For example : To build a kernel we can use make it will take more than one hour , to accelerate the process we can use make -j4 all this may finish almost four times as quickly.
Generally : How to force an application to use a specific nombre nof cpu ?
There is no general solution to this problem - cpu usage of applications is entirely down to what the application does, and how it works internally.
In order to parallelism tasks for efficiency reasons, you often need to restructure the process flow. This varies greatly, depending on algorithm, so it's never as simple as pressing a 'use more processors' button.
