CPU Scheduling :CPU Scheduling


CPU Scheduling :CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling Operating Systems Examples Java Thread Scheduling Algorithm Evaluation


Basic Concepts :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 distribution


Alternating Sequence of CPU And I/O Bursts :Alternating Sequence of CPU And I/O Bursts


Histogram of CPU-burst Times :Histogram of CPU-burst Times


CPU Scheduler :CPU Scheduler Selects from among the 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


Dispatcher :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


Scheduling Criteria :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)


Optimization Criteria :Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time


First-Come, First-Served (FCFS) 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: Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17


FCFS Scheduling (Cont.) :FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P2 , P3 , P1 The Gantt chart for the schedule is: 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


Shortest-Job-First (SJR) Scheduling :Shortest-Job-First (SJR) 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


Example of Non-Preemptive SJF :Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 SJF (non-preemptive) Average waiting time = (0 + 6 + 3 + 7)/4 = 4 Example of Non-Preemptive SJF


Example of Preemptive SJF :Example of Preemptive SJF Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 SJF (preemptive) Average waiting time = (9 + 1 + 0 +2)/4 = 3


Determining Length of Next CPU Burst :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


Prediction of the Length of the Next CPU Burst :Prediction of the Length of the Next CPU Burst


Examples of Exponential Averaging :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


Priority 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 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


Round Robin (RR) :Round Robin (RR) 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 in chunks of at most q time units at once. No process waits more than (n-1)q time units. Performance q large ? FIFO q small ? q must be large with respect to context switch, otherwise overhead is too high


Example of RR with Time Quantum = 20 :Example of RR with Time Quantum = 20 Process Burst Time P1 53 P2 17 P3 68 P4 24 The Gantt chart is: Typically, higher average turnaround than SJF, but better response


Time Quantum and Context Switch Time :Time Quantum and Context Switch Time


Turnaround Time Varies With The Time Quantum :Turnaround Time Varies With The Time Quantum


Multilevel Queue :Multilevel Queue 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


Multilevel Queue Scheduling :Multilevel Queue Scheduling


Multilevel Feedback Queue :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


Example of Multilevel Feedback Queue :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.


Multilevel Feedback Queues :Multilevel Feedback Queues


Multiple-Processor Scheduling :Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Load sharing Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing


Real-Time 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


Thread Scheduling :Thread Scheduling 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


Pthread Scheduling API :Pthread Scheduling API #include #include #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 the threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i],&attr,runner,NULL);


Pthread Scheduling API :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 thread\n"); pthread exit(0); }


Operating System Examples :Operating System Examples Solaris scheduling Windows XP scheduling Linux scheduling


Solaris 2 Scheduling :Solaris 2 Scheduling


Solaris Dispatch Table :Solaris Dispatch Table


Windows XP Priorities :Windows XP Priorities


Linux Scheduling :Linux Scheduling Two algorithms: time-sharing and real-time Time-sharing Prioritized credit-based – process with most credits is scheduled next Credit subtracted when timer interrupt occurs When credit = 0, another process chosen When all processes have credit = 0, recrediting occurs Based on factors including priority and history Real-time Soft real-time Posix.1b compliant – two classes FCFS and RR Highest priority process always runs first


The Relationship Between Priorities and Time-slice length :The Relationship Between Priorities and Time-slice length


List of Tasks Indexed According to Prorities :List of Tasks Indexed According to Prorities


Algorithm Evaluation :Algorithm Evaluation Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload Queueing models Implementation


5.15 :5.15


End of Chapter 5 :End of Chapter 5


5.08 :5.08


In-5.7 :In-5.7


In-5.8 :In-5.8


In-5.9 :In-5.9


Dispatch Latency :Dispatch Latency


Java Thread Scheduling :Java Thread Scheduling JVM Uses a Preemptive, Priority-Based Scheduling Algorithm FIFO Queue is Used if There Are Multiple Threads With the Same Priority


Java Thread Scheduling (cont) :Java Thread Scheduling (cont) JVM Schedules a Thread to Run When: The Currently Running Thread Exits the Runnable State A Higher Priority Thread Enters the Runnable State * Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not


Time-Slicing :Time-Slicing Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used: while (true) { // perform CPU-intensive task . . . Thread.yield(); } This Yields Control to Another Thread of Equal Priority


Thread Priorities :Thread Priorities Priority Comment Thread.MIN_PRIORITY Minimum Thread Priority Thread.MAX_PRIORITY Maximum Thread Priority Thread.NORM_PRIORITY Default Thread Priority Priorities May Be Set Using setPriority() method: setPriority(Thread.NORM_PRIORITY + 2);