Process SchedulingIntroductionTopics for discussionThe CPU-I/O CycleCPU/IO BurstsMotivationSlide 7Scheduling CriteriaOptimization CriteriaTypes of schedulingClassification of Scheduling ActivityFirst-Come, First-Served (FCFS) SchedulingFCFS Scheduling (Cont.)Shortest-Job-First (SJR) SchedulingExample of Non-Preemptive SJFExample of Preemptive SJFDetermining Length of Next CPU BurstExamples of Exponential AveragingMore on Exponential AveragingExponentially Decreasing CoefficientsSlide 21Shortest Process Next: critiquePriority SchedulingRound Robin (RR)Example of RR with Time Quantum = 20Time Quantum and Context Switch TimeTurnaround Time Varies With The Time QuantumOther policiesScheduling in Real-Time SystemsSummary01/14/19 1Process SchedulingB.Ramamurthy01/14/19 2IntroductionAn important aspect of multiprogramming is scheduling. The resources that are scheduled are IO and processors. The goal is to achieveHigh processor utilizationHigh throughputnumber of processes completed per unit timeLow response timetime elapse from the submission of a request to the beginning of the response01/14/19 3Topics for discussionMotivationTypes of schedulingShort-term schedulingVarious scheduling criteriaVarious algorithmsPriority queuesFirst-come, first-servedRound-robinShortest process firstShortest remaining time and othersQueuing Model and Performance Analysis01/14/19 4The CPU-I/O CycleWe observe that processes require alternate use of processor and I/O in a repetitive fashionEach cycle consist of a CPU burst (typically of 5 ms) followed by a (usually longer) I/O burst A process terminates on a CPU burstCPU-bound processes have longer CPU bursts than I/O-bound processes01/14/19 5CPU/IO BurstsBursts of CPU usage alternate with periods of I/O waita CPU-bound processan I/O bound process01/14/19 6MotivationConsider these programs with processing-component and IO-component indicated by upper-case and lower-case letters respectively. A1 a1 A2 a2 A30 30 50 80 120 130 ===> JOB A B1 b1 B20 20 40 60 ====> JOB B C1 c1 C2 c2 C3 c3 C4 c4 C5 0 10 20 60 80 100 110 130 140 150 =>JOB C01/14/19 7MotivationThe starting and ending time of each component are indicated beneath the symbolic references (A1, b1 etc.)Now lets consider three different ways for scheduling: no overlap, round-robin, simple overlap.Compare utilization U = Time CPU busy / Total run time01/14/19 8Scheduling CriteriaCPU utilization – keep the CPU as busy as possibleThroughput – # of processes that complete their execution per time unitTurnaround time – amount of time to execute a particular processWaiting time – amount of time a process has been waiting in the ready queueResponse time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)01/14/19 9Optimization CriteriaMax CPU utilizationMax throughputMin turnaround time Min waiting time Min response time01/14/19 10Types of schedulingLong-term : To add to the pool of processes to be executed.Medium-term : To add to the number of processes that are in the main memory.Short-term : Which of the available processes will be executed by a processor?IO scheduling: To decide which process’s pending IO request shall be handled by an available IO device.01/14/19 11Classification of Scheduling ActivityLong-term: which process to admitMedium-term: which process to swap in or outShort-term: which ready process to execute next01/14/19 12First-Come, First-Served (FCFS) SchedulingProcess Burst TimeP124 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 = 27Average waiting time: (0 + 24 + 27)/3 = 17P1P2P324 27 30001/14/19 13FCFS 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 = 3Average waiting time: (6 + 0 + 3)/3 = 3Much better than previous case.Convoy effect short process behind long processP1P3P263 30001/14/19 14Shortest-Job-First (SJR) SchedulingAssociate 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.01/14/19 15Process Arrival TimeBurst TimeP10.0 7 P22.0 4 P34.0 1 P45.0 4Average waiting time = (0 + 6 + 3 + 7)/4 - 4Example of Non-Preemptive SJFP1P3P273 160P48 1201/14/19 16Example of Preemptive SJFProcess Arrival TimeBurst TimeP10.0 7 P22.0 4 P34.0 1 P45.0 4Average waiting time = (9 + 1 + 0 +2)/4 - 3P1P3P242110P45 7P2P11601/14/19 17Determining Length of Next CPU BurstCan only estimate the length.Can be done by using the length of previous CPU bursts, using exponential averaging.:Define 4.10 , 3.burst CPU next the for value predicted 2.burst CPU of lenght actual 1.1nthnnt .tnnn1101/14/19 18Examples of Exponential Averaging =0n+1 = nRecent history does not count. =1 n+1 = tnOnly the actual last CPU burst counts.If we expand the formula, we get:n+1 = tn+(1 - ) t n -1 + … +(1 - ) j t n -j + … +(1 - )n+1 1Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.01/14/19 19More on Exponential AveragingS[n+1] next burst, s[n] current burstS[n+1] = T[n] + (1-) S[n] ; 0 < < 1more weight is put on recent instances whenever > 1/nBy expanding this eqn, we see that weights of past instances are decreasing exponentiallyS[n+1] = T[n] + (1-)T[n-1] + ... (1-)iT[n-i] + ... + (1-)nS[1]predicted value of 1st instance S[1] is not calculated; usually set to 0 to give priority to to new processes01/14/19 20Exponentially Decreasing Coefficients01/14/19 21Exponentially Decreasing CoefficientsHere S[1] = 0 to give high priority to new processesExponential averaging tracks changes in process behavior much
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