CPU Scheduling PDF - Operating System Concepts

Chapter 6: CPU Scheduling Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013  Chapter 6: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Sc...

Chapter 6: CPU Scheduling Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013  Chapter 6: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time CPU Scheduling Operating Systems Examples 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 To examine the scheduling algorithms of several operating systems Operating System Concepts – 9th Edition 6.3 Silberschatz, Galvin and Gagne ©2013  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 CPU burst followed by I/O burst CPU burst distribution is of main concern Operating System Concepts – 9th Edition 6.4 Silberschatz, Galvin and Gagne ©2013  CPU Scheduler Short-term scheduler selects from among the processes in ready queue, and allocates the CPU to one of them  Queue may be ordered in various ways 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  Consider access to shared data  Consider preemption while in kernel mode  Consider interrupts occurring during crucial OS activities Operating System Concepts – 9th Edition 6.5 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 6.6 Silberschatz, Galvin and Gagne ©2013  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 time-sharing environment) Operating System Concepts – 9th Edition 6.7 Silberschatz, Galvin and Gagne ©2013  Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Operating System Concepts – 9th Edition 6.8 Silberschatz, Galvin and Gagne ©2013  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 P3 0 24 27 30 Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 9th Edition 6.9 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 process  Consider one CPU-bound and many I/O-bound processes Operating System Concepts – 9th Edition 6.10 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  Could ask the user Operating System Concepts – 9th Edition 6.11 Silberschatz, Galvin and Gagne ©2013  Example of SJF ProcessArriva l 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 Operating System Concepts – 9th Edition 6.12 Silberschatz, Galvin and Gagne ©2013  Example of Shortest-remaining-time-first Now we add the concepts of varying arrival times and preemption to the analysis ProcessA arri 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 Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec Operating System Concepts – 9th Edition 6.13 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  highest priority)  Preemptive  Nonpreemptive SJF is priority scheduling where priority is the inverse of 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 Concepts – 9th Edition 6.14 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 Average waiting time = 8.2 msec Operating System Concepts – 9th Edition 6.15 Silberschatz, Galvin and Gagne ©2013  Round Robin (RR) 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 Performance  q large  FIFO  q small  q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 9th Edition 6.16 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 Typically, higher average turnaround than SJF, but better response q should be large compared to context switch time q usually 10ms to 100ms, context switch < 10 usec Operating System Concepts – 9th Edition 6.17 Silberschatz, Galvin and Gagne ©2013  Multilevel Queue Ready queue is partitioned into separate queues, eg:  foreground (interactive)  background (batch) 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). 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 Operating System Concepts – 9th Edition 6.18 Silberschatz, Galvin and Gagne ©2013  Multilevel Queue Scheduling Operating System Concepts – 9th Edition 6.19 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 6.20 Silberschatz, Galvin and Gagne ©2013  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 FCFS  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 FCFS and receives 16 additional milliseconds  If it still does not complete, it is preempted and moved to queue Q2 Operating System Concepts – 9th Edition 6.21 Silberschatz, Galvin and Gagne ©2013  Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes  Currently, most common Processor affinity – process has affinity for processor on which it is currently running  soft affinity  hard affinity  Variations including processor sets Operating System Concepts – 9th Edition 6.22 Silberschatz, Galvin and Gagne ©2013  Multicore Processors Recent trend to place multiple processor cores on same physical chip Faster and consumes less power Multiple threads per core also growing  Takes advantage of memory stall to make progress on another thread while memory retrieve happens Operating System Concepts – 9th Edition 6.23 Silberschatz, Galvin and Gagne ©2013  Multithreaded Multicore System Operating System Concepts – 9th Edition 6.24 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 6.25 Silberschatz, Galvin and Gagne ©2013  Real-Time CPU Scheduling (Cont.) Conflict phase of dispatch latency: 1. Preemption of any process running in kernel mode 2. Release by low- priority process of resources needed by high- priority processes Operating System Concepts – 9th Edition 6.26 Silberschatz, Galvin and Gagne ©2013  Priority-based Scheduling For real-time scheduling, scheduler must support preemptive, priority- based 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 Operating System Concepts – 9th Edition 6.27 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 6.28 Silberschatz, Galvin and Gagne ©2013  Operating System Examples Linux scheduling Windows scheduling Operating System Concepts – 9th Edition 6.29 Silberschatz, Galvin and Gagne ©2013  Linux Scheduling in Version 2.6.23 + Completely Fair Scheduler (CFS) Scheduling classes  Each has specific priority  Scheduler picks highest priority task in highest scheduling class  Rather than quantum based on fixed time allotments, based on proportion of CPU time  2 scheduling classes included, others can be added 1. default 2. real-time Quantum calculated based on nice value from -20 to +19  Lower value is higher priority  Calculates target latency – interval of time during which task should run at least once  Target latency can increase if say number of active tasks increases CFS scheduler maintains per task virtual run time in variable vruntime  Associated with decay factor based on priority of task – lower priority is higher decay rate  Normal default priority yields virtual run time = actual run time To decide next task to run, scheduler picks task with lowest virtual run time Operating System Concepts – 9th Edition 6.30 Silberschatz, Galvin and Gagne ©2013  Linux Scheduling (Cont.) Real-time scheduling according to POSIX.1b  Real-time tasks have static priorities Real-time plus normal map into global priority scheme Nice value of -20 maps to global priority 100 Nice value of +19 maps to priority 139 Operating System Concepts – 9th Edition 6.31 Silberschatz, Galvin and Gagne ©2013  Windows Scheduling Windows uses priority-based preemptive scheduling Highest-priority thread runs next Dispatcher is scheduler Thread runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority thread Real-time threads can preempt non-real-time 32-level priority scheme Variable class is 1-15, real-time class is 16-31 Priority 0 is memory-management thread Queue for each priority If no run-able thread, runs idle thread Operating System Concepts – 9th Edition 6.32 Silberschatz, Galvin and Gagne ©2013  Windows Priorities Operating System Concepts – 9th Edition 6.33 Silberschatz, Galvin and Gagne ©2013