Chapter06.pdf

Chapter 6: CPU Scheduling Chapter 6: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Thread Scheduling Multiple-Processor Scheduling Real-Time CPU Scheduling Operating Systems Examples Algorithm Evaluation 2 Objectives...

Chapter 6: CPU Scheduling Chapter 6: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Thread Scheduling Multiple-Processor Scheduling Real-Time CPU Scheduling Operating Systems Examples Algorithm Evaluation 2 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 3 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 4 Histogram of CPU-burst Times 5 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 6 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 7 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) 8 Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time 9 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 10 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 11 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 12 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 13 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.  n +1 = predicted value for the next CPU burst 3.  , 0    1 4. Define : 𝜏𝑛+1 = 𝛼 𝑡𝑛 + 1 − 𝛼 𝜏𝑛. Commonly, α set to ½ Preemptive version called shortest-remaining-time-first 14 Prediction of the Length of the Next CPU Burst 15 Examples of Exponential Averaging  =0 n+1 = n 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 16 Example of Shortest-remaining-time-first Now we add the concepts of varying arrival times and preemption to the analysis 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 Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5 msec 17 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 18 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 19 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 20 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 21 Time Quantum and Context Switch Time 22 Turnaround Time Varies With The Time Quantum 80% of CPU bursts should be shorter than q 23 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 24 Multilevel Queue Scheduling 25 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 26 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 27 Thread Scheduling Distinction between user-level and kernel-level threads When threads supported, threads scheduled, not processes 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 Typically done via priority set by programmer Kernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system 28 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 Can be limited by OS – Linux and Mac OS X only allow PTHREAD_SCOPE_SYSTEM 29 Pthread Scheduling API #include #include #define NUM_THREADS 5 int main(int argc, char *argv[]) { int i, scope; pthread_t tid[NUM THREADS]; pthread_attr_t attr; pthread_attr_init(&attr); if (pthread_attr_getscope(&attr, &scope) != 0) fprintf(stderr, "Unable to get scheduling scope\n"); else { if (scope == PTHREAD_SCOPE_PROCESS) printf("PTHREAD_SCOPE_PROCESS"); else if (scope == PTHREAD_SCOPE_SYSTEM) printf("PTHREAD_SCOPE_SYSTEM"); else fprintf(stderr, "Illegal scope value.\n"); } 30 Pthread Scheduling API pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM); for (i = 0; i < NUM_THREADS; i++) pthread_create(&tid[i],&attr,runner,NULL); for (i = 0; i < NUM_THREADS; i++) pthread_join(tid[i], NULL); } void *runner(void *param) { pthread_exit(0); } 31 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 32 NUMA and CPU Scheduling Note that memory-placement algorithms can also consider affinity 33 Multiple-Processor Scheduling – Load Balancing If SMP, need to keep all CPUs loaded for efficiency Load balancing attempts to keep workload evenly distributed Push migration – periodic task checks load on each processor, and if found pushes task from overloaded CPU to other CPUs Pull migration – idle processors pulls waiting task from busy processor 34 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 35 Multithreaded Multicore System 36