Process Scheduling

Scheduling Queue

[ ] The operating System must allocate resources to processes, enable processes to share and exchange information, protect the resources of each process from other processes and enable synchronization amongst process.

[ ] 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.


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.


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.