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Old 2005-08-26, 05:42   #1
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Exclamation Prime95 using only one half of CPU

I checked the Task Manager and I found that Prime95 is using only about half of the CPU. The other half is being used by "System Idle process". What should I do?

Thank you
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Old 2005-08-26, 07:27   #2
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it is really using the full processor if thats a pentium iv. most likely you have the hyperthreading on. you can go into the bios to turn it off.
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Old 2005-08-26, 07:38   #3
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I'm getting per-iteration times of about 0.054s for a 2.992 GHz Pentium 4 processor. Is this about normal?
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Old 2005-08-26, 10:05   #4
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Quote:
Originally Posted by Unregistered
I'm getting per-iteration times of about 0.054s for a 2.992 GHz Pentium 4 processor. Is this about normal?
Well, that depends very much on what exponent you are crunching. You can compare with the bench marks here:
http://www.mersenne.org/bench.htm
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Old 2005-09-08, 23:30   #5
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Quote:
Originally Posted by Unregistered
I checked the Task Manager and I found that Prime95 is using only about half of the CPU. The other half is being used by "System Idle process". What should I do?

Thank you
In Task Manager click on the "Performance" tab, click on "View", "CPU History", and "One Graph Per CPU". This will show you the usage of each of your P4 threads.

I run a 3EGHz Pentium processor with HTT active.

With only essential processes running - firewall, anti-virus, etc - along with Prime95 working on any task from LL testing of a 10,000,000 digit Mersenne candidate to factor checking, my processor use averages about 51%, 50% being used by Prime95. This gives a consistent "Per iteration time" of 0.033 sec.

In general, one thread will be running at around 65-66%, the other at around 35-36%, with the thread usages mirroring each other to maintain the overall average of 51%.

Out of curiosity, and in the hope of potentially halving the "Per iteration time" by having Prime95 use all my processing power, I turned off HTT. Processor usage increased to 100% at all times, and, amazingly, the "Per iteration time" nearly doubled to 0.063 sec.

You will not be surprised to learn that my processor again has HTT enabled.
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Old 2005-09-09, 02:18   #6
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I would recommend that if you haven't already done so you should upgrade to the latest version of Prime95.

I recently upgraded from version 23.8 to version 24.14 and got a reduction in "Per iteration time" from around 0.054 sec. to 0.033 sec.
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Old 2005-10-08, 15:40   #7
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I'm getting per-iteration times of about 0.072s for a 3.0 GHz Pentium 4 processor with HTT active when working on a 10 000 000 digit Mersenne candidate. How do you get 0.033 per-iteration times?
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Old 2005-11-14, 01:27   #8
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I get 0.062 for a 3.2 GHz.... I don't understand either. (I have HT on)
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Old 2005-11-14, 02:49   #9
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There are differences between Northwood P4@3Ghz and Prescott P4@3Ghz which will also be reflecting in your iteration times.

When comparing you need to say what chip you have not just "P4".
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Old 2005-11-14, 08:52   #10
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It helps to say also which exponent you are referring to.
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Old 2005-11-17, 14:39   #11
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Default copy of a response I posted in September 2004

Some earlier threads in this and other forums discussed why P4s with hyperthreading may seem to report using only 50% for Prime95, when they're actually using about 100% of the real processor power.


For instance, see "Prime95 runs on my CPU only 50%" at http://www.mersenneforum.org/showthread.php?t=3064.
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