Chapter 2 Memory and process Management

(1)

Chapter 2

Memory and process

Management

Part 2: Process

management in OS

Prepared by: Nor Zuraida Bt Mohd Gaminan

Department of IT & Communication

(2)

Learning Outcome

 By the end of this chapter, student will be able to:

1)Explain role of control blocks and interrupt in

the dispatching process

2)Describe various types of scheduling processes 3)Explain diferent types of scheduling algorithms 4)Explain how queuing and the scheduler work

together

5)Diferences between multiprogramming and

time sharing

(3)

Control blocks

 There are several control felds that must be maintained in

support of each active program.

 Often, a control block is created to hold:

1)a partition’s key control fags, 2)constants

3) variables

 The control blocks (one per partition) are linked to form a linked

list.

 The dispatcher typically determines which program is to start by

following the chain of pointers from control block to control block.

 A given control block’s relative position in the linked list might

be determined by its priority or computed dynamically, perhaps taking into account such factors as:

1) program size, 2) time in memory,

3) peripheral device requirements

(4)

Control blocks

•_Information

about each program is stored in the program’s control block.

•_{The dispatcher}

determines which

program to start next by

(5)

Interrupts

 An interrupt is an electronic signal.

 Hardware senses the signal, saves key control information for the currently executing

program, and starts the operating system’s

interrupt handler routine. At that instant, the interrupt ends.

 The operating system then handles the interrupt.

 Subsequently, after the interrupt is processed, the dispatcher starts an application program.  Eventually, the program that was executing at

(6)

Example of how interrupt

work

(7)

work

(8)

work

(9)

work

(10)

Example of how interrupt

work

(11)

work

(12)

Process

 A program execution, process execution must progress

in sequential fashion

 Process include:

1)Program counter 2)Stack

3)Data section

 Process state:

- _New_{: The process is being created}

- _Running:_{Instruction are being executed}

(13)

(14)

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

(15)

CPU Scheduler

 _{Preemptive scheduling policy interrupts processing of a}

job and transfers the CPU to another job.

- The process may be pre-empted by the operating system when:

1)a new process arrives (perhaps at a higher priority), or 2)an interrupt or signal occurs, or

3)a (frequent) clock interrupt occurs.

 Non-preemptive scheduling policy functions without

external interrupts.

- once a process is executing, it will continue to execute until it terminates, or

(16)

CPU Scheduling

 Scheduling criteria:

1)CPU utilization

 The ratio of busy time of the processor to the total time passes for processes to fnish.

Processor Utilization = (Processor buy time) /

(Processor busy time + Processor idle time)

 Maximize, to keep as busy as possible

2) Throughput

 The measure of work done in a unit time interval.

(17)

CPU Scheduling

3) Turnaround time

 The sum of time spent in the ready queue, execution time and I/O time.

tat = t(process completed) – t(process submitted)

 minimize, time of submission to time of completion. 4) Waiting time

 minimize, time spent in ready queue - afected solely by scheduling policy

5) Response time

 The amount of time it takes to start responding to a request. This criterion is important for interactive systems.

rt = t(frst response) – t(submission of request)

 minimize

(18)

CPU Scheduling

 Types of scheduling:

1)long-term scheduling

(19)

Long-term scheduling

 Determine which programs admitted to system for processing - controls degree of

multiprogramming

 Once admitted, program becomes a process, either:

– added to queue for short-term scheduler

(20)

Medium –term scheduling

 Part of swapping function between main memory and disk

- based on how many processes the OS wants available at any one time

(21)

Short –term scheduling

(dispatcher)

 Executes most frequently, to decide which process to execute next

– Invoked whenever event occurs that interrupts current process or provides an opportunity to preempt current one in favor of another

(22)

CPU Scheduling

 Types of scheduling algorithm: Basic strategies

1)First In First Out (FIFO) 2)Shortest Job First (SJF)

3)Shortest Remaining Time First (SRTF) 4)Round Robin (RR)

5)Priority

Combined strategies

1)Multi-level queue

(23)

First Come First Serve

(FIFO)

 Non-preemptive.

 Handles jobs according to their arrival time -- the earlier they arrive, the sooner they’re

served.

 Simple algorithm to implement -- uses a FIFO queue.

 Good for batch systems; not so good for interactive ones.

(24)

Shortest Job First (SJF)

 Non-preemptive.

 Handles jobs based on length of their CPU cycle time.

◦ Use lengths to schedule process with shortest time.

 Optimal – gives minimum average waiting time for a given set of processes.

◦ optimal only when all of jobs are available at same time and the CPU estimates are available and accurate.

(25)

Shortest Remaining Time First

(SRTF)

 Preemptive version of the SJF algorithm.

 Processor allocated to job closest to completion.

◦ This job can be preempted if a newer job in READY queue has a “time to completion” that's shorter.

 Can’t be implemented in interactive system --

requires advance knowledge of CPU time required to fnish each job.

 SRT involves more overhead than SJN.

(26)

Round Robin (RR)

 FCFS with Preemption.

 Used extensively in interactive systems

because it’s easy to implement.

 Isn’t based on job characteristics but on a

predetermined slice of time that’s given to each job.

◦ Ensures CPU is equally shared among all active processes and isn’t monopolized by any one job.

 Time slice is called a time quantum

(27)

Priority scheduling

 Non-preemptive.

 Gives preferential treatment to important jobs.

◦ Programs with highest priority are processed frst.

◦ Aren’t interrupted until CPU cycles are completed or a natural wait occurs.

 If 2+ jobs with equal priority are in READY queue, processor is allocated to one that arrived frst

(frst come frst served within priority).

(28)

(29)

(30)

(31)

Multi-level feedback queue

(32)

Multi-level feedback queue

(33)

Queuing and scheduler

 As one program fnishes processing and space becomes available, which program is loaded into memory next?

 This decision typically involves two separate modules, a queuing routine and a

(34)

Queuing and scheduler

1) As programs enter the system, they are placed on a queue by the queuing routine.

(35)

Multiprogramming and time

sharing

 A timesharing system allows multiple users to interact with a computer at the same time

 Multiprogramming allowed multiple processes to be active at once, which gave rise to the

ability for programmers to interact with the

computer system directly, while still sharing its resources