Process A program in execution

But, What does it mean to be “in execution”?

Process States New ReadyRunning Terminated Waiting Admitted Exit I/O or Event Completion Scheduler Dispatch I/O or Event Wait Interrupt

What to keep track of?

Process Control Block Process Number Program Counter I/O Status Info Registers Memory Limits Accounting Info Scheduling Info

Process Scheduling Choose processes for execution (ready Q) Decide which process goes from ready to running

Context Switch Change “running” process Move info from running job to PCB? Move info for new job from PCB into registers, PC etc. Typical time about 1  sec

Types of Schedulers Short term CPU Scheduler Selects from ready Q Runs every few milliseconds Must be quick/small Long term scheduler Controls degree of multiprogramming Balance CPU-bound, I/O-bound processes Decide what goes into ready Q?

Scheduling Algorithms Metrics CPU utilization (%) Throughput – jobs completed/time Turnaround time Submission to completion Waiting time Time in ready queue Response time Interactive systems

Scheduling Algorithms First-Come, First-Served Shortest-Job-First Priority Round-Robin Multilevel Queue

First-Come, First-Served Non-preemptive Process must voluntarily relinquish by terminating or by making a system call Intrinsic priority: Arrival time Expected performance ? Limitations ? Benefits ?

Shortest-Job-First Non-preemptive Better response for short jobs Provably optimal min avg. waiting time for a pool of jobs Disadvantage(s) ?

Priority Associate priority with each process One queue for each priority level FCFS within each level Problem(s) with priority? SJF is special case

Priority Starvation possible Solution? Increase priority with “age” Implement aging ? Nice (Unix)

Round-Robin Preemptive Process “runs” time-slice (q) units FCFS special case of RR (q = infinity) Time sharing Each process “executes” at speed 1/n. q = ??? Balance q with context switch time

Multilevel Queues Partition Ready Queue Each Q has own schedule method E.g. foreground (interactive, RR) and background (batch, FCFS) queues Approaches Higher queues have absolute priority over lower level queues OR Time slice among queues

Evaluation Modeling Deterministic - Requires exact numbers Queueing - Uses distribution of bursts Simulation Create software models of systems. Lots of work, time consuming Actual implementation & test Cost May change behavior