Outline Announcement Process Scheduling– continued Preemptive scheduling algorithms Round robin Multiple-level queue Please turn in your lab 1 report in hardcopy with source code and running results attached I will let you know next Tuesday how to assign up for demonstration Please pick up your first quiz if you have not done so

Announcement Dr. Andy Wang will give a guest lecture this Thursday He is going to talk about his research on operating systems There will be no office hours this Thursday I will be attending a symposium in DC area We will have recitation session tomorrow I will go over the code I have for Lab 2 I will go over two examples on how to synchronize threads 4/9/2019 COP4610

Process Scheduler - review Ready List Scheduler CPU Resource Manager Resources Preemption or voluntary yield Allocate Request Done New Process job “Ready” “Running” “Blocked” 4/9/2019 COP4610

The Scheduler Organization Ready Process Enqueuer Ready List Dispatcher Context Switcher Process Descriptor CPU From Other States Running Process 4/9/2019 COP4610

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) 4/9/2019 COP4610

First-Come-First-Served Assigns priority to processes in the order in which they request the processor p0 p1 p2 p3 p4 1275 1200 900 475 350 4/9/2019 COP4610

Shortest-Job-Next Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. SJN is optimal gives minimum average waiting time for a given set of processes. 4/9/2019 COP4610

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 Average time spent in ready queue = (9 + 1 + 0 +2)/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16 4/9/2019 COP4610

Priority Scheduling In priority scheduling, processes/threads are allocated to the CPU based on the basis of an externally assigned priority A commonly used convention is that lower numbers have higher priority SJN is a priority scheduling where priority is the predicted next CPU burst time. FCFS is a priority scheduling where priority is the arrival time 4/9/2019 COP4610

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. 4/9/2019 COP4610

Round Robin (TQ=50) – cont. i t(pi) 0 350 1 125 2 475 3 250 4 75 p0 TTRnd(p0) = 1100 TTRnd(p1) = 550 TTRnd(p2) = 1275 TTRnd(p3) = 950 TTRnd(p4) = 475 W(p0) = 0 W(p1) = 50 W(p2) = 100 W(p3) = 150 W(p4) = 200 Wavg = (0+50+100+150+200)/5 = 500/5 = 100 475 400 300 200 100 Equitable Most widely-used Fits naturally with interval timer p4 p1 p3 p2 550 650 750 850 950 1050 1150 1250 1275 TTRnd_avg = (1100+550+1275+950+475)/5 = 4350/5 = 870 4/9/2019 COP4610

Round-robin Time Quantum 4/9/2019 COP4610

Process/Thread Context Rn . . . Status Registers Functional Unit Left Operand Right Operand Result ALU PC IR Ctl Unit 4/9/2019 COP4610

Context Switching - review CPU New Thread Descriptor Old Thread Descriptor 4/9/2019 COP4610

Cost of Context Switching Suppose there are n general registers and m control registers Each register needs b store operations, where each operation requires K time units (n+m)b x K time units Note that at least two pairs of context switches occur when application programs are multiplexed each time 4/9/2019 COP4610

Round Robin (TQ=50) – cont. Overhead must be considered i t(pi) 0 350 1 125 2 475 3 250 4 75 p0 TTRnd(p0) = 1320 TTRnd(p1) = 660 TTRnd(p2) = 1535 TTRnd(p3) = 1140 TTRnd(p4) = 565 W(p0) = 0 W(p1) = 60 W(p2) = 120 W(p3) = 180 W(p4) = 240 Wavg = (0+60+120+180+240)/5 = 600/5 = 120 540 480 360 240 120 p4 p1 p3 p2 575 790 910 1030 1150 1270 1390 1510 1535 TTRnd_avg = (1320+660+1535+1140+565)/5 = 5220/5 = 1044 635 670 4/9/2019 COP4610

Multi-Level Queues All processes at level i run before Preemption or voluntary yield Ready List0 New Process Scheduler Ready List1 CPU Done Ready List2 All processes at level i run before any process at level j At a level, use another policy, e.g. RR Ready List3 4/9/2019 COP4610

Multilevel Queues Ready queue is partitioned into separate queues foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground – RR background – FCFS 4/9/2019 COP4610

Multilevel Queues – cont. 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 4/9/2019 COP4610

Multilevel Queue Scheduling 4/9/2019 COP4610

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 4/9/2019 COP4610

Multilevel Feedback Queues – cont. 4/9/2019 COP4610

Example of Multilevel Feedback Queue Three queues: Q0 – time quantum 8 milliseconds Q1 – 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. 4/9/2019 COP4610

Two-queue scheduling 4/9/2019 COP4610

Three-queue scheduling 4/9/2019 COP4610

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. 4/9/2019 COP4610

Algorithm Evaluation Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload. Queuing models Implementation 4/9/2019 COP4610

Evaluation of CPU Schedulers by Simulation 4/9/2019 COP4610

Contemporary Scheduling Involuntary CPU sharing -- timer interrupts Time quantum determined by interval timer -- usually fixed for every process using the system Sometimes called the time slice length Priority-based process (job) selection Select the highest priority process Priority reflects policy With preemption Usually a variant of Multi-Level Queues 4/9/2019 COP4610

Scheduling in real OSs All use a multiple-queue system UNIX SVR4: 160 levels, three classes time-sharing, system, real-time Solaris: 170 levels, four classes time-sharing, system, real-time, interrupt OS/2 2.0: 128 level, four classes background, normal, server, real-time Windows NT 3.51: 32 levels, two classes regular and real-time Mach uses “hand-off scheduling” 4/9/2019 COP4610

BSD 4.4 Scheduling Involuntary CPU Sharing Preemptive algorithms 32 Multi-Level Queues Queues 0-7 are reserved for system functions Queues 8-31 are for user space functions nice influences (but does not dictate) queue level 4/9/2019 COP4610

Windows NT/2K Scheduling Involuntary CPU Sharing across threads Preemptive algorithms 32 Multi-Level Queues Highest 16 levels are “real-time” Next lower 15 are for system/user threads Range determined by process base priority Lowest level is for the idle thread 4/9/2019 COP4610

Scheduler in Linux In file kernel/sched.c The policy is a variant of RR scheduling 4/9/2019 COP4610

Summary The scheduler is responsible for multiplexing the CPU among a set of ready processes / threads It is invoked periodically by a timer interrupt, by a system call, other device interrupts, any time that the running process terminates It selects from the ready list according to its scheduling policy Which includes non-preemptive and preemptive algorithms 4/9/2019 COP4610