CSE 331 Operating Systems Design 1 Scheduling Scheduling is divided into various levels. These levels are defined by the location of the processes A process can be –available to be executed by the processor –partially or fully in main memory –in secondary memory –is not started yet

CSE 331 Operating Systems Design 2 Types of Scheduling Long Term Scheduling (batch processing) –The decision to add to the pool of processes to be executed. Medium Term Scheduling (swapping) –The decision to add to the process in main memory. Short Term Scheduling(CPU Scheduling) –The decision as to which process will gain the processor. I/O Scheduling –The decision as to which process's I/O request shall be handled by a device.

CSE 331 Operating Systems Design 3 CPU Scheduling Select process(es) to run on processor(s) Process state is changed from “ready” to “running” The component of the OS which does the scheduling is called the scheduler

CSE 331 Operating Systems Design 4 Basic Concepts (CPU Scheduling) 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

CSE 331 Operating Systems Design 5 Alternating Sequence of CPU And I/O Bursts

CSE 331 Operating Systems Design 6 Histogram of CPU-burst Times

CSE 331 Operating Systems Design 7 Types of CPU Scheduling A scheduling algorithm is NON-PREEMPTIVE (run to completion) if the CPU cannot be taken away by the OS. A scheduling algorithm is PREEMPTIVE if the CPU can be taken away by the OS.

CSE 331 Operating Systems Design 8 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.

CSE 331 Operating Systems Design 9 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. (context switch overhead)

CSE 331 Operating Systems Design 10 The Interrupting Clock- Timer The OS sets the interrupting clock to generate an interrupt at some specified future time. This interrupt time is the process quantum(time slice-ts, time quantum-tq). Provides reasonable response times and prevents the system being held up by processes in infinite loops.

CSE 331 Operating Systems Design 11 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)

CSE 331 Operating Systems Design 12 Optimization Criteria Fairness : each process should get a fair share of the CPU Efficiency: keep CPU 100% utilized Response time : should be minimized for interactive users Turnaround : minimize batch turnaround times Throughput : maximize number of jobs processed per hour

CSE 331 Operating Systems Design 13 User-Oriented, Performance Criteria CriteriaAim Response Timelow response time, maximum number of interactive users Turnaround Timetime between submission and completion Deadlinesmaximize deadlines met

CSE 331 Operating Systems Design 14 System-oriented, Performance Criteria CriteriaAim Throughputallow maximum number of jobs to complete Processormaximize percentage of time processor is busy utilization Overheadminimize time processor busy executing OS

CSE 331 Operating Systems Design 15 System oriented, other criteria CriteriaAim Fairnesstreat processes the same avoid starvation Enforcing Prioritiesgive preference to higher priority processes Balancing Resourceskeep the system resources busy

CSE 331 Operating Systems Design 16 Important Factors I/O / CPU boundedness of a process Is the process interactive or batch? Process priority Page fault frequency (Virtual Memory ) Preemption frequency (time slice) Execution time received Execution time required to complete

CSE 331 Operating Systems Design 17 Popular research area in 1970’s.. Assumptions One program per user One thread per program Programs are independent

CSE 331 Operating Systems Design 18 Scheduling Algorithms FCFS FCFS Shortest Job First SJF Shortest Remaining Time Highest Response Ratio Next Round Robin RR Virtual Round Robin Priority Priority Priority Classes(Multilevel Queues) Feedback Queues

CSE 331 Operating Systems Design 19 FCFS (First Come First Serve) Implementation: –As each process becomes ready, it joins the ready queue. –When the current process finishes, the oldest process is selected next. Characteristics: –Simple to implement –Non-premptive –Penalizes short and I/O-bound processes

CSE 331 Operating Systems Design 20 First-Come, First-Served (FCFS) Scheduling ProcessBurst 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 Chart for the schedule is: Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

CSE 331 Operating Systems Design 21 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P 2, P 3, P 1. The Gantt chart for the schedule is: Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case. Convoy effect short process behind long process P1P1 P3P3 P2P

CSE 331 Operating Systems Design 22 Approximating Load Let λ = mean arrival rate So 1/ λ = mean time between arrivals And µ = mean service rate So 1/ µ = mean service time (avg t(p i )) CPU busy = ρ = λ * 1/ µ = λ / µ Notice must have λ / µ (i.e., ρ < 1) What if ρ approaches 1?

CSE 331 Operating Systems Design 23 Predicting Wait Time in FCFS In FCFS, when a process arrives, all in ready list will be processed before this job Let µ be the service rate Let L be the ready list length W avg (p) = L*1/ µ + 0.5* 1/ µ = L/ µ +1/(2 µ ) Compare predicted wait with actual in earlier examples

CSE 331 Operating Systems Design 24 Shortest-Job-First (SJF) Sometimes known as Shortest Process Next (SPN) Implementation: –The process with the shortest expected execution time is given priority on the processor

CSE 331 Operating Systems Design 25 ProcessArrival TimeBurst Time P P P P SJF (non-preemptive) Average waiting time = ( )/4 = 4 Example of Non-Preemptive SJF P1P1 P3P3 P2P P4P4 812

CSE 331 Operating Systems Design 26 SJF-continued Characteristics: –Non-premptive –Reduces average waiting time over FIFO –Always produces the minimum average turnaround time –Must know how long a process will run –Possible user abuse –Suitable for batch environments. Not useful in a timesharing environment

CSE 331 Operating Systems Design 27 Shortest Remaining Time (SRT) Preemptive counterpart of SPN Implementation: –Process with the smallest estimated run- time to completion is run next –A running process may be preempted by a new process with a shorter estimate run- time

CSE 331 Operating Systems Design 28 Example of Preemptive SJF ProcessArrival TimeBurst Time P P P P SJF (preemptive) Average waiting time = ( )/4 =3 P1P1 P3P3 P2P P4P4 57 P2P2 P1P1 16

CSE 331 Operating Systems Design 29 SRT-continued Characteristics: –Still requires estimates of the future –Higher overhead than SJF –No additional interrupts are generated as in Round Robin –Elapsed service times must be recorded

CSE 331 Operating Systems Design 30 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. .t nnn  +1 1

CSE 331 Operating Systems Design 31 Prediction of the Length of the Next CPU Burst

CSE 331 Operating Systems Design 32 Examples of Exponential Averaging  =0 –  n+1 =  n –Recent history does not count.  =1 –  n+1 = t n –Only the actual last CPU burst counts. If we expand the formula, we get:  n+1 =  t n +(1 -  )  t n-1 + … +(1 -  ) j  t n-1 + … +(1 -  ) n=1 t n  0 Since both  and (1 -  ) are less than or equal to 1, each successive term has less weight than its predecessor.

CSE 331 Operating Systems Design 33 Highest Response Ratio Next (HRRN) How do you get around the problem of Indefinite postponement? Implementation: –Once a job gets the CPU, it runs to completion –The priority of a job is a function of the job's service time and the time it has been waiting for service priority = (time waiting + service time) / service time

CSE 331 Operating Systems Design 34 Characteristics: –Nonpremptive –Shorter jobs still get preference over longer jobs –However aging ensures long jobs will eventually gain the processor –Estimation still involved

CSE 331 Operating Systems Design 35 Round Robin (RR) Implementation: –Processes are dispatched FIFO. But are given a fixed time on the CPU (quantum - time slice). Characteristics: –Preemptive –Effective in time sharing environments –Penalizes I/O bound processes

CSE 331 Operating Systems Design 36 Quantum Size Some Options: –Large or small quantum –Fixed or variable quantum –Same for everyone or different –If quantum is to large RR degenerates into FCFS –If quantum is to small context switching becomes the primary job being executed A good guide is: quantum should be slightly larger than the time required for a typical interaction (overhead 10%) For 100ms ts - context switch time < 10ms

CSE 331 Operating Systems Design 37 Example of RR with Time Quantum = 20 ProcessBurst Time P 1 53 P 2 17 P 3 68 P 4 24 The Gantt chart is: Typically, higher average turnaround than SJF, but better response. P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P

CSE 331 Operating Systems Design 38 Time Quantum and Context Switch Time

CSE 331 Operating Systems Design 39 Turnaround Time Varies With The Time Quantum

CSE 331 Operating Systems Design 40 Virtual Round Robin (VRR) A modification to the RR algorithm to remove the bias towards CPU bound processes. Implementation: –Two “ready” queues, one called an AUX queue for storing “completed” IO processes –AUX queue has priority over READY queue –IO processes only runs for remaining tim e Characteristics: –Performance studies indicate fairer than RR

CSE 331 Operating Systems Design 41 Priority Implementation: –Each process is assigned a priority and the scheduler always selects the highest priority process first Characteristics: –High priority processes may run indefinitely, so decrease the priority of these processes at regular intervals –Assign high priority to system processes with known characteristics such as being I/O bound

CSE 331 Operating Systems Design 42 Priority Classes Priority Class 4 Priority Class 2 Priority Class 1 Priority Class 3 Highest Lowest

CSE 331 Operating Systems Design 43 Implementation: –Processes are grouped into priority classes –Round Robin is used within a class –When selecting process start with the highest class. If the class is empty, use a lower class Characteristics: –If priorities are not adjusted from time to time, lower classes may starve to death

CSE 331 Operating Systems Design 44 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

CSE 331 Operating Systems Design 45 Multilevel Queue Scheduling

CSE 331 Operating Systems Design 46 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

CSE 331 Operating Systems Design 47 Example of Multilevel Feedback Queue Three queues: –Q 0 – time quantum 8 milliseconds –Q 1 – time quantum 16 milliseconds –Q 2 – FCFS Scheduling –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. –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.

CSE 331 Operating Systems Design 48 Multilevel Feedback Queues

CSE 331 Operating Systems Design 49 I/O processes: –If the job requires I/O before quantum expiration it leaves the network and comes back at the same level queue CPU bound processes: –If the quantum expires first, the process is placed on the next lower queue –This continues until it reaches the bottom queue

CSE 331 Operating Systems Design 50 Dispatching: –A process is only placed on the CPU if all higher level queues are empty –A running process is preempted by a process arriving in a higher queue –Processes from lower level queues receive a larger quantum

CSE 331 Operating Systems Design 51 Summary: A CPU Scheduling Mechanism Should Favour short jobs Favour I/O bound jobs to get good I/O device utilization Determine the nature of a job and schedule accordingly