1CSC 4103 - Operating SystemsFall 2009Tevfik Ko!arLouisiana State UniversitySeptember 10th, 2009Lecture - VCPU Scheduling - I2Roadmap• CPU Scheduling– Basic Concepts– Scheduling Criteria & Metrics– Different Scheduling Algorithms• FCFS• SJF• Priority• RR3Basic Concepts• Multiprogramming is needed for efficient CPU utilization • CPU Scheduling: deciding which processes to execute when• Process execution begins with a CPU burst, followed by an I/O burst• CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait4Alternating Sequence of CPU And I/O Bursts5Histogram of CPU-burst Durations6Process State• As 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 execution7CPU Scheduler• Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them! short-term scheduler• CPU scheduling decisions may take place when a process:1. Switches from running to waiting state2. Switches from running to ready state3. Switches from waiting to ready4. Terminates• Scheduling under 1 and 4 is nonpreemptive/cooperative– Once a process gets the CPU, keeps it until termination/switching to waiting state/release of the CPU• All other scheduling is preemptive– Most OS use this– Cost associated with access to shared data– i.e. time quota expires8Dispatcher• Dispatcher module gives control of the CPU 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• Dispatch latency – time it takes for the dispatcher to stop one process and start another running9Scheduling Criteria• CPU utilization – keep the CPU 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) -->minimize10Optimization Criteria• Maximize CPU utilization• Maximize throughput• Minimize turnaround time • Minimize waiting time • Minimize response timeScheduling Metrics11"Scheduling metrics#arrival time ta = time the process became “Ready” (again)#wait time Tw = time spent waiting for the CPU#service time Ts = time spent executing in the CPU#turnaround time Tr = total time spent waiting and executing#5 executedTr#5 arrivedArrival timesExecution times= Tw + TsTsTwTr / Ts = 2.5taFirst-Come, First-Served (FCFS) Scheduling12A B C D E MeanFCFS scheduling policy#processes are assigned the CPU 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 queueABCDEArrival timesStallings, W. (2004) Operating Systems:Internals and Design Principles (5th Edition). A B C D E Mean13FCFS Scheduling - Example 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 = 17P1P2P324 27 30014FCFS Scheduling - ExampleSuppose 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 processP1P3P263 30015Shortest-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• 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 processesNon-Preemptive SJF16ABCDEArrival timesSJF scheduling policy#nonpreemptive, assumes the run times are known in advance#among several equally important “Ready” jobs (or CPU bursts), the scheduler picks the one that will finish the earliestShortest Job First (SJF)A B C D E MeanSJFStallings, W. (2004) Operating Systems:Internals and Design Principles (5th Edition).17 Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4• SJF (non-preemptive) Gantt Chart• Average waiting time = (0 + 6 + 3 + 7)/4 = 4Non-Preemptive SJF - ExampleP1P3P273 160P48 12Preemptive SJF (SRT)18A B C D E MeanSRT scheduling policy"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 serviceABCDEArrival timesStallings, W. (2004) Operating Systems:Internals and Design Principles (5th Edition).19Example 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) Gantt ChartP1P3P242110P45 7P2P11620Priority 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 process21Example of Priority Process Arrival Time Burst Time Priority P1 0.0 7 2 P2 2.0 4 1 P3 4.0 1 4 P4 5.0 4 3• Priority (non-preemptive)– P1 --> P2 --> P4 --> P3• Priority (preemptive)– ??22Round