1. Chapter 2
Memory and process
Management
PART 2: PROCESS
MANAGEMENT IN OS
DEPARTMENT OF IT & COMMUNICATION
POLITEKNIK TUANKU SYED SIRAJUDDIN

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 different types of scheduling algorithms
4) Explain how queuing and the scheduler work together
5) Differences between multiprogramming and time
sharing
6) Explain how to handle deadlock

3. Process
Process – a program in execution.
A process – need resources (CPU time, files, I/O
devices) to accomplish task.
process execution must progress in sequential
fashion

4. Process include:
1)Program counter
2)Process Stack – containing temp data (subroutine
parameters, return addresses, temp variables)
3)Data section – containing global variables.

5. Process state

6. ……………………….Process state
As a processes executes, it changes state.
Each process – in on of the following state :
 NEW – The process is being created.
 READY – The process is waiting to be assigned to a pocessor.
 RUNNING – Instruction are being executed.
 WAITING – The process are waiting for some event to occur
(I/O completion or reception of a signal)
 TERMINATED – The process has finished execution.

7. Control blocks
Each process is representer in the OS by a process
control blok (PCB).
There are several control fields that must be
maintained in support of each active program.
Often, a control block is created to hold:
1) a partition’s key control flags,
2) constants
3) variables
The control blocks (one per partition) are linked to
form a linked list.

8. ………………………..Control blocks
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) andother measures of the program’s impact on
system resources.

9. Control blocks
•Information about
each program is
stored in the
program’s control
block.
•The dispatcher
determines which
program to start
next by
following a linked
list of
control blocks.

10. 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 the time
of the interrupt resumes processing.

11. Example of how interrupt work
Step 1:

12. Example of how interrupt work
Step 2:

13. Example of how interrupt work
Step 3:

14. Example of how interrupt work
Step 4:

15. Example of how interrupt work
Step 5:

16. Example of how interrupt work
Step 6:

17. CPU SCHEDULING
Basic Conceptts :
Objective of multiprogramming  to have some
process running at all time, to maximize CPU
utilization.
When one process in wait state  OS will take the
CPU away from that process and gives the CPU to
another one. This pattern continues…
Uniprecess ???  Only ONE running process. If
more than one ???

18. ……………………………….CPU Scheduler
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
 Scheduling under 1 and 4 is nonpreemptive
All other scheduling is preemptive

19. ………………………………...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 by switching o the waiting state.

20. …………………………………………CPU Scheduling
Scheduling criteria:
1)CPU utilization
The ratio of busy time of the processor to the total time
passes for processes to finish.
Processor Utilization =
(Processor busy time) /
(Processor busy time + Processor idle time)
To keep the CPU as busy as possible

21. …………………………………………CPU Scheduling
2) Throughput
If the CPU is busy executing process  then work
is being done.
The measure of work done in a unit time interval.
Throughput =
(Number of processes completed) / (Time Unit)
Long process would take one process/hour
Short process might take 10 processes/second.

22. …………………………………………….CPU Scheduling
3) Turnaround time
 How long it takes to execute a process.
 The sum of time spent waiting to get into memory, waiting
in ready queue, execution time on CPU and doing I/O time.
tat = time(process completed) – time(process submitted)
4) Waiting time
 The sum of periods spent waiting in the ready queue only.
5) Response time
 Time from the submission of a request until the first
response is produced.
 This criterion is important for interactive systems.
rt = t(first response) – t(submission of request)

23. ………………………………………….CPU Scheduling
Types of scheduling:
1)long-term scheduling
2)Medium-term scheduling
3)Short-term scheduling

24. 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
– swapped out (to disk), so added to queue for medium-term
scheduler

25. 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
- must consider memory management if no virtual memory
(VM), so look at memory requirements of swapped out
processes

26. 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
– Events: clock interrupt, I/O interrupt, OS call, signal

27. Scheduling Algorithm
CPU scheduling deals with the
problem of deciding which of
the processes in the ready
queue is to be allocated the
CPU.

28. …………………………………….Scheduling Algorithm
Types of scheduling algorithm:
Basic strategies
1)First In First Out (FIFO) / First –Come, First-Served.
2)Shortest Job First (SJF)
3)Shortest Remaining Time First (SRTF)
4)Round Robin (RR)
5)Priority
Combined strategies
1)Multi-level queue
2)Multi-level feedback queue

29. First Come First Serve (FIFO)
 Non-preemptive.
 Handles jobs according to their arrival time -- the earlier they
arrive, the sooner they’re served.
 The process the request the CPU first is allocated the CPU
first.
 Simplest algorithm to implement -- uses a FIFO queue.
 Good for batch systems; not so good for interactive ones.
 Turnaround time is unpredictable.

30. 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.
 Doesn’t work in interactive systems because users don’t
estimate in advance CPU time required to run their jobs.

31. 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 finish each job.
 SRT involves more overhead than SJN.
 OS monitors CPU time for all jobs in READY queue and
performs “context switching”.

32. 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
 size crucial to system performance (100 ms to 1-2 secs)

33. Priority scheduling
 Non-preemptive.
 Gives preferential treatment to important jobs.
 Programs with highest priority are processed first.
 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 first (first come first served
within priority).
 Many different methods of assigning priorities by system
administrator or by Processor Manager.

34. Multi-level queue

35. Multi-level queue

36. Multi-level queue

37. Multi-level feedback queue (MLFQ)

38. Multi-level feedback queue (MLFQ) : Example

39. Queuing and scheduler
As one program finishes processing and space
becomes available, which program is loaded
into memory next?
This decision typically involves two separate
modules, a queuing routine and a
scheduler

40. Queuing and scheduler
1) As programs enter the
system, they are placed on a
queue by the queuing
routine.
2) When space becomes
available, the scheduler
selects a program from the
queue and loads it into
memory.

41. 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
 In a timesharing system, each user has his or her own virtual
machine, in which all system resources are (in effect)
available for use