I have a udp server that receive data and computing it.
I have two thread for each role.
In my cpu is a 8 multi-core and I send data in varius speed.
but
You cannot make one thread use more than one core. To achieve better CPU utilization you need to redesign your program to create more threads and let the OS schedule them for you. There's no need to manually restrict the threads to specific cores. OSes are really good at figuring out how to allocate cores to threads.
In your case, if the data computing tasks are CPU heavy, you could spawn a new thread per request or have a worker thread pool that would be picking incoming tasks and processing them. This is just one of ideas. It's difficult to say without knowing more about your application architecture and the problems it's trying to solve.