ITEC 502 Chapter 5 CPU Scheduling MiJung Choi

ITEC 502 컴퓨터 시스템 및 실습 Chapter 5: CPU Scheduling Mi-Jung Choi mjchoi@postech. ac. kr DPNM Lab. , Dept. of CSE, POSTECH

Contents 1. Basic Concepts 2. Scheduling Criteria 3. Scheduling Algorithms 4. Multiple-Processor Scheduling 5. Real-Time Scheduling 6. Thread Scheduling ITEC 502 컴퓨터 시스템 및 실습 2

Basic Concepts § Process Scheduling is – the basis of multi-programmed operating system § Terminology – CPU scheduling, Process scheduling, Kernel Thread scheduling – used interchangeably, we use process scheduling ITEC 502 컴퓨터 시스템 및 실습 3

Basic Concepts § In a single-processor system – Only one process can run at a time – Any others must wait until the CPU is free and can be rescheduled – When the running process goes to the waiting state, • the OS may select another process to assign CPU to improve CPU utilization – Every time one process has to wait, another process can take over use of the CPU § Process scheduling is – a fundamental function of operating-system – to select a process from the ready queue and assign the CPU § Maximum CPU utilization obtained with multiprogramming – by process scheduling ITEC 502 컴퓨터 시스템 및 실습 4

Diagram of Process State § It is important to realize that only one process can be running on any processor at any instant § Many processes may be ready and waiting states ITEC 502 컴퓨터 시스템 및 실습 5

Process Scheduling § Two types of queues: one ready queue, a set of device queues § Two types of resources: CPU, I/O § Arrow indicates the flow of processes in the system ITEC 502 컴퓨터 시스템 및 실습 6

CPU - I/O Burst Cycle § Process execution consists of – a cycle of CPU execution (CPU burst) and I/O wait (I/O burst) § Process alternate between these two states – Process execution begins with a CPU burst, which is followed by an I/O burst, and so on – Eventually, the final CPU burst ends with a system call to terminate execution § CPU burst distribution of a process – varies greatly from process to process and from computer to computer ITEC 502 컴퓨터 시스템 및 실습 7

Scheduling: Introduction to Scheduling § Bursts of CPU usage alternate with periods of I/O wait – a CPU-bound process – an I/O bound process ITEC 502 컴퓨터 시스템 및 실습 8

Alternating Sequence of CPU & I/O Bursts CPU burst time I/O burst time CPU burst time ITEC 502 컴퓨터 시스템 및 실습 9

Histogram of CPU-burst Times § CPU burst distribution is generally characterized – as exponential or hyper-exponential – with large number of short CPU burst and small number of long CPU burst § I/O bound process has many short CPU bursts § CPU bound process might have a few long CPU bursts ITEC 502 컴퓨터 시스템 및 실습 10

Process Scheduler § § § is one of OS modules selects one of the processes in memory that are ready to execute, and allocates the CPU to the selected process CPU scheduling decisions may take place when a process: 1. switches from running to waiting state: I/O request, invocation of wait() for the termination of other process 2. switches from running to ready state: when interrupt occurs 3. switches from waiting to ready: at completion of I/O 4. terminates § § Scheduling under 1 and 4 is non-preemptive Scheduling under 2 and 3 is preemptive ITEC 502 컴퓨터 시스템 및 실습 11

Non-preemptive vs. Preemptive § Non-preemptive scheduling – Once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU – either by terminating or by switching to the waiting state – used by Windows 3. x § Preemptive scheduling – Current running process can be switched with another at any time • because interrupt can occur at any time – Most of modern OS provides this scheme (Windows XP, Mac OS X, UNIX) – incurs a cost associated with access to shared data among processes – affects the design of the OS kernel • Certain OS (UNIX) waits either for a system call to complete or for an I/O block to take place before doing a context switch • protects critical kernel code by disabling and enabling the interrupt at the entry and exit of the code ITEC 502 컴퓨터 시스템 및 실습 12

Dispatcher § Dispatcher module is a part of a Process Scheduler § 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 ITEC 502 컴퓨터 시스템 및 실습 13

Context Switch ITEC 502 컴퓨터 시스템 및 실습 14

Scheduling Criteria § Based on the scheduling criteria, the performance of various scheduling algorithm could be evaluated – Different scheduling algorithms have different properties § CPU utilization – ratio (%) of the amount of time while the CPU was busy per time unit § Throughput – # of processes that complete their execution per time unit § Turnaround time – the interval from the time of submission of a process to the time of completion. Sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O § Waiting time – Amount of time a process has been waiting in the ready queue, which is affected by scheduling algorithm § Response time – In an interactive system, amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) ITEC 502 컴퓨터 시스템 및 실습 15

Optimization Criteria § It is desirable to maximize: – The CPU utilization – The throughput § It is desirable to minimize: – The turnaround time – The waiting time – The response time § However in some circumstances, it is desirable to optimize the minimum or maximum values rather than the average – Interactive systems, it is more important to minimize the variance in the response time than minimize the average response time ITEC 502 컴퓨터 시스템 및 실습 16

Process Scheduling Algorithms § § First-Come, First-Served Scheduling (FCFS) Shortest-Job-First Scheduling (SJF) Priority Scheduling Round-Robin Scheduling § Our measure of comparison is the average waiting time – CPU utilization, Throughput, Turnaround time ITEC 502 컴퓨터 시스템 및 실습 17

First-Come, First-Served (FCFS) Scheduling Process Burst 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: P 1 0 P 2 24 P 3 27 30 § Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 § Average waiting time: (0 + 24 + 27)/3 = 17 ITEC 502 컴퓨터 시스템 및 실습 18

FCFS Scheduling Suppose that the processes arrive in the order P 2 , P 3 , P 1 § The Gantt chart for the schedule is: P 2 0 P 3 3 P 1 6 30 § Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 § Average waiting time: (6 + 0 + 3)/3 = 3 § Much better than previous case ITEC 502 컴퓨터 시스템 및 실습 19

FCFS Scheduling 0 P 1 P 2 10 CPU (10) CPU(6) CPU(4) I/O(4) CPU (4) ITEC 502 컴퓨터 시스템 및 실습 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) 20

FCFS Scheduling § FCFS scheduling algorithm is non-preemptive – Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O – is particularly troublesome for time-sharing systems § Convoy effect occurs – When one CPU-bound process with long CPU burst and many I/O-bound process which short CPU burst – All I/O bound process waits for the CPU-bound process to get off the CPU while I/O is idle – All I/O- and CPU- bound processes executes I/O operation while CPU is idle – results in low CPU and device utilization ITEC 502 컴퓨터 시스템 및 실습 21

Shortest-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: – non-preemptive – 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 known as the Shortest-Remaining-Time-First (SRTF) § SJF is optimal – gives minimum average waiting time for a given set of processes ITEC 502 컴퓨터 시스템 및 실습 22

Example of Non-Preemptive SJF Process Arrival Time Burst Time P 1 0. 0 7 P 2 2. 0 4 P 3 4. 0 1 P 4 5. 0 4 § SJF (non-preemptive) P 1 0 3 P 3 7 P 2 8 P 4 12 16 § Average waiting time = (0 + 6 + 3 + 7)/4 = 4 ITEC 502 컴퓨터 시스템 및 실습 23

Example of Preemptive SJF Process P 1 0. 0 P 2 2. 0 P 3 4. 0 P 4 5. 0 § SJF (preemptive) P 1 0 P 2 2 Arrival Time Burst Time 7 4 1 4 P 3 4 P 2 5 P 4 7 P 1 11 16 § Average waiting time = (9 + 1 + 0 +2)/4 = 3 ITEC 502 컴퓨터 시스템 및 실습 24

Preemptive SJF Scheduling 0 P 1 P 2 10 CPU (10) CPU(6) CPU(4) I/O(4) CPU (4) ITEC 502 컴퓨터 시스템 및 실습 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) 25

Non-preemptive SJF Scheduling 0 P 1 P 2 10 CPU (10) CPU(6) CPU(4) I/O(4) CPU (4) ITEC 502 컴퓨터 시스템 및 실습 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) 26

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 § The value of tn contains our most recent information § n stores the past history § The parameter controls the relative weight of recent and past history in our prediction ITEC 502 컴퓨터 시스템 및 실습 27

Prediction of the Length of the Next CPU Burst § In this example, 0 = 10, = ½ § 1 = x t 0 + (1 - ) x 0 = ½ x 6 + ½ x 10 = 8 § 2 = x t 1 + (1 - ) x 1 = ½ x 4 + ½ x 8 = 6 ITEC 502 컴퓨터 시스템 및 실습 28

Examples of Exponential Averaging § =0 – n+1 = n-1 = n-2. … = 0 – Recent history does not count § =1 – n+1 = tn – Only the actual last CPU burst counts § If we expand the formula, we get: n+1 = tn + (1 - ) tn - 1 + … + ( 1 - ) j tn - j + … + (1 - )n +1 0 § Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor ITEC 502 컴퓨터 시스템 및 실습 29

Priority 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 – Non-preemptive § 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 process ITEC 502 컴퓨터 시스템 및 실습 30

Example of Non-Preemptive Priority Process P 1 10 P 2 1 P 3 2 P 4 1 P 5 5 Burst Time 3 1 4 5 2 Priority § Priority Scheduling (non-preemptive) P 2 0 1 P 5 P 1 6 P 3 16 P 4 18 19 § Average waiting time = (0 + 1 + 6 + 18)/5 = 8. 2 ITEC 502 컴퓨터 시스템 및 실습 31

Preemptive Priority Scheduling 0 P 0 (1) P 1 (2) P 2 (3) 10 CPU (10) CPU(6) CPU(4) I/O(4) CPU (4) ITEC 502 컴퓨터 시스템 및 실습 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) 32

Non-preemptive Priority Scheduling 0 P 0 (1) P 1 (2) P 2 (3) 10 CPU (10) CPU(6) CPU(4) I/O(4) CPU (4) ITEC 502 컴퓨터 시스템 및 실습 50 20 30 I/O (20) 40 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) 33

Round Robin (RR) Scheduling § Each process gets a small unit of CPU time (time quantum), usually 10100 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 (= q time units) in chunks of at most n x q time units at once § No process waits more than (n-1) x q time units § Performance depends on the size of the time quantum – q large RR is same as FIFO – q small provides high concurrency: each of n processes has its own processor running at 1/n the speed of the real processor ITEC 502 컴퓨터 시스템 및 실습 34

Example of RR with Time Quantum = 20 Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 § The Gantt chart is: P 1 0 P 2 20 37 P 3 P 4 57 P 1 77 P 3 P 4 P 1 P 3 97 117 121 134 154 162 § Typically, higher average turnaround than SJF, but better response ITEC 502 컴퓨터 시스템 및 실습 35

RR Scheduling (1) 0 P 0 (1) P 1 (2) P 2 (3) 10 CPU (10) CPU(6) CPU(4) 20 30 I/O (20) 50 CPU(11) CPU(9) I/O(17) I/O(4) CPU (4) 40 I/O(24) CPU (4) § Time Quantum = 2 ITEC 502 컴퓨터 시스템 및 실습 36

RR Scheduling (2) 0 P 0 (1) P 1 (2) P 2 (3) 10 CPU (10) CPU(6) CPU(4) 20 30 I/O (20) 50 CPU(11) CPU(9) I/O(17) I/O(4) CPU (4) 40 I/O(24) CPU (4) § Time Quantum = 5 ITEC 502 컴퓨터 시스템 및 실습 37

Time Quantum and Context Switch Time § The effect of context switching on the performance of RR scheduling, for example one process of 10 time quantum – quantum = 12 time units, finished in less than 1 time quantum – quantum = 6 time units, requires 2 quanta, 1 context switch – quantum = 1 time units, requires 10 quanta, 9 context switch ITEC 502 컴퓨터 시스템 및 실습 38

Round Robin (RR) Scheduling § The time quantum q must be large with respect to context switch, otherwise overhead is too high § If the context switching time is 10% of the time quantum, then about 10% of the CPU time will be spent in context switching § Most modern OS have time quanta ranging from 10 to 100 milliseconds, § The time required for a context switch is typically less than 10 microseconds; thus the context-switch time is a small fraction of the time quantum ITEC 502 컴퓨터 시스템 및 실습 39

Turnaround Time varies with the Time Quantum § The turnaround time depends on the size of the time quantum § The average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum § When SJF and RR used § If quantum = 6 and 7, average turnaround time = 10. 5 § If quantum = 1, average turnaround time = 11 ITEC 502 컴퓨터 시스템 및 실습 40

Scheduling Algorithm with multi-Queues § Multi-level Queue Scheduling § Multi-level Feedback Queue Scheduling ITEC 502 컴퓨터 시스템 및 실습 41

Multilevel Queue § Ready queue is partitioned into separate queues: foreground (interactive) background (batch) – The processes are permanently assigned to one queue, generally based on some property, or process type § Each queue has its own scheduling algorithm – foreground – RR – background – FCFS § Scheduling must be done between the queues – Fixed priority scheduling - serve all from foreground then from background, Possibility of starvation – Time slice scheduling – 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 ITEC 502 컴퓨터 시스템 및 실습 42

Multilevel Queue Scheduling § No process in the batch queue could run unless the queues with high priority were all empty § If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted ITEC 502 컴퓨터 시스템 및 실습 43

Multi-level Queue Scheduling 0 P 0 (1) P 1 (0) P 2 (0) 10 CPU (10) CPU(6) CPU(4) I/O(4) CPU (4) 20 30 I/O (20) 40 50 CPU(11) CPU(9) I/O(17) I/O(24) CPU (4) § Two-level Queues: SJF with priority 0, FCFS with priority 1 § Fixed Priority Queue § Preemptive ITEC 502 컴퓨터 시스템 및 실습 44

Multilevel Feedback Queue § A process can move between the various queues; aging can be implemented in 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 ITEC 502 컴퓨터 시스템 및 실습 45

Example of Multilevel Feedback Queue § Three queues: – Q 0 – RR with time quantum 8 milliseconds – Q 1 – RR time quantum 16 milliseconds – Q 2 – FCFS § Scheduling – A new job enters queue Q 0 which is served RR. 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 RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2 – The job is serverd based on FCFS in queue Q 2 ITEC 502 컴퓨터 시스템 및 실습 46

Multilevel Feedback Queues ITEC 502 컴퓨터 시스템 및 실습 47

Multi-level Feedback Queue Scheduling 0 P 0 (1) P 1 (0) P 2 (0) 10 CPU (10) CPU(6) CPU(4) 20 30 I/O (20) 50 CPU(11) CPU(9) I/O(17) I/O(4) CPU (4) 40 I/O(24) CPU (4) § Three-level Queues: – RR with quantum 3, RR with quantum 6, FCFS with priority 1 ITEC 502 컴퓨터 시스템 및 실습 48

Scheduling Algorithms Goals ITEC 502 컴퓨터 시스템 및 실습 49

Scheduling in Batch Systems (1) An example of shortest job first scheduling ITEC 502 컴퓨터 시스템 및 실습 50

Scheduling in Batch Systems (2) Three level scheduling ITEC 502 컴퓨터 시스템 및 실습 51

Scheduling in Interactive Systems (1) § Round Robin Scheduling – list of runnable processes after B uses up its quantum ITEC 502 컴퓨터 시스템 및 실습 52

Scheduling in Interactive Systems (2) A scheduling algorithm with four priority classes ITEC 502 컴퓨터 시스템 및 실습 53

Multiple-Processor Scheduling § CPU scheduling more complex when multiple CPUs are available § Homogeneous processors within a system or heterogeneous processors within a system § Asymmetric multiprocessing vs. Symmetric multiprocessing (SMP) – Symmetric Multiprocessing (SMP) – each processor makes its own scheduling decisions – Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing § Load sharing on SMP system – keeps the workload evenly distributed across all processors in an SMP system – Push migration vs. Pull migration ITEC 502 컴퓨터 시스템 및 실습 54

Real-Time Scheduling § Hard real-time systems – required to complete a critical task within a guaranteed amount of time § Soft real-time computing – requires that critical processes receive priority over less fortunate ones ITEC 502 컴퓨터 시스템 및 실습 55

Scheduling in Real-Time Systems Schedulable real-time system § Given – m periodic events – event i occurs within period Pi and requires Ci seconds § Then the load can only be handled if ITEC 502 컴퓨터 시스템 및 실습 56

Policy versus Mechanism § Separate what is allowed to be done with how it is done – a process knows which of its children threads are important and need priority § Scheduling algorithm parameterized – mechanism in the kernel § Parameters filled in by user processes – policy set by user process ITEC 502 컴퓨터 시스템 및 실습 57

Thread Scheduling (1) § On operating systems that support kernel-level thread, it is kernel-level threads, not processes, that are being scheduled by the operating system. § Local Scheduling – How the threads library decides which thread to put onto an available LWP § Global Scheduling – How the kernel decides which kernel thread to run next ITEC 502 컴퓨터 시스템 및 실습 58

Thread Scheduling (2) Possible scheduling of user-level threads § 50 -msec process quantum § threads run 5 msec/CPU burst ITEC 502 컴퓨터 시스템 및 실습 59

Thread Scheduling (3) Possible scheduling of kernel-level threads § 50 -msec process quantum § threads run 5 msec/CPU burst ITEC 502 컴퓨터 시스템 및 실습 60

Summary § CPU scheduling is the task of selecting a waiting process from the ready queue and allocating the CPU to it § The CPU is allocated to the selected process by the dispatcher § FCFS scheduling is simple, cause short processes to wait for long time § SJF scheduling is provably optimal, providing the shortest averaging waiting time. But predicting the length of the next CPU bursts is difficult § Priority scheduling allocates the CPU to the heights priority process § Both priority and SJF may suffer from starvation. Aging is a technique to prevent starvation § RR scheduling is more appropriate for a time-shared system § Major problem of RR scheduling is the selection of the time quantum § FCFS is non-preemptive, RR is preemptive, SJF and Priority may be preemptive and non-preemptive § Multilevel queue allows different scheduling algorithm for each queue § Multilevel feedback queue allow process to move from one queue to another ITEC 502 컴퓨터 시스템 및 실습 61

Review § § § Basic Concepts Scheduling Criteria Scheduling Algorithms § § § Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling ITEC 502 컴퓨터 시스템 및 실습 62