22 CPU Scheduling

Question

Which process ready to execute commands get the CPU ?

Key function of the operating system

Prerequisites for successful scheduling :

1. CPU-I/O-Burst Cycle Experience shows : I/O occurs after fixed amount of time in $\ge 90\%$. $\Rightarrow$ appropriate time for re-scheduling
2. Pre-emptive Scheduling : Processes can be forces to relinquish processor.

Scheduling Criteria

Have various, often conflicting criteria to measure success of scheduling :

• CPU utilisation
• Throughput : Number or processes completed within a given time
• Turnaround time : Time it takes for each process to be executed
• Waiting time : Amount of time spent in the ready-queue
• Response time : time between submission of request and production of first response.

Have two categories of processes :

• I/O-bound process- spends more time doing I/O than computation, many short CPU bursts
• CPU-bound process - spends more time doing computations; few very long CPU bursts