Process Management – CPU Scheduling

Non- preemptive:

Once a process is on CPU it cannot be interrupted by any other process until the current CPU burst has been completed

Non- preemptive examples

FCFS (First come first serve)

SJF (shortest job first)

Priority

Preemptive

a process running on the CPU can be interrupted for the arrival of a higher priority process or for a time quantum.

Preemptive examples

Round Robin (FCFS with Time Quantum)

SRT- Shortest remaining time first (Preemptive of SJF)

Preemptive priority

Dispatcher module

gives control of the CPU to the process selected by the CPU 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

CPU utilization

keep the CPU as busy as possible

Throughput

of processes that complete their execution per time unit

Turnaround time

amount of time to execute a particular process

Waiting time

amount of time a process has been waiting in the ready queue

Response time

amount of time it takes from when a request was submitted until the first response is produced, not output

SJF Advantages

gives minimum average response time

SJF Disadvantages

long-running jobs may starve if too many short jobs

Shortest Remaining Time First

preemptive version of SJF

SJF Scheduling - Prediction formula

𝑆𝑛+1 = 𝛼𝑇𝑛 + (1 − 𝛼)𝑆�

Priority Scheduling

A priority is associated with each process, and the CPU is allocated to the process with the highest priority.

priority scheduling problem

Starvation - low priority processes may never execute

priority scheduling solution

Aging - as time progresses increase the priority of the process

time quantum q

The small unit of CPU time allocated to each process in Round Robin scheduling.

q large

FIFO

q small

q must be large with respect to context switch, otherwise overhead is too high