PROCESS SCHEDULING

SCHEDULING QUEUE'S

: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) => (must be fast).
: Long-term scheduler is invoked very infrequently (seconds, minutes)=> (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 thancomputations, 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.