Saturday, 10 November 2012

CPU Scheduling


CPU Scheduler
Ø  Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.
Ø  CPU scheduling decisions may take place when a process:
1.            Switches from running to waiting state.
2.            Switches from running to ready state.
3.            Switches from waiting to ready.
4.            Terminates.
Ø  Scheduling under 1 and 4 is nonpreemptive.
Ø  All other scheduling is preemptive.

Dispatcher
Ø  Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:
·         switching context
·         switching to user mode
·         jumping to the proper location in the user program to restart that program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running.

Scheduling Criteria
Ø  CPU utilization – keep the CPU as busy as possible
Ø  Throughput – # of processes that complete their execution per time unit
Ø  Turnaround time – amount of time to execute a particular process
Ø  Waiting time – amount of time a process has been waiting in the ready queue
Ø  Response time – amount of time it takes from when a request was submitted until the first response is produced, not output  (for time-sharing environment)

Optimization Criteria
Ø  Max CPU utilization
Ø  Max throughput
Ø  Min turnaround time
Ø  Min waiting time
Ø  Min response time

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