announcement - michigan state universityforsati/cse410s15/lectures/chapter 9.pdf · • ofﬁce...

• Midterm Exam is on Monday June 8th.

• Office hours on Friday 2-3 pm.

• Exam Material will be posted on Piazza.

Announcement

Chapter 9 Uniprocessor Scheduling

Seventh Edition By William Stallings

Modified by Rana Forsati for CSE 410

Operating Systems: Internals

and Design

Principles

Learning Objective

• Identify different goals of process scheduling algorithms

• Identify different types of scheduling and their role

• Given a set of processes, evaluate them to identify the next process to run

• Identify effect of different scheduling algorithms on CPU bound/I/O bound processes

Scheduling and Process State Transitions

Types of Scheduling

■ Broken down into three separate functions:

long term scheduling

medium term scheduling

short term scheduling

Long-Term Scheduling■ Determines when to create a new process and which

programs are admitted next to the system for processing■ Once admitted a user program becomes a process

and is added to the queue for short term scheduler■ Controls the degree of multiprogramming■ The more processes that are created, the smaller the

percentage of time that each process can be executed

Medium-Term Scheduling■ Part of the swapping function

■ Swapping-in decisions are based on the need to manage the degree of multiprogramming

■ Considers the memory requirements of the swapped-out processes

Short-Term Scheduling■ Known as the dispatcher

■ The main objective of short-term scheduling is to allocate CPU time to processes

■ Executes most frequently [minimum overhead?]

■ Invoked when an event occurs that may lead to the blocking of the current process or that may provide an opportunity to preempt a currently running process in favor of another

- Clock interrupts– I/O interrupts– Operating system calls Short-term success criteria?

9

Short Term Scheduling Algorithms

Aim of Short-term-Scheduling

■ Assign processes to be executed by the processor in a way that meets system (performance) objectives:

■ 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)

■ Minimize response time■ Throughput: # of processes that complete their execution per time unit

■ Maximize throughput■ Processor efficiency: keep the CPU as busy as possible

■ Maximize CPU utilization

Short-Term Scheduling

Dispatcher OR

short-term scheduler

■ Short-Term Scheduling: Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them

Scheduling Objectives

The scheduling function should:

– Share time fairly among processes

– Prevent starvation of processes

– Have low overhead

– Prioritize processes when necessary [flexible]

• First-Come-First-Served (FCFS)

• Round-Robin (RR)

• Shortest-Process-Next (SPN)

• Shortest-Remaining Time (SRT)

• Highest Response Ratio Next (HRRN)

Scheduling Policies

Selection Function

■ Determines which process, among ready processes, is selected next for execution

■ This function may be based on priority, resource requirements, or the execution characteristics of the process:▪ time spent in system so far, waiting ▪ time spent in execution so far▪ total service time required by the process [generally,

this quantity must be estimated or supplied by the user]

Decision Mode▪ Two categories:▪Nonpreemptive

✴Once a process is in the running state, it will continue to execute until • it terminates OR• it blocks itself for I/O or some OS service

▪Preemptive✴ Currently running process may be interrupted and moved to the Ready state by the operating system.

• a new process arrives• interrupt occurs• periodically based on a clock interrupt

Process Scheduling Example

Whatʼs the execution order?

First-Come-First-Served (FCFS)

ABCDE

• Each process joins the Ready queue

• When the current process ceases to execute, the oldest process in the Ready queue is selected

What are the potential consequences of this algorithm?


• Favors long processes: performs much better for long processes than short ones

• A short process may have to wait a very long time before it can execute

• Favors CPU-bound processes (mostly uses the processor)

• I/O processes have to wait until CPU-bound process completes


Process Service Time (ms) P1 24

P2 3 P3 3

• Suppose that the processes arrive in the order: P1 , P2 , P3

• Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 ms

P1 P2 P3

24 27 300


Suppose that the processes arrive in the order P2 , P3 , P1

• Waiting time for P1 = 6; P2 = 0; P3 = 3• Average waiting time: (6 + 0 + 3)/3 = 3 ms• Much better than previous case• Convoy effect: short process behind long process

P1P3P2

63 300

Round Robin• Clock interrupt is generated at periodic intervals

• When an interrupt occurs, the currently running process is placed in the ready queue

• Next ready job is selected

• This technique is also known as time slicing ,because each process is given a slice of time before being preempted.

• Principal design issue is the length of the time quantum, or slice, to be used [too large —> ?, too short —> ?]

Round Robin• Each process gets a small unit of CPU time known as time

slicing (time quantum).• After this time has elapsed, the process is preempted and added

to the end of the Ready queue.

Process Service Time P1 24

P2 3

P3 3

P1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30

q=4

• An amount of time is determined that allows each process to use the processor for that length of time

Round Robin

ABCDE


q = 4

Round Robin, q = 4

Round Robin, q =1

Round Robin

Finish time of each process? a) Round Robin q=30b) Round Robin q=10

Example

Example

Solution

A

B

C

D

E

Shortest Process Next (SPN)

• Another approach to reducing the bias in favor of long processes inherent in FCFS

• Associate with each process the length of its next CPU service. Use these lengths to schedule the process with the shortest time

■ Process with the shortest expected processing time is selected next

■ A short process jumps ahead of longer processes

■ Nonpreemptive policy

Shortest Process Next (SPN)- Issues

■ The difficulty is knowing or estimating the length of the required processing time of each process.

■ A risk with SPN is the possibility of starvation for longer processes, as long as there is a steady supply of shorter processes.

■ It still is not desirable for a time-sharing or transaction processing environment because of the lack of preemption.

Process Arrival Time Service Time P1 0.0 6 P2 0.0 8 P3 0.0 7 P4 0.0 3

• SPN:

• Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 ms

P4 P3P1

3 160 9

P2

24


ABCDE



■ One difficulty is the need to know, or at least estimate, the required processing time of each process (Predictability of longer processes is reduced)

■ Possibility of starvation for longer processes

■ If estimated time for process not correct, the operating system may abort it

• Can be done by using the length of previous CPU services, using exponential averaging

Shortest Process Next (SPN)- Service Estimation

Shortest Remaining Time (SRT)■ Preemptive version of SPN

■ Scheduler always chooses the process that has the shortest expected remaining processing time

• While a process A is running, if a new process B comes whose length is shorter than the remaining time of process A, then B preempts A and B is started to run.

■ Must estimate processing time


ABCDE

■ Should give superior turnaround time performance to SPN because a short job is given immediate preference to a running longer job

■ Risk of starvation of longer processes

Shortest Remaining Time (SRT)

Process Arrival Time Burst Time P1 0.0 8 P2 1.0 4 P3 2.0 9 P4 3.0 5 • SRT:

• Average waiting time = (9 + 0 + 2 + 15) / 4 = 6.5 ms

P2P1

1 170 5

P1

26

P4

10

P3

Shortest Remaining Time (SRT)

Highest Response Ratio Next (HRRN)

■ Chooses next process with the greatest value of response ratio

■ Attractive because it accounts for the age of the process

■ While shorter jobs are favored, aging without service increases the ratio so that a longer process will eventually get past competing shorter jobs

Highest Response Ratio Next (HRRN)

The algorithms with CPU-I/O cycle• The previous examples included processes with just a single CPU service.Each process execution is a CPU-I/O cycle– First, it executes on CPU– Then, it waits for I/O– After completion of I/O, executes on CPU again and so on.– It ends with execution on CPU.• Example– <3, 2, 1, 4, 5>• CPU for 3 time units then I/O for 2, …

• We will make the following assumptions while considering these algorithms with I/O services as well.

• In this class, we will assume that all I/O is concurrent.

FCFS:– A process that finishes its I/O enters at the end of the ready queue. No other change required.

Round Robin:– Same as above– We will assume the process enters the ready queue as soon as its I/O is finished.– If a process finishes its CPU service, another process can start executing immediately.– No history is maintained• E.g., if the time quantum is 10 and the process CPU service is 4 then the process will release the CPU after time 4.– After this process releases the CPU, another process can start right away and will get time quantum of 10– When the first process comes back again, it will get the entire time quantum of 10.

The algorithms with CPU-I/O cycle

The algorithms with CPU-I/O cycle

• SPN and SRT

– When a process is being considered, we will focus on the next CPU service to decide who runs first

• HRRN– We will consider total service time and total waiting time in deciding the process to be chosen


Consider a system where there are 3 processes, A, B, and C. Their details are as follows

A: Arrival time = 0, CPU-I/O cycle = <4,3,2,6,6> B: Arrival time = 2, CPU-I/O cycle = <2,5,4,5,4> C: Arrival time = 4, CPU I/O cycle = <5,2,3,3,5>

Show the execution with FCFS:

Feedback Scheduling

• Preemptive with time quantum

• Demoted to the next lower-priority queue

• With each queue (except the lowest priority queue), FCFS

• Lowest-priority queue: RR

• Penalize jobs that have been running longer

• Donʼt know remaining time process needs to

• execute

Traditional UNIX Scheduling• Multilevel feedback using round robin within each ofthe priority queues• Priorities are recomputed once per second• Base priority divides all processes into fixed bands ofpriority levels• Adjustment factor used to keep process in its assignedband (decreasing order of priority):– Swapper– Block I/O device control– File manipulation– Character I/O device control– User processes

Summary

■ The operating system must make three types of scheduling decisions with respect to the execution of processes:

■ Long-term – determines when new processes are admitted to the system■ Medium-term – part of the swapping function and determines when a program is

brought into main memory so that it may be executed■ Short-term – determines which ready process will be executed next by the

processor

■ From a user’s point of view, response time is generally the most important characteristic of a system; from a system point of view, throughput or processor utilization is important

■ Algorithms:

■ FCFS, Round Robin, SPN, SRT, HRRN, Feedback

announcement - michigan state universityforsati/cse410s15/lectures/chapter 9.pdf · • ofﬁce...

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