why 45% CPU utilisation?
 

I am running MPrime (the Linux version of Prime95) on a computer with 512MB RAM.

Top (similar to task manager in Windows) reported MPrime used about 45% of the CPU. But no other process was using the CPU, so I thought something went wrong.

Top reported high memory utilisation by MPrime so I understood MPrime was performing P1 calculations which require too much memory.

I found quickly the misconfiguration: Because the computer had 1GB of memory when I installed MPrime on it, I configured it to use a maximum of 888MB RAM. Now with only 512MB RAM, MPrime was eating all available memory and somehow it was not using the full power of the CPU. After I updated the configuration of MPrime to use 384MB RAM maximum, I checked again through top and it reported near 98-99% CPU utilisation. Problem solved.

I posted this in order to let other people know of this behaviour and fix their systems, if they encounter the same problem sometime.

And now the question: Why only 45% CPU utilisation (with no other process eating cpu power)? I know the problem was the memory, but how it is explained technicaly?

MPrime has to wait (idled) while the OS retreives paged out memory.

Your hard disk was getting a workout.