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Silberschatz, Galvin and Gagne ©2006 Operating System Principles Chapter 5: Process Scheduling Mi-Jung Choi [email protected] Dept. of Computer and Science Ch05 - Process Scheduling Chapter 5: Process Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling Operating System -2- Fall 2014 Ch05 - Process Scheduling Basic Concepts Process Scheduling is the basis of multi-programmed operating system Terminology CPU scheduling, Process scheduling, Kernel Thread scheduling used interchangeably, we use process scheduling Operating System -3- Fall 2014 Ch05 - Process Scheduling Basic Concepts In a single-processor system Only one process can run at a time Any others must wait until the CPU is free and can be rescheduled. When the running process goes to the waiting state, the OS may select another process to assign CPU to improve CPU utilization. Every time one process has to wait, another process can take over use of the CPU Process scheduling is a fundamental function of operating-system. to select a process from the ready queue and assign the CPU Maximum CPU utilization obtained with multiprogramming by process scheduling Operating System -4- Fall 2014 Ch05 - Process Scheduling Diagram of Process State from ch.3 It is important to realize that only one process can be running on any processor at any instant. Many processes may be ready and waiting states. Operating System -5- Fall 2014 Ch05 - Process Scheduling Process Scheduling from ch.3 Two types of queues: one ready queue, a set of device queues Two types of resources: CPU, I/O Arrow indicates the flow of processes in the system Operating System -6- Fall 2014 Ch05 - Process Scheduling CPU - I/O burst Cycle Process execution consists of a cycle of CPU execution (CPU burst) and I/O wait (I/O burst) Process alternate between these two states Process execution begins with a CPU burst, which is followed by an I/O burst, and so on. Eventually, the final CPU burst ends with an system call to terminate execution. CPU burst distribution of a process varies greatly from process to process and from computer to computer Operating System -7- Fall 2014 Ch05 - Process Scheduling Alternating Sequence of CPU & I/O Bursts CPU burst time I/O burst time CPU burst time CPU burst time Operating System -8- Fall 2014 Ch05 - Process Scheduling Histogram of CPU-burst Times CPU burst distribution is generally characterized as exponential or hyper-exponential with large number of short CPU burst and small number of long CPU burst I/O bound process has many short CPU bursts CPU bound process might have a few long CPU bursts. Operating System -9- Fall 2014 Ch05 - Process Scheduling Process Scheduler is one of OS modules. selects one of the processes in memory that are ready to execute, and allocates the CPU to the selected process. CPU scheduling decisions may take place when a process: 1. switches from running to waiting state: I/O request, invocation of wait() for the termination of other process 2. switches from running to ready state: when interrupt occurs 3. switches from waiting to ready: at completion of I/O 4. terminates Scheduling under 1 and 4 is non-preemptive Scheduling under 2 and 3 is preemptive Operating System - 10 - Fall 2014 Ch05 - Process Scheduling Non-preemptive vs. Preemptive Non-preemptive scheduling Once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state. used by Windows 3.x Preemptive scheduling Current running process can be switched with another at any time because interrupt can occur at any time Most of modern OS provides this scheme. (Windows XP, Max OS, UNIX) incurs a cost associated with access to shared data among processes affects the design of the OS kernel Certain OS (UNIX) waits either for a system call to complete or for an I/O block to take place before doing a context switch. protects critical kernel code by disabling and enabling the interrupt at the entry and exit of the code. Operating System - 11 - Fall 2014 Ch05 - Process Scheduling Dispatcher Dispatcher module is a part of a Process Scheduler 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 Dispatch latency – time it takes for the dispatcher to stop one process and start another running Operating System - 12 - Fall 2014 Ch05 - Process Scheduling Context Switch from ch. 3 Operating System - 13 - Fall 2014 Ch05 - Process Scheduling Scheduling Criteria Based on the scheduling criteria, the performance of various scheduling algorithm could be evaluated. Different scheduling algorithms have different properties. CPU utilization – ratio (%) of the amount of time while the CPU was busy per time unit. Throughput – # of processes that complete their execution per time unit. Turnaround time – the interval from the time of submission of a process to the time of completion. Sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O Waiting time – Amount of time a process has been waiting in the ready queue, which is affected by scheduling algorithm Response time – In an interactive system, amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) Operating System - 14 - Fall 2014 Ch05 - Process Scheduling Optimization Criteria It is desirable to maximize: The CPU utilization The throughput It is desirable to minimize: The turnaround time The waiting time The response time However in some circumstances, it is desirable to optimize the minimum or maximum values rather than the average. Interactive systems, it is more important to minimize the variance in the response time than minimize the average response time. Operating System - 15 - Fall 2014 Ch05 - Process Scheduling Process Scheduling Algorithms First-Come, First-Served Scheduling (FCFS) Shortest-Job-First Scheduling (SJF) Priority Scheduling Round-Robin Scheduling Our measure of comparison is the average waiting time. CPU utilization, Throughput, Turn arround time, Operating System - 16 - Fall 2014 Ch05 - Process Scheduling First-Come, First-Served (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: P1 P2 0 24 P3 27 30 Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Operating System - 17 - Fall 2014 Ch05 - Process Scheduling FCFS Scheduling Suppose that the processes arrive in the order P2 , P3 , P1 The Gantt chart for the schedule is: P2 0 P3 3 P1 6 30 Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case Operating System - 18 - Fall 2014 Ch05 - Process Scheduling FCFS Scheduling 0 P0 P1 P2 10 20 I/O (20) CPU (10) CPU(6) CPU(4) 10 CPU (4) I/O(24) 14 P1 CPU(6) 50 CPU(9) I/O(17) P2 CPU(4) 40 CPU(11) I/O(4) CPU (4) P0 CPU(10) Operating System 30 P2 CPU(4) 20 P0 I/O(6) 24 30 - 19 - P1 CPU(9) P0 CPU(11) 41 P2 CPU(4) 50 54 Fall 2014 Ch05 - Process Scheduling FCFS Scheduling FCFS scheduling algorithm is non-preemptive Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O. is particularly troublesome for time-sharing systems. Convoy effect occurs. When one CPU-bound process with long CPU burst and many I/O-bound process which short CPU burst. All I/O bound process waits for the CPU-bound process to get off the CPU while I/O is idle All I/O- and CPU- bound processes executes I/O operation while CPU is idle. results in low CPU and device utilization Operating System - 20 - Fall 2014 Ch05 - Process Scheduling 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 Two schemes: non-preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is known as the Shortest-Remaining-Time-First (SRTF) SJF is optimal – gives minimum average waiting time for a given set of processes Operating System - 21 - Fall 2014 Ch05 - Process Scheduling Example of Non-Preemptive SJF Process Arrival Time P1 0.0 P2 2.0 P3 4.0 P4 5.0 SJF (non-preemptive) P1 0 3 Burst Time 7 4 1 4 P3 7 P2 8 P4 12 16 Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Operating System - 22 - Fall 2014 Ch05 - Process Scheduling Example of Preemptive SJF Process Arrival Time P1 0.0 P2 2.0 P3 4.0 P4 5.0 SJF (preemptive) P1 0 P2 2 P3 4 P2 5 Burst Time 7 4 1 4 P4 P1 11 7 16 Average waiting time = (9 + 1 + 0 +2)/4 = 3 Operating System - 23 - Fall 2014 Ch05 - Process Scheduling Preemptive SJF Scheduling 0 P0 P1 P2 10 20 I/O (20) CPU (10) CPU(6) CPU(4) Operating System P1 CPU(6) P2 CPU(4) 40 50 CPU(11) CPU(9) I/O(17) I/O(4) CPU (4) P0 P2 CPU(4) CPU(1) 30 CPU (4) I/O(24) P0 CPU(9) P1 I/O(4) - 24 - P1 CPU(9) P2 P0 P2 I/O(2) CPU(4) I/O(1) P0 CPU(11) Fall 2014 Ch05 - Process Scheduling Non-preemptive SJF Scheduling 0 P0 P1 P2 10 CPU(6) Operating System P2 CPU(4) 50 CPU(9) CPU (4) I/O(24) P1 CPU(6) 40 CPU(11) I/O(17) I/O(4) CPU (4) P0 CPU(10) 30 I/O (20) CPU (10) CPU(4) 20 P2 CPU(4) Idle (6) - 25 - P0 CPU(11) P1 CPU(9) P2 CPU(4) Fall 2014 Ch05 - Process Scheduling 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 1. t n actual lenght of nth CPU burst 2. n 1 predicted value for the next CPU burst 3. is a real number, 0 1 4. Define : n 1 tn 1 n . The value of tn contains our most recent information. n+1 stores the past history The parameter controls the relative weight of recent and past history in our prediction. Operating System - 26 - Fall 2014 Ch05 - Process Scheduling Prediction of the Length of the Next CPU Burst In this example, 0 = 10, = ½ 1 = x t0 + (1- ) x 0 = ½ x 6 + ½ x 10 = 8 2 = x t2 + (1- ) x 2 = ½ x 4 + ½ x 8 = 6 Operating System - 27 - Fall 2014 Ch05 - Process Scheduling Examples of Exponential Averaging =0 n+1 = n = n-1 = n-2 . … = 0 Recent history does not count =1 n+1 = tn Only the actual last CPU burst counts If we expand the formula, we get: n+1 = tn + (1 - ) tn - 1 + … + (1 - )j tn -j + … + (1 - )n +1 0 Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor Operating System - 28 - Fall 2014 Ch05 - Process Scheduling 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) Preemptive Non-preemptive 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 Operating System - 29 - Fall 2014 Ch05 - Process Scheduling Example of Non-Preemptive Priority Process P1 P2 P3 P4 P5 Burst Time 10 1 2 1 5 Priority 3 1 4 5 2 Priority Scheduling (non-preemptive) P2 0 1 P5 P1 P3 16 6 P4 18 19 Average waiting time = (0 + 1 + 6 + 16 + 18)/5 = 8.2 Operating System - 30 - Fall 2014 Ch05 - Process Scheduling Preemptive Priority Scheduling 0 P0(1) P1(2) 10 20 I/O (20) CPU (10) CPU(6) P2(3) CPU(4) P1 CPU(6) 40 50 CPU(11) CPU(9) I/O(17) I/O(4) CPU (4) P0 CPU(10) 30 CPU (4) I/O(24) P0 P2 Idle: P2 CPU(4) P2(I/O4) CPU(4)I/O(2) P0 CPU(11) P1 CPU(9) P2 P2 I/O(2)CPU(4) I/O is same to idle (원래 I/O에는 idle이 들어가야 합니다.) Operating System - 31 - Fall 2014 Ch05 - Process Scheduling Non-preemptive Priority Scheduling 0 10 P0 (1) CPU (10) P1 (2) P2 (3) CPU(6) CPU(4) I/O(4) CPU (4) 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) 이 경우는 특수 case로 preemptive나 non-preemptive나 답이 같습니다. (즉 앞장과 답이 같습니다.) Operating System - 32 - Fall 2014 Ch05 - Process Scheduling Round Robin (RR) Scheduling Each process gets a small unit of CPU time (time quantum), 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 (= q time units) in chunks of at most n x q time units at once. No process waits more than (n-1) x q time units. Performance depends on the size of the time quantum. q large RR is same as FIFO q small provides high concurrency: each of n processes has its own processor running at 1/n the speed of the real processor Operating System - 33 - Fall 2014 Ch05 - Process Scheduling Example of RR with Time Quantum = 20 Process P1 P2 P3 P4 The Gantt chart is: P1 0 P2 20 37 P3 Burst Time 53 17 68 24 P4 57 P1 77 P3 97 117 P4 P1 P3 P3 121 134 154 162 Typically, higher average turnaround than SJF, but better response Operating System - 34 - Fall 2014 Ch05 - Process Scheduling RR Scheduling 0 P0(1) P1(2) 10 CPU (10) CPU(6) P2(3) CPU(4) 20 30 I/O (20) 50 CPU(11) CPU(9) I/O(17) I/O(4) CPU (4) 40 CPU (4) I/O(24) P0 P2 P0 P1 P2 P0 P1 P0 P1 P2 P0 P2 CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) Idle P1 I/O (11) P1 P1 P1 P1 P0 P1 P0 P0 P2 P0 P2 P0 CPU(2) CPU(2) CPU1) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) CPU(2) C CPU Time Quantum = 2 (9번째 P1과 10번째 P2는 바뀌어도 상관 없음) (20번째 P0와 21번째 P2도 바뀌어도 상관 없음) Operating System - 35 - Fall 2014 Ch05 - Process Scheduling RR Scheduling 0 P0(1) P1(2) 10 CPU(6) P0 CPU(5) CPU (4) I/O(24) P0 CPU(5) 50 CPU(9) I/O(17) P1 CPU(5) 40 CPU(11) I/O(4) CPU (4) P2 CPU(4) 30 I/O (20) CPU (10) P2(3) CPU(4) 20 Idle P1 I/O(15) P2 P1 CPU(1) CPU(4) P0 CPU(5) P1 CPU(5) P0 P2 CPU(5) CPU(4 Time Quantum = 5 Operating System - 36 - Fall 2014 Ch05 - Process Scheduling Time Quantum and Context Switch Time The effect of context switching on the performance of RR scheduling, for example one process of 10 time quantum. quantum = 12 time units, finished in less than 1 time quantum quantum = 6 time units, requires 2 quanta, 1 context switch quantum = 1 time units, requires 10 quanta, 9 context switch Operating System - 37 - Fall 2014 Ch05 - Process Scheduling Round Robin (RR) Scheduling The time quantum q must be large with respect to context switch, otherwise overhead is too high If the context switching time is 10% of the time quantum, then about 10% of the CPU time will be spent in context switching Most modern OS have time quanta ranging from 10 to 100 milliseconds, The time required for a context switch is typically less than 10 microseconds; thus the context-switch time is a small fraction of the time quantum. Operating System - 38 - Fall 2014 Ch05 - Process Scheduling Turnaround Time varies with the Time Quantum The turnaround time depends on the size of the time quantum The average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum. When SJF and RR used If quantum = 6 and 7, average turnaround time = 10.5 If quantum = 1, average turnaround time = 11 Operating System - 39 - Fall 2014 Ch05 - Process Scheduling Scheduling Algorithm with multi-Queues Multi-level Queue Scheduling Multi-level Feedback Queue Scheduling Operating System - 40 - Fall 2014 Ch05 - Process Scheduling Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) The processes are permanently assigned to one queue, generally based on some property, or process type. Each queue has its own scheduling algorithm foreground – RR background – FCFS Scheduling must be done between the queues Fixed priority scheduling - serve all from foreground then from background, Possibility of starvation. Time slice scheduling – 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 - 41 - Fall 2014 Ch05 - Process Scheduling Multilevel Queue Scheduling No process in the batch queue could run unless the queues with high priority were all empty. If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted. Operating System - 42 - Fall 2014 Ch05 - Process Scheduling Multi-level Queue Scheduling 0 10 P0 (1) CPU (10) P1 (0) P2 (0) CPU(6) CPU(4) I/O(4) CPU (4) 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) Two-level Queues: SJF with priority 0, FCFS with priority 1 Fixed Priority Queue Preemptive Operating System - 43 - Fall 2014 Ch05 - Process Scheduling Multilevel Feedback Queue A process can move between the various queues; aging can be implemented in this way 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 - 44 - Fall 2014 Ch05 - Process Scheduling Example of Multilevel Feedback Queue Three queues: Q0 – RR with time quantum 8 milliseconds Q1 – RR time quantum 16 milliseconds Q2 – FCFS Scheduling A new job enters queue Q0 which is served RR. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. At Q1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. The job is serverd based on FCFS in queue Q2 Operating System - 45 - Fall 2014 Ch05 - Process Scheduling Multilevel Feedback Queues Operating System - 46 - Fall 2014 Ch05 - Process Scheduling Multi-level Feedback Queue Scheduling 0 10 P0 (1) CPU (10) P1 (0) CPU(6) P2 (0) CPU(4) I/O(4) CPU (4) 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) Three-level Queues: RR with quantum 3, RR with quantum 6, FCFS with priority 1 Operating System - 47 - Fall 2014 Ch05 - Process Scheduling Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a system or heterogeneous processors within a system Asymmetric multiprocessing vs. Symmetric multiprocessing (SMP) Symmetric Multiprocessing (SMP) – each processor makes its own scheduling decisions. Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing. Load sharing on SMP system keeps the workload evenly distributed across all processors in an SMP system. Push migration vs. Pull migration Operating System - 48 - Fall 2014 Ch05 - Process Scheduling Real-Time Scheduling Hard real-time systems – required to complete a critical task within a guaranteed amount of time Soft real-time computing – requires that critical processes receive priority over less fortunate ones Operating System - 49 - Fall 2014 Ch05 - Process Scheduling Thread Scheduling On operating systems that support kernel-level thread, it is kernel-level threads, not processes, that are being scheduled by the operating system. Local Scheduling – How the threads library decides which thread to put onto an available LWP Global Scheduling – How the kernel decides which kernel thread to run next Operating System - 50 - Fall 2014 Ch05 - Process Scheduling Summary CPU scheduling is the task of selecting a waiting process from the ready queue and allocating the CPU to it. The CPU is allocated to the selected process by the dispatcher. FCFS scheduling is simple, cause short processes to wait for long time SJF scheduling is provably optimal, providing the shortest averaging waiting time. But predicting the length of the next CPU bursts is difficult. Priority scheduling allocates the CPU to the heights priority process. Both priority and SJF may suffer from starvation. Aging is a technique to prevent starvation. RR scheduling is more appropriate for a time-shared system. Major problem of RR scheduling is the selection of the time quantum. FCFS is non-preemptive, RR is preemptive, SJF and Priority may be preemptive and non-preemptive. Multilevel queue allows different scheduling algorithm for each queue. Multilevel feedback queue allow process to move from one queue to another. Operating System - 51 - Fall 2014 Silberschatz, Galvin and Gagne ©2006 Operating System Principles End of Chapter 5 Mi-Jung Choi [email protected] Dept. of Computer and Science