CPU Scheduling PDF

Chapter 6: CPU Scheduling Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013  Chapter 6: CPU Scheduling ! Basic Concepts ! Scheduling Criteria ! Scheduling Algorithms ! Sections from the textbook: 6.1...

Chapter 6: CPU Scheduling Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013  Chapter 6: CPU Scheduling ! Basic Concepts ! Scheduling Criteria ! Scheduling Algorithms ! Sections from the textbook: 6.1, 6.2, and 6.3 Operating System Concepts – 9th Edition 6.2 Silberschatz, Galvin and Gagne ©2013  Objectives ! To introduce CPU scheduling, which is the basis for multiprogrammed operating systems ! To describe various CPU-scheduling algorithms ! To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system Operating System Concepts – 9th Edition 6.3 Silberschatz, Galvin and Gagne ©2013  Basic Concepts ! Maximum CPU utilization obtained with multiprogramming load store add store CPU burst read from file ! Process execution consists of cycles of CPU execution and I/O wait for I/O I/O burst wait store increment index CPU burst ! CPU burst followed by I/O burst, write to file and so on. wait for I/O I/O burst ! Process execution begins and ends with CPU burst. load store add store CPU burst ! CPU burst distribution is of main read from file concern wait for I/O I/O burst Operating System Concepts – 9th Edition 6.4 Silberschatz, Galvin and Gagne ©2013  Histogram of CPU-burst Times Short CPU-bursts Long CPU-bursts Operating System Concepts – 9th Edition 6.5 Silberschatz, Galvin and Gagne ©2013  CPU Scheduler ! When the CPU is idle, Short-term scheduler (CPU scheduler) selects from the processes in ready queue (memory), and allocates the CPU to one of them ! Queue may be ordered in various ways (not only “FIFO”). ! CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state (I/O request or wait()) 2. Switches from running to ready state (interrupts) 3. Switches from waiting to ready 4. Terminates ! Scheduling under 1 and 4 is nonpreemptive (cooperative) ! All other scheduling is preemptive, which can result in race conditions: ! Consider access to shared data ! Consider preemption while in kernel mode ! Consider interrupts occurring during crucial OS activities Operating System Concepts – 9th Edition 6.6 Silberschatz, Galvin and Gagne ©2013  Dispatcher ! Another component involved in the CPU-scheduling function is the dispatcher. ! Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: ! switching context ! switching to user mode ! jumping to the proper location in the user program to restart that program ! The dispatcher should be fast! ! Dispatch latency – time it takes for the dispatcher to stop one process and start another running Operating System Concepts – 9th Edition 6.7 Silberschatz, Galvin and Gagne ©2013  Scheduling Criteria ! Criteria have been suggested for comparing CPU-scheduling algorithms. ! 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, from submission to completion (TAT=CT-AT) ! Waiting time – amount of time a process spends waiting in the ready queue (WT=CT-AT-BT)è (WT=TAT-BT) ! Response time – amount of time it takes from when a request was submitted until the first response is produced, not output. RT = (The first time to be executed by CPU) - AT Operating System Concepts – 9th Edition 6.8 Silberschatz, Galvin and Gagne ©2013  Scheduling Algorithm Optimization Criteria ! It is better to: ! Maximizing CPU utilization ! Maximizing throughput ! Minimizing turnaround time ! Minimizing waiting time ! Minimizing response time Operating System Concepts – 9th Edition 6.9 Silberschatz, Galvin and Gagne ©2013  Scheduling Algorithms ! CPU scheduling algorithm decides on which of the processes in the ready queue is to be allocated the CPU. ! First- Come, First-Served (FCFS) Scheduling. ! Shortest-Job-First (SJF) Scheduling. ! Priority Scheduling. ! Round Robin (RR). ! Multilevel Queue. Operating System Concepts – 9th Edition 6.10 Silberschatz, Galvin and Gagne ©2013  First- Come, First-Served (FCFS) Scheduling - The simplest, the process that requests the CPU first is allocated the CPU first. - With the help of FIFO queue. - nonpreemptive 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: P1 P2 P3 0 24 27 30 ! Waiting time for P1 = 0; P2 = 24; P3 = 27 milliseconds ! Average waiting time: (0 + 24 + 27)/3 = 17 milliseconds Operating System Concepts – 9th Edition 6.11 Silberschatz, Galvin and Gagne ©2013  First- Come, First-Served (FCFS) Scheduling P1 P2 P3 0 24 27 30 ! Turnaround Time for P1 = 24; P2 = 27; P3 = 30 ! Average Turnaround Time : (24 + 27 + 30)/3 = 30 Operating System Concepts – 9th Edition 6.12 Silberschatz, Galvin and Gagne ©2013  First- Come, First-Served (FCFS) Scheduling Process Arrive Time Burst Time P1 0 24 P2 1 3 P3 2 3 ! P1 P2 P3 0 24 27 30 Waiting time? ! Turnaround Time for P1 = 24-0; P2 = 27-1; P3 = 30-2 ! Average Turnaround Time: (24 + 26 + 28)/3 = 26 Operating System Concepts – 9th Edition 6.13 Silberschatz, Galvin and Gagne ©2013  FCFS Scheduling (Cont.) Suppose that the processes arrive in the order: P2 , P3 , P1 ! The Gantt chart for the schedule is: P2 P3 P1 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 processè Lower CPU utilization ! Consider one CPU-bound and many I/O-bound processes ⌦ Troublesome for time-sharing systems. Operating System Concepts – 9th Edition 6.14 Silberschatz, Galvin and Gagne ©2013  Shortest-Job-First (SJF) Scheduling ! Associate with each process the length of its next CPU burst ! Use these lengths to schedule the process with the shortest time ! SJF is optimal – gives minimum average waiting time for a given set of processes ⌦ The difficulty is knowing the length of the next CPU request Operating System Concepts – 9th Edition 6.15 Silberschatz, Galvin and Gagne ©2013  Example of SJF ProcessArrival Time Burst Time P1 0.0 6 P2 2.0 8 P3 4.0 7 P4 5.0 3 ! SJF scheduling chart P4 P1 P3 P2 0 3 9 16 24 ! Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 ! Turnaround Time for P1 = 9 ; P2 = 24; P3 = 16; P4 = 3 ! Average Turnaround Time: (9+24+16+3)/4 = 13 ! What is the average waiting time using FCFS scheduling ? Operating System Concepts – 9th Edition 6.16 Silberschatz, Galvin and Gagne ©2013  Determining Length of Next CPU Burst ! Can only estimate the length – should be similar to the previous one ! Then pick process with shortest predicted next CPU burst ! Can be done by using the length of previous CPU bursts, using exponential averaging 1. t n = actual length of n th CPU burst 2. t n +1 = predicted value for the next CPU burst 3. a , 0 £ a £ 1 4. Define : ! tn contains our most recent information, while stores the past history. ! Commonly, α set to ½ Operating System Concepts – 9th Edition 6.17 Silberschatz, Galvin and Gagne ©2013  Prediction of the Length of the Next CPU Burst 12 τi 10 8 ti 6 4 2 time CPU burst (ti) 6 4 6 4 13 13 13 … "guess" (τi) 10 8 6 6 5 9 11 12 … Prove that diagram !!! Operating System Concepts – 9th Edition 6.18 Silberschatz, Galvin and Gagne ©2013  Examples of Exponential Averaging ! a =0 ! tn+1 = tn ! Recent history does not count ! a =1 ! tn+1 = a tn Only the actual last CPU burst counts ! ! If we expand the formula, we get: tn+1 = a tn+(1 - a)a tn -1 + … +(1 - a )j a tn -j + … +(1 - a )n +1 t0 ! Since both a and (1 - a) are less than or equal to 1, each successive term has less weight than its predecessor Operating System Concepts – 9th Edition 6.19 Silberschatz, Galvin and Gagne ©2013  Examples of Exponential Averaging ! Problem: Calculate the predicted burst time using exponential averaging for the fifth process if the predicted burst time for the first process is 10 units and actual burst time of the first four processes is 4, 8, 6 and 7 units respectively. Given α = 0.5. ! Solution: ! Predicted Burst Time for 2nd Process- ! Predicted Burst Time for 3rd Process- = α x Actual burst time of 1st process + (1-α) x = α x Actual burst time of 2nd process + (1-α) x Predicted burst time for 1st process Predicted burst time for 2nd process = 0.5 x 4 + 0.5 x 10 = 2 + 5 = 7 units = 0.5 x 8 + 0.5 x 7 = 4 + 3.5 = 7.5 units ! Predicted Burst Time for 4th Process- ! Predicted Burst Time for 5th Process- = α x Actual burst time of 3rd process + (1-α) x = α x Actual burst time of 4th process + (1-α) x Predicted burst time for 3rd process Predicted burst time for 4th process = 0.5 x 6 + 0.5 x 7.5 = 3 + 3.75 = 6.75 units = 0.5 x 7 + 0.5 x 6.75 = 3.5 + 3.375 = 6.875 units https://www.gatevidyalay.com/predicting-burst-time-sjf-scheduling/ Operating System Concepts – 9th Edition 6.20 Silberschatz, Galvin and Gagne ©2013  Shortest-remaining-time-first ! Now we add the concepts of varying arrival times and preemption to the analysis ! SJF can be preemptive or nonpreemptive. ! The next CPU burst of the newly arrived process may be shorter than what is left of the currently executing process. " A preemptive SJF algorithm will preempt the currently executing process,. " a nonpreemptive SJF algorithm will allow the currently running process to finish its CPU burst. ! Preemptive SJF è shortest-remaining-time-first Operating System Concepts – 9th Edition 6.21 Silberschatz, Galvin and Gagne ©2013  Example of Shortest-remaining-time-first ProcessAarri Arrival TimeT Burst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 ! Preemptive SJF Gantt Chart P1 P2 P4 P1 P3 0 1 5 10 17 26 ! Waiting time(WT)= total waiting time – # of milliseconds process executed – arrival time Ø WT for P1= (10-1-0)= 9 ms Ø WT for P2=(1-0-1)= 0 ms Ø WT for P3=(17-0-2)= 15 ms Ø WT for P4=(5-0-3)= 2 ms ! Average waiting time = (9+0+15+2)/4 = 26/4 = 6.5 ms ! Turnaround Time for P1 = 17-0 ; P2 = 5-1; P3 = 26-2; P4 = 10-3 ! Average Turnaround Time: (17+4+24+7)/4 Operating System Concepts – 9th Edition 6.22 Silberschatz, Galvin and Gagne ©2013  Priority Scheduling ! A priority number (integer) is associated with each process ! The CPU is allocated to the process with the highest priority (smallest integer º highest priority) ! Processes with Equal-priority are scheduled in FCFS. ! When a process arrives at the ready queue, its priority is compared with the priority of the currently running process. ! Preemptive, preempts the CPU if the priority of the newly arrived process is higher ! Nonpreemptive, puts the new process at the head of the ready queue. ! SJF is a special case of priority scheduling where priority is the inverse of predicted next CPU burst time (i.e., the larger the CPU burst, the lower the priority, and vice versa). ⌦ Problem º Starvation (indefinite blocking) – low priority processes may never execute ! Solution º Aging – as time progresses increase the priority of the process Operating System Concepts – 9th Edition 6.23 Silberschatz, Galvin and Gagne ©2013  Example of Priority Scheduling ProcessA arri Burst TimeT Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 ! Priority scheduling Gantt Chart WT for P1=6 ms WT for P2=0 ms WT for P3=16 ms WT for P4=18 ms WT for P5=1 ms ! Average waiting time = (6+0+16+18+1)/5= 8.2 ms ! What is the average waiting time using FCFS and SJF? Operating System Concepts – 9th Edition 6.24 Silberschatz, Galvin and Gagne ©2013  Round Robin (RR) ! Designed for time-sharing systems. ! It is similar to FCFS scheduling, but preemption is added to enable the system to switch between processes. ! Each process gets a small unit of CPU time (time quantum q), usually 10-100 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 (n-1)q time units. ! Timer interrupts every quantum to schedule next process " The CPU scheduler picks the first process from the ready queue, sets a timer to interrupt after 1 time quantum, and dispatches the process. ! Performance ! q large Þ FCFS ! q small Þ more context switches, overhead is too high Operating System Concepts – 9th Edition 6.25 Silberschatz, Galvin and Gagne ©2013  Round Robin (RR) tail head ! New processes are added to the tail of the ready queue. First In - First Out One of two things will then happen The process has a CPU burst of less than 1 time The process has a CPU burst longer than 1 quantum time quantum process will release the CPU the timer will go off and will cause an voluntarily. interrupt to the operating system The scheduler will then proceed to the A context switch will be executed, and next process in the ready queue the process will be put at the tail of the ready queue. The CPU scheduler will then select the next process in the ready queue. Operating System Concepts – 9th Edition 6.26 Silberschatz, Galvin and Gagne ©2013  Example of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3 P3 3 ! The Gantt chart is: P1 P2 P3 P1 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30 ! Average turnaround time= (30+ 7+ 10)/3= 15.67 msec ! Typically, higher average turnaround time than SJF, but better response ! q should be large compared to context switch time ! q usually 10ms to 100ms, context switch < 10 microseconds (μs) Operating System Concepts – 9th Edition 6.27 Silberschatz, Galvin and Gagne ©2013  Example of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3 P3 3 ! The Gantt chart is: P1 P2 P3 P1 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30 Turnaround time= Completion time - Arrival time Waiting time= Turnaround time – Burst time Process Completion Turnaround Waiting Average Waiting time=(6+4+7)/3 time time time =17/3= 5.66 ms P1 30 30-0=30 30-24=6 P2 7 7-0=7 7-3=4 P3 10 10-0=10 10-3=7 Operating System Concepts – 9th Edition 6.28 Silberschatz, Galvin and Gagne ©2013  Time Quantum and Context Switch Time Operating System Concepts – 9th Edition 6.29 Silberschatz, Galvin and Gagne ©2013  Turnaround Time Varies With The Time Quantum For Q=1 Average turn around time = 11 msec (prove) 80% of CPU bursts should be shorter For Q=5 than q Average turn around time = 12.25 msec (prove) and so on. Note that the average turnaround time of a set of processes does not necessarily improve as the time-quantum size increases. In general, the average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum Operating System Concepts – 9th Edition 6.30 Silberschatz, Galvin and Gagne ©2013  Preemptive/ Non preemptive Scheduling Preemptive/ nonpreemptive FCFS nonpreemptive SJF may be either preemptive/ nonpreemptive Priority may be either preemptive/ nonpreemptive RR preemptive Operating System Concepts – 9th Edition 6.31 Silberschatz, Galvin and Gagne ©2013  Multilevel Queue ! Ready queue is partitioned into separate queues based on their properties, for example: ! foreground (interactive), ! background (batch) These two have different response-time requirements è different scheduling needs. Also, foreground processes may have higher priority over background processes. ! Process permanently in a given queue ! Each queue has its own scheduling algorithm: ! foreground – RR ! background – FCFS ! Scheduling must be done between the queues: ! Fixed priority scheduling; (i.e., serve all from foreground then from background) èPossibility of starvation. ! 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 20% to background in FCFS Operating System Concepts – 9th Edition 6.32 Silberschatz, Galvin and Gagne ©2013  Multilevel Queue Scheduling Operating System Concepts – 9th Edition 6.33 Silberschatz, Galvin and Gagne ©2013  Multilevel Feedback Queue ! Multilevel feedback queue scheduling algorithm allows a process to move between queues. ! The idea is to separate processes according to the characteristics of their CPU bursts. ! If a process uses too much CPU time, it will be moved to a lower-priority queue àI/O-bound and interactive processes in the higher-priority queues. ! A process that waits too long in a lower-priority queue may be moved to a higher-priority queue à this form of aging prevents starvation. ! Multilevel-feedback-queue scheduler defined by the following parameters: ! number of queues ! scheduling algorithms for each queue ! method used to determine when to upgrade a process ! method used to determine when to demote a process ! method used to determine which queue a process will enter when that process needs service Operating System Concepts – 9th Edition 6.34 Silberschatz, Galvin and Gagne ©2013  Example of Multilevel Feedback Queue ! Three queues: ! Q0 – RR with time quantum 8 ms ! Q1 – RR time quantum 16 ms Q0 ! Q2 – FCFS ! Scheduling ! A new job enters queue Q0 Q1 4 When it gains CPU, job receives 8 ms 4 If it does not finish in 8 ms, job is moved to queue Q1 ! At Q1 job is again served RR and receives Q2 16 additional ms 4 If it still does not complete, it is preempted and moved to queue Q2 Operating System Concepts – 9th Edition 6.35 Silberschatz, Galvin and Gagne ©2013  End of Chapter 6 Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013