FCFS Scheduling (Cont.)

First-Come, First-Served (FCFS) Scheduling

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