Question 7693 – CPU-Scheduling

CPU scheduling decisions take place under one of four conditions:
1. When a process switches from the running state to the waiting state, such as for an I/O request or invocation of the wait( ) system call.
2. When a process switches from the running state to the ready state, for example in response to an interrupt.
3. When a process switches from the waiting state to the ready state, say at completion of I/O or a return from wait( ).
4. When a process terminates.
For conditions 1 and 4 there is no choice – A new process must be selected.
For conditions 2 and 3 there is a choice – To either continue running the current process, or select a different one.
Note: If scheduling takes place only under conditions 1 and 4, the system is said to be non-preemptive, or cooperative. Under these conditions, once a process starts running it keeps running, until it either voluntarily blocks or until it finishes. Otherwise the system is said to be preemptive.