Process Scheduling. Process Scheduling. CPU and I/O Bursts. CPU - I/O Burst Cycle. Variations in Bursts. Histogram of CPU Burst Times

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1 Scheduling The objective of multiprogramming is to have some process running all the time The objective of timesharing is to have the switch between processes so frequently that users can interact with each program as it is running Bottom line is that the OS plays a shell game and needs to switch from process to process Scheduling refers to the algorithms used to implement the switching 3/21/99 ICSS44 - Scheduling 1 running running trap/intr idle Scheduling P OS P 1 Save state in PCB Reload state from PCB 1 Save state in PCB 1 Reload state from PCB trap/intr 3/21/99 ICSS44 - Scheduling 2 idle running idle - I/O Burst Cycle The success of scheduling depends on the following observed property of processes processes execution consists of a cycle of execution and I/O wait execution alternates back and forth between these two states and I/O Bursts load add read from file wait for I/O inc write to file wait for I/O load add read from file wait for I/O Burst I/O Burst Burst I/O Burst Burst I/O Burst 3/21/99 ICSS44 - Scheduling 3 3/21/99 ICSS44 - Scheduling 4 Histogram of Burst Times Variations in Bursts frequency Burst Times burst duration (milliseconds) The duration of bursts vary greatly from process to process an I/O bound process spends most of its time doing I/O a bound process spends most of its time doing computations The distribution can be important when selecting a scheduling algorithm 3/21/99 ICSS44 - Scheduling 5 3/21/99 ICSS44 - Scheduling 6 1

2 s The selection process is carried out by the shortterm scheduler (or scheduler) The scheduling policy determines when it is time for a process to be removed form the and which process should be allocated next The scheduling mechanism determines how scheduling actually happens long-term scheduler ready queue I/O medium-term scheduler short-term scheduler I/O waiting queues 3/21/99 ICSS44 - Scheduling 7 3/21/99 ICSS44 - Scheduling 8 Context Switch Preemptive/Non-Preemptive Scheduling A context switch saves the state of the current process and loads the saved state of the new process Context switch time is pure overhead Speed varies from machine-to-machine, from OSto-OS typical speeds range from 1 to 1 microseconds Non-preemptive scheduling decisions are made when a process switches from the running to the waiting state when a process terminates Preemptive scheduling adds when a process switches from the running to the ready state (interrupt occurs) when a process switches from the waiting to the ready state 3/21/99 ICSS44 - Scheduling 9 3/21/99 ICSS44 - Scheduling 1 Non-Preemptive Programming Execution Every process periodically yields to the scheduler Relies on correct process behavior Malicious Accidental Need a mechanism to override running process Allocate Resource Manager Request Resources 3/21/99 ICSS44 - Scheduling 11 3/21/99 ICSS44 - Scheduling 12 2

3 From Other States The Scheduling & Performance Descriptor Enqueue Allocate Resource Manager Request Dispatch Context Switch can control: utilization Average time a process waits for service Average amount of time to complete a job 3/21/99 ICSS44 - Scheduling 13 3/21/99 ICSS44 - Scheduling 14 Scheduling Criteria Common criteria used to compare scheduling algorithms include utilization Throughput the number of processes completed per unit time Turnaround time the time it takes to complete the job ( wait time + run time ) Wait time Response time Strategy Strategy = policy the scheduler mechanism uses to choose from the ready list Strive for any of: Equitability Favor very short or long jobs Meet priority requirements Different policies for different requirements Mechanism never changes 3/21/99 ICSS44 - Scheduling 15 3/21/99 ICSS44 - Scheduling 16 Contemporary Scheduling Mechanism Involuntary 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 May or may not have preemption Optimal Scheduling Suppose the scheduler knows each process p i s service time, τ(p i ). Could look at every p i and τ(p i ) in the ready list and choose the scheduler that optimized on any desired criteria But Other processes may arrive while these processes are being serviced The τ(p i ) are almost certainly just estimates Algorithm to choose optimal schedule is O(n 2 ) 3/21/99 ICSS44 - Scheduling 17 3/21/99 ICSS44 - Scheduling 18 3

4 Talking About Scheduling... Let P = {p i i < n} = set of processes Let S(p i ) {running, ready, blocked} Let τ(p i ) = Time process needs to be in running state (the service time) Let W(p i ) = Time p i is in ready state before first transition to running (wait time) Let T TRnd (p i ) = Time from p i first enter ready to last exit ready (turnaround time) Batch Throughput rate = inverse of avg T TRnd Timesharing response time = W(p i ) Simple Scheduling Model Ignore the resource manager Only consider running and ready states Ignores time in blocked state created when it enters ready is destroyed when it enters blocked Really just looking at small phases of a process 3/21/99 ICSS44 - Scheduling 19 3/21/99 ICSS44 - Scheduling 2 Nonpreemptive s Easy to build and analyze Easy to analyze performance No issue of voluntary/involuntary sharing First-Come-First-Served p T TRnd (p ) = τ(p ) = 35 3/21/99 ICSS44 - Scheduling 21 3/21/99 ICSS44 - Scheduling 22 First-Come-First-Served First-Come-First-Served p p p p 1 p 2 T TRnd (p ) = τ(p ) = 35 T TRnd (p 1 ) = (τ(p 1 ) +T TRnd (p )) = = 475 W(p 1 ) = T TRnd (p ) = 35 T TRnd (p ) = τ(p ) = 35 T TRnd (p 1 ) = (τ(p 1 ) +T TRnd (p )) = = 475 T TRnd (p 2 ) = (τ(p 2 ) +T TRnd (p 1 )) = = 95 W(p 1 ) = T TRnd (p ) = 35 W(p 2 ) = T TRnd (p 1 ) = 475 3/21/99 ICSS44 - Scheduling 23 3/21/99 ICSS44 - Scheduling 24 4

5 First-Come-First-Served First-Come-First-Served p p 1 p 2 p 3 p p 1 p 2 p 3 T TRnd (p ) = τ(p ) = 35 T TRnd (p 1 ) = (τ(p 1 ) +T TRnd (p )) = = 475 T TRnd (p 2 ) = (τ(p 2 ) +T TRnd (p 1 )) = = 95 T TRnd (p 3 ) = (τ(p 3 ) +T TRnd (p 2 )) = = 12 W(p 1 ) = T TRnd (p ) = 35 W(p 2 ) = T TRnd (p 1 ) = 475 W(p 3 ) = T TRnd (p 2 ) = 95 T TRnd (p ) = τ(p ) = 35 T TRnd (p 1 ) = (τ(p 1 ) +T TRnd (p )) = = 475 T TRnd (p 2 ) = (τ(p 2 ) +T TRnd (p 1 )) = = 95 T TRnd (p 3 ) = (τ(p 3 ) +T TRnd (p 2 )) = = 12 T TRnd ( ) = (τ( ) +T TRnd (p 3 )) = = 1275 W(p 1 ) = T TRnd (p ) = 35 W(p 2 ) = T TRnd (p 1 ) = 475 W(p 3 ) = T TRnd (p 2 ) = 95 W( ) = T TRnd (p 3 ) = 12 3/21/99 ICSS44 - Scheduling 25 3/21/99 ICSS44 - Scheduling 26 FCFS Average Wait Time Easy to implement Ignores service time, etc Not a great performer p p 1 p 2 p 3 75 T TRnd (p ) = τ(p ) = 35 T TRnd (p 1 ) = (τ(p 1 ) +T TRnd (p )) = = 475 T TRnd (p 2 ) = (τ(p 2 ) +T TRnd (p 1 )) = = 95 T TRnd (p 3 ) = (τ(p 3 ) +T TRnd (p 2 )) = = 12 T TRnd ( ) = (τ( ) +T TRnd (p 3 )) = = 1275 W(p 1 ) = T TRnd (p ) = 35 W(p 2 ) = T TRnd (p 1 ) = 475 W(p 3 ) = T TRnd (p 2 ) = 95 W( ) = T TRnd (p 3 ) = 12 W avg = ( )/5 = 2974/5 = 595 3/21/99 ICSS44 - Scheduling 27 T TRnd ( ) = τ( ) = 75 W( ) = 3/21/99 ICSS44 - Scheduling p 1 p p 1 T TRnd (p 1 ) = τ(p 1 )+τ( ) = = 2 T TRnd (p 1 ) = τ(p 1 )+τ( ) = = 2 T TRnd ( ) = τ( ) = 75 W( ) = T TRnd (p 3 ) = τ(p 3 )+τ(p 1 )+τ( ) = = 45 T TRnd ( ) = τ( ) = 75 W(p 3 ) = 2 W( ) = 3/21/99 ICSS44 - Scheduling 29 3/21/99 ICSS44 - Scheduling 3 5

6 p 1 p 3 p p 1 p 3 p p 2 T TRnd (p ) = τ(p )+τ(p 3 )+τ(p 1 )+τ( ) = = 8 T TRnd (p 1 ) = τ(p 1 )+τ( ) = = 2 T TRnd (p 3 ) = τ(p 3 )+τ(p 1 )+τ( ) = = 45 T TRnd ( ) = τ( ) = 75 W(p ) = 45 W(p 3 ) = 2 W( ) = T TRnd (p ) = τ(p )+τ(p 3 )+τ(p 1 )+τ( ) = = 8 T TRnd (p 1 ) = τ(p 1 )+τ( ) = = 2 T TRnd (p 2 ) = τ(p 2 )+τ(p )+τ(p 3 )+τ(p 1 )+τ( ) = = 1275 T TRnd (p 3 ) = τ(p 3 )+τ(p 1 )+τ( ) = = 45 T TRnd ( ) = τ( ) = 75 W(p ) = 45 W(p 2 ) = 8 W(p 3 ) = 2 W( ) = 3/21/99 ICSS44 - Scheduling 31 3/21/99 ICSS44 - Scheduling 32 Minimizes wait time May starve large jobs Must know service times Optimal (for min wait time)!! p 1 p 3 p p 2 T TRnd (p ) = τ(p )+τ(p 3 )+τ(p 1 )+τ( ) = = 8 T TRnd (p 1 ) = τ(p 1 )+τ( ) = = 2 T TRnd (p 2 ) = τ(p 2 )+τ(p )+τ(p 3 )+τ(p 1 )+τ( ) = = 1275 T TRnd (p 3 ) = τ(p 3 )+τ(p 1 )+τ( ) = = 45 T TRnd ( ) = τ( ) = 75 W avg = ( )/5 = 1525/5 = W(p ) = 45 W(p 2 ) = 8 W(p 3 ) = 2 W( ) = 3/21/99 ICSS44 - Scheduling 33 Predicting Bursts SJF can be approximated by guessing what the service time will be τ n+1 = αt n + ( 1 - α) τ n Where α 1 This formula defines an exponential average τ n s most recent information α controls the relative weight of recent and past history 3/21/99 ICSS44 - Scheduling 34 burst length α =.5 Predicting Burst time Actual Predicted 3/21/99 ICSS44 - Scheduling 35 Pri Priority Scheduling Reflects importance of external use May cause starvation Can address starvation with aging p 3 p 1 p 2 p T TRnd (p ) = τ(p )+τ( )+τ(p 2 )+τ(p 1 ) )+τ(p 3 ) = = 1275 T TRnd (p 1 ) = τ(p 1 )+τ(p 3 ) = = 375 T TRnd (p 2 ) = τ(p 2 )+τ(p 1 )+τ(p 3 ) = = 85 T TRnd (p 3 ) = τ(p 3 ) = 25 T TRnd ( ) = τ( )+ τ(p 2 )+ τ(p 1 )+τ(p 3 ) = = 925 W avg = ( )/5 = 24/5 = W(p ) = 925 W(p 1 ) = 25 W(p 2 ) = 375 W(p 3 ) = W( ) = 85 3/21/99 ICSS44 - Scheduling 36 6

7 Deadline (none) 2 Deadline Scheduling Must receive service by deadline May not be feasible p 1 p p 2 p 3 p 1 p p 2 p 3 p p 1 p 2 p 3 Preemptive s Highest priority process is guaranteed to be running at all times Or at least at the beginning of a time slice Dominant form of contemporary scheduling But complex to build & analyze 3/21/99 ICSS44 - Scheduling 37 3/21/99 ICSS44 - Scheduling 38 Round Robin (TQ=5) Round Robin (TQ=5) 5 p p 1 p 1 W(p 1 ) = 5 3/21/99 ICSS44 - Scheduling 39 3/21/99 ICSS44 - Scheduling 4 Round Robin (TQ=5) Round Robin (TQ=5) p 1 p 1 p 2 p 1 p 1 p 2 2 p 3 W(p 1 ) = 5 W(p 2 ) = 1 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 3/21/99 ICSS44 - Scheduling 41 3/21/99 ICSS44 - Scheduling 42 7

8 Round Robin (TQ=5) Round Robin (TQ=5) p 1 p 1 p 2 2 p 3 p 1 p 1 p 2 2 p 3 3 p W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 3/21/99 ICSS44 - Scheduling 43 3/21/99 ICSS44 - Scheduling 44 Round Robin (TQ=5) Round Robin (TQ=5) p p 1 p 2 p 3 p p 1 p 2 p p p 1 p 2 p 3 p p 1 p 2 p 3 p p 1 T TRnd ( ) = 475 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 T TRnd (p 1 ) = 55 T TRnd ( ) = 475 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 3/21/99 ICSS44 - Scheduling 45 3/21/99 ICSS44 - Scheduling 46 Round Robin (TQ=5) Round Robin (TQ=5) p p 1 p 2 p 3 p p 1 p 2 p 3 p p 1 p 2 p p p 1 p 2 p 3 p p 1 p 2 p 3 p p 1 p 2 p p p 2 p 3 p p 2 p 3 p p 2 p 3 p p 2 p 3 p p 2 p T TRnd (p 1 ) = 55 T TRnd (p 3 ) = 95 T TRnd ( ) = 475 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 T TRnd (p ) = 11 T TRnd (p 1 ) = 55 T TRnd (p 3 ) = 95 T TRnd ( ) = 475 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 3/21/99 ICSS44 - Scheduling 47 3/21/99 ICSS44 - Scheduling 48 8

9 Round Robin (TQ=5) p p 1 p 2 p 3 p p 1 p 2 p 3 p p 1 p 2 p p Round Robin (TQ=5) Equitable Most widely-used Fits naturally with interval timer p 1 p 2 p 3 p p 1 p 2 p 3 p p 1 p 2 p p p 2 p 3 p p 2 p 3 p p 2 p p 2 p 2 p 2 p 2 p p 2 p 3 p p 2 p 3 p p 2 p p 2 p 2 p 2 p 2 T TRnd (p ) = 11 T TRnd (p 1 ) = 55 T TRnd (p 2 ) = 1275 T TRnd (p 3 ) = 95 T TRnd ( ) = 475 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 3/21/99 ICSS44 - Scheduling 49 T TRnd (p ) = 11 T TRnd (p 1 ) = 55 T TRnd (p 2 ) = 1275 T TRnd (p 3 ) = 95 T TRnd ( ) = 475 T TRnd _ avg = 87 W(p 1 ) = 5 W(p 2 ) = 1 W(p 3 ) = 15 W( ) = 2 W avg = 1 3/21/99 ICSS44 - Scheduling 5 p RR with Overhead=1 (TQ=5) p 1 p 2 p 3 p p 1 p 2 p 3 p p 1 p 2 p 3 T TRnd (p ) = 132 T TRnd (p 1 ) = 66 T TRnd (p 2 ) = 1535 T TRnd (p 3 ) = 114 T TRnd ( ) = 565 Overhead must be considered p p 2 p 3 p p 2 p 3 p p 2 p p 2 p 2 p 2 p 2 T TRnd _ avg = 144 W(p 1 ) = 6 W(p 2 ) = 12 W(p 3 ) = 18 W( ) = 24 W avg = 12 3/21/99 ICSS44 - Scheduling Multi-Level Queues All processes at level i run before any process at level j At a level, use RR BSD UNIX has 32 run queues Queues -7 are for system Queues 8-31are for user nice influences run queue 3/21/99 ICSS44 - Scheduling 52 BSD Priorities es have two priorities; one for user-mode execution, and one for kernel-mode execution Priorities range from to 127 ( is highest priority) User mode priorities range from PUSER (5) to 127 Priorities in UNIX are not fixed, they change based on usage by the process and system load 3/21/99 ICSS44 - Scheduling 53 BSD Priority A process s scheduling priority is determined by two values found in the PCB p_cpu - estimate of recent utilization p_nice - user-settable weighting factor (-2 to 2) A process s user-mode priority is calculated every 4 clock ticks (clamped to 127) p _ usrpri PUSER p _ cpu 4 2 p _ nice 3/21/99 ICSS44 - Scheduling 54 9