2) Process Scheduling

Representation of Process Scheduling



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SCHEDULING QUEUES
~ 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 among the various queues

Ready Queue and Various I/O Device 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

Addition of Medium Term Scheduling



~ 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 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