Prime95 fails to recognize more than 2 cores?

[MrLittleTexas]

I'm running Fedora on an Intel i5 quad-core, attempting to run P95 multithreading on all four cores. However, when I attempt to set the number of CPUs I am told to "please select a value between 1 and 2". What is going on? How can I get access to all four cores?

[S485122]

Can you specify the model of processor ?
If you are sure the processor is a quad core you can force mprime to use a certain number of cores by a setting documented in the file undoc.txt.

Jacob

"The program automatically computes the number of CPUs, hyperthreading, and speed.
This information is used to calculate how much work to get.
If the program did not correctly figure out your CPU information, you can override the info in local.txt:
NumCPUs=n
CpuNumHyperthreads=1 or 2
CpuSpeed=s
Where n is the number of physical CPUs or cores, not logical CPUs created by hyperthreading. Choose 1 for non-hyperthreaded and 2 for hyperthreaded. Finally, s is the speed in MHz."

[MrLittleTexas]

Thanks for your help. I have an i5 2520M which evidently has two physical cores with four total logical cores. Does that mean I can only use two threads? For some reason my CPU is only running at about 45% capacity according to system monitor which makes me think I should be able to crank it up more.

[CRGreathouse]

Originally Posted by MrLittleTexas

Thanks for your help. I have an i5 2520M which evidently has two physical cores with four total logical cores. Does that mean I can only use two threads? For some reason my CPU is only running at about 45% capacity according to system monitor which makes me think I should be able to crank it up more.

You can run as many threads as you like, but after the first two the total work won't increase and may well decrease. The system monitor is lying through its teeth, I don't know why it is programmed that way.

[Prime95]

Look in undoc.txt for an option that allows you to allocate your 2 hyperthreads from the dialog boxes. Something like confiurehyperthreads IIRC.

The reason prime95 does not automatically make hyperthreads available is so that only those that have been warned that hyperthreading may actually hurt performance will be tempted to give it a try.

[Dubslow]

Originally Posted by MrLittleTexas

Thanks for your help. I have an i5 2520M which evidently has two physical cores with four total logical cores. Does that mean I can
only use two threads?

Although there is no code to prevent you from using 4 threads, it is strongly recommended against using hyperthreads for Prime95. See e.g. this thread for explanation (especially the last post).

Originally Posted by MrLittleTexas

For some reason my CPU is only running at about 45% capacity according to system monitor which makes me think I should be able to crank it up more.

That's because Windows isn't very smart about hyperthreads versus actual cores. Two of the cores it shows are "virtual cores", the hyperthreads that don't actually help Prime95. So when you run 2 threads, Prime95 is fully utilizing the silicon (one thread for each physical core), even though two of the available (hyper)threads seem to be inactive.