Chapter 5: Process Scheduling

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1 Chapter 5: Process Scheduling

2 Chapter 5: Process Scheduling 5.1 Basic Concepts 5.2 Scheduling Criteria 5.3 Scheduling Algorithms First-Come, First-Served Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling 5.4 Thread Scheduling 5.5 Multiple-Processor Scheduling 5.6 Real-Time CPU Scheduling Priority-Based, Rate-Monotonic, EDF, Proportional Sharing 5.7 Operating Systems Examples 5.8 Algorithm Evaluation 5.2

3 5.1 Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait Alternating Sequence of CPU And I/O Bursts Histogram of CPU-burst Times 5.3

4 Short-Term CPU Scheduler Selects from among the (ready-state) processes in memory that are ready to execute, and allocates the CPU to one of them CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates Scheduling under 1 and 4 is nonpreemptive All other scheduling is preemptive 5.4

5 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 Dispatch latency time it takes for the dispatcher to stop one process and start another running 5.5

6 5.2 Scheduling Criteria 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 Waiting time amount of time a process has been waiting in the ready queue Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for timesharing environment) 5.6

7 5.3 Scheduling Algorithms First-Come, First-Served Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling 5.7

8 5.3.1 First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is: P 1 P 2 P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 =

9 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: P 2 P 3 P Waiting time for P 1 = 6;P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case Convoy effect short process behind long process All the other processes wait for the one big process to get off the CPU The average waiting time under FCFS is often quite long 5.9

10 5.3.2 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: nonpreemptive 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 know as the Shortest-Remaining-Time-First (SRTF) SJF is optimal gives minimum average waiting time for a given set of processes 5.10

11 Example of Non-Preemptive SJF Process Arrival Time Burst Time P P P P SJF (non-preemptive) P 1 P 3 P 2 P Average waiting time = ( )/4 =

12 Example of Preemptive SJF (SRTF) Process Arrival Time Burst Time P P P P SJF (preemptive) P 1 P 2 P 3 P 2 P 4 P Average waiting time = ( )/4 =

13 5.3.3 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 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 5.13

14 5.3.4 Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually 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. Performance q large FCFS / FIFO q small q must be large with respect to context switch, otherwise overhead is too high 5.14

15 Example of RR with Time Quantum = 20 Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 The Gantt chart is: P 1 P 2 P 3 P 4 P 1 P 3 P 4 P 1 P 3 P Typically, higher average turnaround than SJF, but better response 5.15

16 Time Quantum and Context Switch Time 5.16

17 Turnaround Time Varies With The Time Quantum Average turnaround time does not necessarily improve as the time-quantum size increases Rule of thumb: 80 percent of CPU bursts should be shorter than the time quantum 5.17

18 Algorithm Evaluation Consider the following processes that arrive at time 0, in the order given, with the length of the CPU burst given in milliseconds. Process P 1 P 2 P 3 P 4 P 5 Burst Time For each of the first-come, first-served (FCFS), shortest job first (SJF), and round robin (RR, quantum = 10 milliseconds) scheduling algorithms for this set of processes, (a) draw the Gantt chart, (b) determine the average waiting time, and (c) determine the average turnaround time. 5.18

19 5.3.5 Multilevel Queue Scheduling 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 Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility 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 5.19

20 Multilevel Queue Scheduling 5.20

21 Multilevel Feedback Queue A process can move between the various queues; aging can be implemented 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 5.21

22 Example of Multilevel Feedback Queue Three queues: Q 0 RR with time quantum 8 milliseconds Q 1 RR time quantum 16 milliseconds Q 2 FCFS Scheduling A new job enters queue Q 0 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 Q 1. At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q

23 5.4 Thread Scheduling Distinction between user-level and kernel-level threads Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP Known as process-contention scope (PCS) since scheduling competition is within the process Kernel thread scheduled onto available CPU is systemcontention scope (SCS) competition among all threads in system 5.23

24 Pthread Scheduling API allows specifying either PCS or SCS during thread creation PTHREAD SCOPE PROCESS schedules threads using PCS scheduling PTHREAD SCOPE SYSTEM schedules threads using SCS scheduling. 5.24

25 #include <pthread.h> #include <stdio.h> #define NUM THREADS 5 Pthread Scheduling API int main(int argc, char *argv[]) { int i; pthread t tid[num THREADS]; pthread attr t attr; /* get the default attributes */ pthread attr init(&attr); /* set the scheduling algorithm to PROCESS or SYSTEM */ pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM); /* set the scheduling policy - FIFO, RT, or OTHER */ pthread attr setschedpolicy(&attr, SCHED OTHER); /* create the threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i],&attr,runner,null); /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) pthread join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { printf("i am a thread\n"); pthread exit(0); } 5.25

26 5.5 Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Asymmetric multiprocessing only one processor (master server) accesses the system data structures, alleviating the need for data sharing symmetric multiprocessing: each processor is self-scheduling Processor affinity: keep a process running on the same processor soft affinity: attempt but not guarantee hard affinity: allow a process to specify not to migrate to other processors Load sharing : keep the workload evenly distributed across all processors push migration: a specific task checks loads by moving processes from overloaded to idle or less-busy processors pull migration: an idle processor pulls a waiting process from a busy processor 5.26

27 NUMA and CPU Scheduling 5.27

28 Symmetric Multithreading (SMT) Symmetric multithreading (SMT): provide multiple logical processors by hardware Hyperthreading technology on Intel processors Each logical processor has its own architecture state responsible for interrupt handling 5.28

29 Multithreaded Multicore System Recent trend to place multiple processor cores on same physical chip Faster and consume less power Multiple threads per core also growing Takes advantage of memory stall to make progress on another thread while memory retrieve happens 5.29

30 5.6 Real-Time CPU Scheduling Can present obvious challenges Soft real-time systems no guarantee as to when critical real-time process will be scheduled Hard real-time systems task must be serviced by its deadline Two types of latencies affect performance 1. Interrupt latency time from arrival of interrupt to start of routine that services interrupt 2. Dispatch latency time for schedule to take current process off CPU and switch to another 5.30

31 Real-Time CPU Scheduling (Cont.) Conflict phase of dispatch latency: 1. Preemption of any process running in kernel mode 2. Release by lowpriority process of resources needed by highpriority processes 5.31

32 Priority-based Scheduling For real-time scheduling, scheduler must support preemptive, prioritybased scheduling But only guarantees soft real-time For hard real-time must also provide ability to meet deadlines Processes have new characteristics: periodic ones require CPU at constant intervals Has processing time t, deadline d, period p 0 t d p Rate of periodic task is 1/p 5.32

33 Virtualization and Scheduling Virtualization software schedules multiple guests onto CPU(s) Each guest doing its own scheduling Not knowing it doesn t own the CPUs Can result in poor response time Can effect time-of-day clocks in guests Can undo good scheduling algorithm efforts of guests 5.33

34 Rate Montonic Scheduling A priority is assigned based on the inverse of its period Shorter periods = higher priority; Longer periods = lower priority P 1 is assigned a higher priority than P 2. Missed Deadlines with Rate Monotonic Scheduling 5.34

35 Earliest Deadline First Scheduling (EDF) Priorities are assigned according to deadlines: the earlier the deadline, the higher the priority; the later the deadline, the lower the priority 5.35

36 Proportional Share Scheduling T shares are allocated among all processes in the system An application receives N shares where N < T This ensures each application will receive N / T of the total processor time 5.36

37 POSIX Real-Time Scheduling The POSIX.1b standard API provides functions for managing real-time threads Defines two scheduling classes for real-time threads: 1. SCHED_FIFO - threads are scheduled using a FCFS strategy with a FIFO queue. There is no time-slicing for threads of equal priority 2. SCHED_RR - similar to SCHED_FIFO except time-slicing occurs for threads of equal priority Defines two functions for getting and setting scheduling policy: 1. pthread_attr_getsched_policy(pthread_attr_t *attr, int *policy) 2. pthread_attr_setsched_policy(pthread_attr_t *attr, int policy) 5.37

38 Summary 5.1 Basic Concepts 5.2 Scheduling Criteria 5.3 Scheduling Algorithms First-Come, First-Served Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling 5.4 Thread Scheduling 5.5 Multiple-Processor Scheduling 5.6 Real-Time CPU Scheduling Priority-Based, Rate-Monotonic, EDF, Proportional Sharing 5.7 Operating Systems Examples 5.8 Algorithm Evaluation 5.38

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