Process Concepts, Performance Measures and Evaluation Techniques

1
Lecture 2

• Process Concepts, Performance Measures and
  Evaluation Techniques

2
Lecture Highlights

• What is a process
• Process Control Block
• Process states and process life cycle
• Performance Measures
• Evaluation Techniques

3
What is a process

• A process is a pre-written set of ordered
  instructions which when executed causes the
  computer to behave in a manner determined by the
  code
• A process is a program/part of machine code in
  execution
• New process are spawned either by a user or by
  other processes.

4
Process and the operating system

• When a new process is created, the operating
  system typically does the following
• Assigns the process a unique process id (PID)
• Allocates a chunk of memory from the available
  heap for the process
• Initializes a new PCB (Process Control Block)
• Adds the process to appropriate scheduling queues
• Other maintenance tasks

5
Process Control Block (PCB)

• A PCB is a data structure which stores certain
  information about each process. A typical PCB
  looks as follows
• Process ID (PID)
• Time of Arrival (TOA)
• Execution Time
• Priority/ Process Type
• Size (Location in memory)
• Program Counter
• Registers/Threads
• Needed Resources

6
PCB components Process ID

• The system assigns each process a unique
  identifier which is used by other processes for
  scheduling, communication and any other purpose.

7
PCB components Time of Arrival

• The system keeps track of the time a process
  enters the process queue for scheduling purposes.

8
PCB components Execution Time

• This parameter is used by scheduling algorithms
  which order processes by the amount of time they
  need to complete execution. Mathematical formulae
  are used to calculate the estimated execution
  time of a process.

9
PCB components Priority

• Some processes, such as system processes, have a
  higher priority than others and the operating
  system uses this priority during scheduling and
  memory management.

10
PCB components Size

• It is the size of the process in bytes. This
  parameter indicates the memory location of a
  process also.

11
PCB components Program Counter

• The program counter value stores the address of
  the next instruction to be executed.

12
PCB components Registers/Threads

• This PCB parameter saves the state of different
  registers used by that particular process.

13
PCB components Needed Resources

• This PCB parameter indicates the quantities of
  system resources needed by that particular
  process.

14
Process States

• The state of a process is defined in part by its
  current activity. As it executes, it changes
  state. Each process may be in one of the
  following states
• New The process is being created
• Running Instructions are being executed
• Waiting The process is waiting for some event to
  occur
• Ready The process is waiting to be assigned to a
  processor
• Terminated The process has finished execution

15
Process Life Cycle
16
Uploading/Downloading PCBs
Process P0 operating system Process P1
Interrupt or system call
executing
Save state into PCB0
. . . . .
. . . . . . . . . . . . .
. .
idle
Reload state from PCB1
executing
Interrupt or system call
idle
Save state into PCB1
. . . . .
. .
idle
Reload state from PCB0
executing
17
Performance Measures

• It is important to quantify performance so that
  it can be measured. Commonly used performance
  measures
• CPU Utilization
• Turnaround time
• Waiting time
• Throughput
• Response time
• Before we discuss the individual performance
  measures, we need to understand the related
  concepts of context switching and starvation.

18
Performance MeasuresRelated Concept - Context
Switch

• Switching the CPU to another process requires
  saving the state of the old process and loading
  the state of the new process. This task is known
  as a context switch. (it is labeled in the
  diagram uploading/downloading PCBs)
• Increased context switching affects performance
  adversely because the CPU spends more time
  switching between tasks than it does with the
  tasks itself.

19
Performance MeasuresRelated Concept - Starvation

• This is the situation where a process waits
  endlessly for CPU attention.
• As a result of starvation, the starved process
  may never complete its designated task.

20
Performance MeasuresCPU Utilization

• It is the ratio of time that the CPU is doing
  actual processing to the total CPU time observed.
  
• This is a true measure of performance since it
  measures efficiency of the system. An idle CPU
  has 0 CPU utilization since it offers null
  performance per unit cost. The higher the CPU
  utilization, the better the efficiency of the
  system

21
Performance MeasuresTurnaround Time

• The time between a processs arrival into the
  system and its completion.
• Two related parameters that can be studied
  include average turnaround time and maximum
  turnaround time.
• The turnaround time includes the context
  switching times and execution times.
• The turnaround time is inversely related to the
  system performance i.e. lower turnaround times
  imply better system performance.

22
Performance Measures Waiting Time

• Waiting time is the sum of the periods spent
  waiting in the ready queue.
• Mathematically, it is the difference between the
  turnaround time and execution time.
• It inversely affects system performance.
• It has two related forms average waiting time
  and maximum waiting time.
• As a point of interest, the CPU scheduling
  algorithm does not affect the execution time of a
  process but surely determines the waiting time.

23
Performance MeasuresThroughput

• The average number of processes completed per
  unit time.
• Higher throughput is generally considered as
  indicative of increased performance.
• However, it should not be the sole performance
  criterion taken into account because throughput
  does not take into account loss of performance
  caused by starvation.

24
Performance MeasuresResponse Time

• The time difference between submission of the
  process and the first I/O operation is termed
  response time.
• It affects performance inversely.
• However, it is not considered to be a reasonable
  measure and is rarely used.

25
Evaluation Techniques

• When developing an operating system or the
  modules thereof, evaluation of its performance is
  needed before it is installed for real usage.
  Evaluation provides useful clues to which
  algorithms would best serve the cases of
  application.

26
Evaluation TechniquesThree Common Techniques

• All evaluation techniques can be classified into
  the following three types
• Analytic method
• Implementation in real time systems
• Simulation Method

27
Evaluation TechniquesAnalytic Method

• In the analytic method, a mathematical formula
  is developed to represent a computing system.
  This method provides clear and intuitive
  evaluation of system performance, and is most
  useful to a specific algorithm. However, it is
  too simple to examine a complex and near-real
  system.

28
Evaluation TechniquesImplementation in real-time
systems

• Another way is to implement an operating system
  in a real machine. This method produces a
  complete and accurate evaluation.
• A disadvantage with it is its dramatic cost.
• Also evaluation is dependent on the environment
  of machine in which evaluation is carried out.

29
Evaluation TechniquesSimulation Method

• Simulation is a method that uses programming
  technique to develop a model of a real system.
• Implementation of the model with prescribed jobs
  shows how the system works.
• Furthermore, the model contains a number of
  algorithms, variables, and parameters.
• Changing the above factors in simulation, one is
  able to know how the system performance would be
  affected and, therefore, to predict possible
  changes in performance of the real system.
• This method has a balanced complexity and cost.
  It was viewed as the most potentially powerful
  and flexible of the evaluation techniques

30
Simulation Method The evaluation technique used
in this course

• The simulation method entails development of a
  model of a real system which contains a number of
  algorithms, variables and parameters. Changing
  these factors in simulation enables one to know
  its affect on system performance. This method,
  thus, is best suited for our purpose of studying
  the operating systems design.

31
Lecture Summary

• What is a process
• Process Control Block
• Process states and process life cycle
• Performance Measures
• Evaluation Techniques
• Justification of simulation method as the chosen
  technique for this course

32
Preview of next lecture

• The following topics shall be covered in the
  next lecture
• Introduction to CPU scheduling
• What is CPU scheduling
• Related Concepts of Starvation, Context Switching
  and Preemption
• Scheduling Algorithms
• Parameters Involved
• Parameter-Performance Relationships
• Some Sample Results