Chapter 5 CPU Scheduling Chapter 5 CPU Scheduling
- Slides: 41
Chapter 5: CPU Scheduling
Chapter 5: CPU Scheduling n Basic Concepts n Scheduling Criteria n Scheduling Algorithms n Multiple-Processor Scheduling n Real-Time Scheduling n Thread Scheduling n Operating Systems Examples n Java Thread Scheduling n Algorithm Evaluation AE 4 B 33 OSS 5. 2 Silberschatz, Galvin and Gagne © 2005
Basic Concepts n Maximum CPU utilization obtained with multiprogramming n CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait n CPU burst distribution AE 4 B 33 OSS 5. 3 Silberschatz, Galvin and Gagne © 2005
Alternating Sequence of CPU And I/O Bursts AE 4 B 33 OSS 5. 4 Silberschatz, Galvin and Gagne © 2005
Histogram of CPU-burst Times AE 4 B 33 OSS 5. 5 Silberschatz, Galvin and Gagne © 2005
CPU Scheduler n Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them n 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 n Scheduling under 1 and 4 is nonpreemptive n All other scheduling is preemptive AE 4 B 33 OSS 5. 6 Silberschatz, Galvin and Gagne © 2005
Dispatcher n Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: l switching context l switching to user mode l jumping to the proper location in the user program to restart that program n Dispatch latency – time it takes for the dispatcher to stop one process and start another running AE 4 B 33 OSS 5. 7 Silberschatz, Galvin and Gagne © 2005
Scheduling Criteria n CPU utilization – keep the CPU as busy as possible n Throughput – # of processes that complete their execution per time unit n Turnaround time – amount of time to execute a particular process n Waiting time – amount of time a process has been waiting in the ready queue n 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) AE 4 B 33 OSS 5. 8 Silberschatz, Galvin and Gagne © 2005
Optimization Criteria n Max CPU utilization n Max throughput n Min turnaround time n Min waiting time n Min response time AE 4 B 33 OSS 5. 9 Silberschatz, Galvin and Gagne © 2005
First-Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 n 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 0 24 P 3 27 30 n Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 n Average waiting time: (0 + 24 + 27)/3 = 17 AE 4 B 33 OSS 5. 10 Silberschatz, Galvin and Gagne © 2005
FCFS Scheduling (Cont. ) Suppose that the processes arrive in the order P 2 , P 3 , P 1 n The Gantt chart for the schedule is: P 2 0 P 3 3 P 1 6 30 n Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 n Average waiting time: (6 + 0 + 3)/3 = 3 n Much better than previous case n Convoy effect short process behind long process AE 4 B 33 OSS 5. 11 Silberschatz, Galvin and Gagne © 2005
Shortest-Job-First (SJF) Scheduling n Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time n Two schemes: l nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst l 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) n SJF is optimal – gives minimum average waiting time for a given set of processes AE 4 B 33 OSS 5. 12 Silberschatz, Galvin and Gagne © 2005
Example of Non-Preemptive SJF Process Arrival Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 Burst Time n SJF (non-preemptive) P 1 0 3 P 3 7 P 2 8 P 4 12 16 n Average waiting time = (0 + 6 + 3 + 7)/4 = 4 AE 4 B 33 OSS 5. 13 Silberschatz, Galvin and Gagne © 2005
Example of Preemptive SJF Process Arrival Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 Burst Time n SJF (preemptive) P 1 0 P 2 2 P 3 4 P 2 5 P 4 P 1 11 7 16 n Average waiting time = (9 + 1 + 0 +2)/4 = 3 AE 4 B 33 OSS 5. 14 Silberschatz, Galvin and Gagne © 2005
Determining Length of Next CPU Burst n Can only estimate the length n Can be done by using the length of previous CPU bursts, using exponential averaging AE 4 B 33 OSS 5. 15 Silberschatz, Galvin and Gagne © 2005
Prediction of the Length of the Next CPU Burst AE 4 B 33 OSS 5. 16 Silberschatz, Galvin and Gagne © 2005
Examples of Exponential Averaging n =0 n+1 = n l Recent history does not count n =1 l n+1 = tn l Only the actual last CPU burst counts n If we expand the formula, we get: n+1 = tn+(1 - ) tn -1 + … +(1 - )j tn -j + … l +(1 - )n +1 0 n Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor AE 4 B 33 OSS 5. 17 Silberschatz, Galvin and Gagne © 2005
Priority Scheduling n A priority number (integer) is associated with each process n The CPU is allocated to the process with the highest priority (smallest integer highest priority) l Preemptive l nonpreemptive n SJF is a priority scheduling where priority is the predicted next CPU burst time n Problem Starvation – low priority processes may never execute n Solution Aging – as time progresses increase the priority of the process AE 4 B 33 OSS 5. 18 Silberschatz, Galvin and Gagne © 2005
Round Robin (RR) n 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. n 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. n Performance AE 4 B 33 OSS l q large FIFO l q small q must be large with respect to context switch, otherwise overhead is too high 5. 19 Silberschatz, Galvin and Gagne © 2005
Example of RR with Time Quantum = 20 Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 n The Gantt chart is: P 1 0 P 2 20 37 P 3 P 4 57 P 1 77 P 3 P 4 P 1 P 3 97 117 121 134 154 162 n Typically, higher average turnaround than SJF, but better response AE 4 B 33 OSS 5. 20 Silberschatz, Galvin and Gagne © 2005
Time Quantum and Context Switch Time AE 4 B 33 OSS 5. 21 Silberschatz, Galvin and Gagne © 2005
Turnaround Time Varies With The Time Quantum AE 4 B 33 OSS 5. 22 Silberschatz, Galvin and Gagne © 2005
Multilevel Queue n Ready queue is partitioned into separate queues: foreground (interactive) background (batch) n Each queue has its own scheduling algorithm l foreground – RR l background – FCFS n Scheduling must be done between the queues AE 4 B 33 OSS l Fixed priority scheduling; (i. e. , serve all from foreground then from background). Possibility of starvation. l 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 l 20% to background in FCFS 5. 23 Silberschatz, Galvin and Gagne © 2005
Multilevel Queue Scheduling AE 4 B 33 OSS 5. 24 Silberschatz, Galvin and Gagne © 2005
Multilevel Feedback Queue n A process can move between the various queues; aging can be implemented this way n Multilevel-feedback-queue scheduler defined by the following parameters: AE 4 B 33 OSS l number of queues l scheduling algorithms for each queue l method used to determine when to upgrade a process l method used to determine when to demote a process l method used to determine which queue a process will enter when that process needs service 5. 25 Silberschatz, Galvin and Gagne © 2005
Example of Multilevel Feedback Queue n Three queues: l Q 0 – RR with time quantum 8 milliseconds l Q 1 – RR time quantum 16 milliseconds l Q 2 – FCFS n Scheduling AE 4 B 33 OSS l 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. l 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 2. 5. 26 Silberschatz, Galvin and Gagne © 2005
Multilevel Feedback Queues AE 4 B 33 OSS 5. 27 Silberschatz, Galvin and Gagne © 2005
Multiple-Processor Scheduling n CPU scheduling more complex when multiple CPUs are available n Homogeneous processors within a multiprocessor n Load sharing n Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing AE 4 B 33 OSS 5. 28 Silberschatz, Galvin and Gagne © 2005
Real-Time Scheduling n Hard real-time systems – required to complete a critical task within a guaranteed amount of time n Soft real-time computing – requires that critical processes receive priority over less fortunate ones AE 4 B 33 OSS 5. 29 Silberschatz, Galvin and Gagne © 2005
Thread Scheduling n Local Scheduling – How the threads library decides which thread to put onto an available LWP n Global Scheduling – How the kernel decides which kernel thread to run next AE 4 B 33 OSS 5. 30 Silberschatz, Galvin and Gagne © 2005
Pthread Scheduling API #include <pthread. h> #include <stdio. h> #define NUM THREADS 5 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 threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i], &attr, runner, NULL); AE 4 B 33 OSS 5. 31 Silberschatz, Galvin and Gagne © 2005
Pthread Scheduling API /* 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 threadn"); pthread exit(0); } AE 4 B 33 OSS 5. 32 Silberschatz, Galvin and Gagne © 2005
Operating System Examples n Solaris scheduling n Windows XP scheduling n Linux scheduling AE 4 B 33 OSS 5. 33 Silberschatz, Galvin and Gagne © 2005
Solaris 2 Scheduling AE 4 B 33 OSS 5. 34 Silberschatz, Galvin and Gagne © 2005
Solaris Dispatch Table AE 4 B 33 OSS 5. 35 Silberschatz, Galvin and Gagne © 2005
Windows XP Priorities AE 4 B 33 OSS 5. 36 Silberschatz, Galvin and Gagne © 2005
Linux Scheduling n Two algorithms: time-sharing and real-time n Time-sharing Prioritized credit-based – process with most credits is scheduled next l Credit subtracted when timer interrupt occurs l When credit = 0, another process chosen l When all processes have credit = 0, recrediting occurs 4 Based on factors including priority and history n Real-time l Soft real-time l l AE 4 B 33 OSS Posix. 1 b compliant – two classes 4 FCFS and RR 4 Highest priority process always runs first 5. 37 Silberschatz, Galvin and Gagne © 2005
The Relationship Between Priorities and Time-slice length AE 4 B 33 OSS 5. 38 Silberschatz, Galvin and Gagne © 2005
List of Tasks Indexed According to Prorities AE 4 B 33 OSS 5. 39 Silberschatz, Galvin and Gagne © 2005
Algorithm Evaluation n Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload n Queueing models n Implementation AE 4 B 33 OSS 5. 40 Silberschatz, Galvin and Gagne © 2005
End of Chapter 5
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