Modern technology of the era have improved the speed at which processes are executed in a system. As processes enter the system they have to be stored at some place where the system cannot forget. So they are put in a data structure called queue, in fact, all processes are arranged in this job queue. Once the processes know what directions to follow and know exactly what to do they are ready to execute, so they are put in main memory and stored in a ready queue.
It is usual for processes to get interrupted while running, the most common reason interruption happens is because processes require input or output events, in such cases, process have to wait until their request is fulfilled, meanwhile they are placed in a device queue. Since the processes are placed in the ready queue and the many device queues, the system must select a process from the variety of the different queues. The long-term scheduler or job scheduler selects processes from the job queueand places them into main memory raedy for execution. The job scheduler selects processes less frequent than the short-term scheduler or CPU scheduler. The CPU scheduler chooses processes from the ready queue and allocates the CPU one of the processes. The CPU scheduler chooses process at an immense speed of 100 milliseconds and must also lower the overhead time of having to select a process.
In order to have an efficient CPU performance the scheduler must make a balanced combination of CPU-bound and I/O-bound processes. A CPU-bound process makes computations and utilizes the system resources most of the time. On the other hand, I/O-bound devotes majority of its time doinf input or output events. The operating system must use at all times the resources equivalently and not one-side, this is why the scheduler needs to select correctly.
As the CPU switches from processes, the state of the original state is saved and the next process� state is loaded for execution, this function is called context switch
