1. scanf n, a, b /* I-O wait */ for (i=1; i<=n; i++) /* CPU burst */ x = x + a*b; printf x /* I-O wait */ for (i=1; i<=n; i++) /* CPU burst */ x = x + a*b; printf x /* I-O wait */ waiting time Sum of periods spent waiting in ready queue Average is across all visits to ready queue Goal: short waiting time

2.  First-Come, First-Served is simplest scheduling algorithm  Ready queue is a FIFO queue: First-In, First- Out  Nonpreemptive : executes until voluntarily gives up CPU  Problems.

3.  Assume processes arrive in this order: P1, P2, P3  Nonpreemptive scheduling Gantt chart  Average waiting time: (0+24+27)/3 = 17 ms PIDBurst P124 P23 P33 P1P2P3

4.  Assume processes arrive in this order: P2, P3, P1  Nonpreemptive scheduling Gantt chart  Average waiting time: (6+0+3)/3 = 3 ms PIDBurst P23 P33 P124 P2P3P1

5.  Shortest -Job- First  Assume the next burst time of each process is known  SJF selects process which has the shortest burst time  Nonpreemptive or preemptive(Not included in the course project)  Shortest-remaining-time-first  Interrupts running process if a new process enters the queue  new process must have shorter burst than remaining time

6.  Assume all processes arrive at the same time: P1, P2, P3, P4  Nonpreemptive scheduling Gantt chart  Average waiting time: (3+16+9+0)/4=7 ms PIDBurst P16 P28 P37 P43 P1P3P2

7.  Switch between processes  Time quantum is a small unit of time (10 to 100 ms)  Process is executed on the CPU for at most one time quantum  Implemented by using the ready queue as a circular queue  Head process gets the CPU  Uses less than a time quantum IMPLIES gives up the CPU voluntarily  Uses full time quantum IMPLIES timer will cause an interrupt  Context switch will be executed  Process will be put at the tail of queue

8.  Assume processes arrive in this order: P1, P2, P3  Preemptive scheduling  Time quantum : 4 ms Gantt chart  P1 uses a full time quantum; P2, P3 use only a part of a quantum  P1 waits 0+6=6; P2 waits 4; P3 waits 7  Average waiting time: (6+4+7)/3=5.66 ms PIDBurst P124 P23 P33 P1P2P3P1

9. Process Scheduling Simulator