CPU Utilisation

Question

I have a small program which attempts to find the largest prime factor of a very large number. I have written this program in C#. It is brute force and will take quite sometime to complete. This is deliberate as I'm trying to understand how the CPU is utilised.

How can I get it to fully utilise the CPU. At the moment it only ever uses about 25% while for about 68% of the time the CPU is just idle:

Why does the program not use all the available CPU cycles?

Update:

The CPU history for the four cores looks like this

Why are none of these right up at the top?

Answer 1

If your computer has a multicore architecture (which is likely), the only way you'll get a high amount of usage is if you make use of multiple threads.

For example, if your computer is a quad-core, you'll only use 25% of the CPU if you use a single thread.

By using multiple threads (either via explicit multithreading code or via something like Parallel.For) you'll get something close to 100%, but not exactly - there are always overheads that keep programs from achieving full usage. The nature of the algorithm (i.e. how parallelisable it is) also makes a big difference. Something like ray tracing is embarrassingly parallel and the overheads are miminal.

Answer 2

I'm guessing you have 4 CPUs/cores, and since you're running a single threaded process, you'll use only one of those CPUs, hence your 25% usage figure. If you can parallelise this (dependent upon your algorithm) then you can invoke (say) 4 threads and that should distribute across your CPUs (it's dependant upon your OS scheduler, so this is something you can't guarantee completely)

Answer 3

Can it be that you have four CPU cores? In case the programm runs on one/fourth of your system with 100 % it could show up as 25 % in the process list.. what does the multi threat performance bars say?