Roadmap. Basic Concepts. Process State. Histogram of CPU-burst Durations. Tevfik Koşar. Alternating Sequence of CPU And I/O Bursts

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1 S 421/521 - Operating Systems Fall 212 Lecture - V PU Scheduling - I Tevfik Koşar Roadmap PU Scheduling asic oncepts Scheduling riteria & Metrics ifferent Scheduling lgorithms FFS SJF Priority RR Preemptive vs Non-preemptive Scheduling Gantt harts & Performance omparison University at uffalo September 11 th, asic oncepts lternating Sequence of PU nd I/O ursts Multiprogramming is needed for efficient PU utilization PU Scheduling: deciding which processes to execute when Process execution begins with a PU burst, followed by an I/O burst PU I/O urst ycle Process execution consists of a cycle of PU execution and I/O wait 3 4 Histogram of PU-burst urations Process State s a process executes, it changes state new: The process is being created ready: The process is waiting to be assigned to a process running: Instructions are being executed waiting: The process is waiting for some event to occur terminated: The process has finished execution 5 6

2 PU Scheduler Selects from among the processes in memory that are ready to execute, and allocates the PU to one of them! short-term scheduler PU 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 5. new process arrives Scheduling under 1 and 4 is nonpreemptive/cooperative Once a process gets the PU, keeps it until termination/switching to waiting state/release of the PU ll other scheduling is preemptive Most OS use this ost associated with access to shared data i.e. time quota expires ispatcher ispatcher module gives control of the PU to the process selected by the short-term scheduler; Its function involves: switching context switching to user mode jumping to the proper location in the user program to restart that program ispatch latency time it takes for the dispatcher to stop one process and start another running 7 8 Scheduling riteria Optimization riteria PU utilization keep the PU as busy as possible --> maximize Throughput # of processes that complete their execution per time unit -->maximize Turnaround time amount of time passed to finish execution of a particular process --> minimize i.e. execution time + waiting time Waiting time total amount of time a process has been waiting in the ready queue -->minimize 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) -->minimize 9 Maximize PU utilization Maximize throughput Minimize turnaround time Minimize waiting time Minimize response time 1 Scheduling Metrics! Scheduling metrics " arrival time t a = time the process became Ready (again) " wait time T w = time spent waiting for the PU " service time T s = time spent executing in the PU " turnaround time T r = total time spent waiting and executing t a = T w + T s First-ome, First-Served (FFS) Scheduling " processes are assigned the PU in the order they request it " when the running process blocks, the first Ready is run next " when a process gets Ready, it is put at the end of the queue #5 arrived T w T s xecution times T r #5 executed T r / T s = 2.5 Mean FFS scheduling policy 11 12

3 FFS Scheduling - xample Process urst 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 hart for the schedule is: P 1 P 2 P Waiting time for P 1 = ; P 2 = 24; P 3 = 27 verage waiting time: ( )/3 = FFS Scheduling - xample 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 Waiting time for P 1 = 6; P 2 = ; P 3 = 3 verage waiting time: ( )/3 = 3 Much better than previous case onvoy effect short process behind long process P Shortest-Job-First (SJF) Scheduling Non-Preemptive SJF ssociate with each process the length of its next PU burst. Use these lengths to schedule the process with the shortest time Two schemes: nonpreemptive once PU given to the process it cannot be preempted until completes its PU burst preemptive if a new process arrives with PU 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 SJF " nonpreemptive, assumes the run times are known in advance " among several equally important Ready jobs (or PU bursts), the scheduler picks the one that will finish the earliest Shortest Job First (SJF) Mean SJF scheduling policy Non-Preemptive SJF - xample Process rrival Time urst Time P 1. 7 P P P SJF (non-preemptive) Gantt hart Preemptive SJF (SRT)! Shortest Remaining Time (SRT) " preemptive version of SJF, also assumes known run time " choose the process whose remaining run time is shortest " allows new short jobs to get good service P 1 P 3 P 2 P Mean verage waiting time = ( )/4 = 4 17 SRT scheduling policy 18

4 xample of Preemptive SJF Priority Scheduling Process rrival Time urst Time P 1. 7 P P P SJF (preemptive) Gantt hart P 1 P 2 P 3 P 2 P 4 P priority number (integer) is associated with each process The PU 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 PU burst time Problem Starvation low priority processes may never execute Solution ging as time progresses increase the priority of the process 19 2 xample of Priority Process rrival Time urst Time Priority P P P P Priority (non-preemptive) P1 --> P2 --> P4 --> P3 Priority (preemptive)?? 21 ach process gets a small unit of PU time (time quantum), usually 1-1 milliseconds. fter 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 PU 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 22 " preemptive FFS, based on a timeout interval, the quantum q " the running process is interrupted by the clock and put last in a FIFO Ready queue; then, the first Ready process is run instead " a crucial parameter is the quantum q (generally ~1 1ms) # q should be big compared to context switch latency (~1µs) # q should be less than the longest PU bursts, otherwise RR degenerates to FFS Mean Mean RR (q = 1) scheduling policy RR (q = 4) scheduling policy 23 24

5 xample of RR with Time Quantum = 2 Time Quantum and ontext Switch Time Process urst Time P 1 53 P 2 17 P 3 68 P 4 24 For q=2, 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 Turnaround Time Varies With The Time Quantum xercise raw gantt charts, find average turnaround and waiting times for above processes, considering: 1) First ome First Served Scheduling 2) Shortest Job First Scheduling (non-preemptive) 3) Shortest Job First Scheduling (preemptive) 4) Round-Robin Scheduling 5) Priority Scheduling (non-preemptive) 6) Priority Scheduling (preemptive) Summary PU Scheduling asic oncepts Scheduling riteria & Metrics ifferent Scheduling lgorithms FFS SJF Priority RR Next Lecture: ontinue PU Scheduling Hmm.. Reading ssignment: hapter 5 from Silberschatz. cknowledgements Operating Systems oncepts book and supplementary material by. Silberschatz, P. Galvin and G. Gagne Operating Systems: Internals and esign Principles book and supplementary material by W. Stallings Modern Operating Systems book and supplementary material by. Tanenbaum R. oursat and M. Yuksel from UNR 29 3

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