Scheduling Algorithms FirstCome, FirstServed (FCFS) Scheduling
Process Burst Time P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
0 24 27 30
Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Suppose that the processes arrive in the order P2 , P3 , P1 . The Gantt chart for the schedule is:
0 3 6 30
Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. Convoy effect short process behind long process
ShortestJobFirst (SJR) Scheduling
Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. Two schemes: 1. non pre emptive once CPU given to the process it cannot be preempted until completes its CPU burst. 2. preemptive if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the ShortestRemainingTimeFirst (SRTF). SJF is optimal gives minimum average waiting time for a given set of processes.
Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 SJF (nonpreemptive)
0 7 8 12 16
Average waiting time = [0 +(82)+(74) +(125)] /4 =4
Example of Preemptive SJF
Proces Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 SJF (preemptive)
0 2 4 5 7 11 16
Average waiting time = (9 + 1 + 0 +2)/4 =3 Determining Length of Next CPU Burst
Can only estimate the length. Can be done by using the length of previous CPU bursts, using exponential averaging.
Prediction of the Length of the Next CPU Burst Pn+1 = a tn +(1a)Pn This formula defines an exponential average Pn stores the past history tn contents are most recent information the parameter a controls the relative weight of recent and past history of in our prediction If a =0 then Pn +1 =Pn That is prediction is constant If a = 1 then Pn +1 = tn Prediction is last cpu burst
Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer ≡ highest priority). 1. Preemptive 2. nonpreemptive SJF is a priority scheduling where priority is the predicted next CPU burst time. Problem ≡ Starvation low priority processes may never execute. Solution ≡ Aging as time progresses increase the priority of the process.
Round Robin (RR)
Each process gets a small unit of CPU time (time quantum), usually 10100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n1)q time units. Performance 1. q large _ FIFO 2. q small _ q must be large with respect to context switch, otherwise overhead is too high. Example of RR with Time Quantum = 4
Process Burst Time P1 24 P2 3 P3 3
The Gantt chart is:
0 4 7 10 14 18 22 26 30
Average waiting time = [(3024)+4+7]/3 = 17/3 =5.66
Multilevel Queue
Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm, foreground RR background FCFS Scheduling must be done between the queues. 1. Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. 2. Time slice each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR 1. 20% to background in FCFS
Multilevel Queue Scheduling
Multilevel Feedback Queue
A process can move between the various queues; aging can be implemented this way. Multilevelfeedbackqueue scheduler defined by the following parameters: 1. number of queues 2. scheduling g algorithms for each queue 3. method used to determine when to upgrade a process 4. method used to determine when to demote a process 5. method used to determine which queue a process will enter when that process needs service
Example of Multilevel Feedback Queue
Three queues: 1. Q0 time quantum 8 milliseconds 2. Q1 time quantum 16 milliseconds 3. Q2 FCFS Scheduling 1. A new job enters queue Q0 which is served FCFS . When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. 2. At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
