 121.698 ms

3.2 mS faster than if hyperthreading is enabbled.

Conclusion - hyperthreading adds 3% overhead on the so called dual core processors - food for thought

[kdelisle2005]

With your dual core, have you tried the Affinity parameter with multiple executions of Prime95??

I have a P4 HT (1 cpu) and I run two executions of Prime95 (I renamed to prime95a.exe and prime95b.exe, in separate subdirectories) - using the Affinity parameter, CPU 0 and CPU 1, respectively.

I know the one CPU can only work on one thing at a time but it seems I get better total throughput (be it very small) when trending for a couple of days.

The downside to running two executions of Prime95 is slower response from other applications (Excel, IE, etc). Windows XP does a good job of handling the priority but it does take some time especially with two Primes going.

For kicks, it sounds like you could run four executions of PRIME95.



kevin