2.) PROCESS SHEDULING

A.) SCHEDULING QUEUSE

>Job queue – set of all processes in the system.
>Ready queue – set of all processes residing in main memory, ready and waiting to >Execute.Device queues – set of processes waiting for an I/O device.
>Processes migrate between the various queues.

B.) SCHEDULERS

>Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.
>Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

>Short-term scheduler is invoked very frequently (milliseconds) fi (must be fast).
>Long-term scheduler is invoked very infrequently (seconds, minutes) fi (may be slow).
>The long-term scheduler controls the degree of multiprogramming.
>Processes can be described as either:
>I/O-bound process – spends more time doing I/O than computations, many short CPU bursts.
>CPU-bound process – spends more time doing computations; few very long CPU bursts.


C.) CONTEXT SWITCH

>When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process.
>Context-switch time is overhead; the system does no useful work while switching.
>Time dependent on hardware support.