Operating System -- Process Description And Control

Operating System

1. Process Description And Control

contents

• Role of the Operating System

• What is a process

• Process Control Block

• Advantages of Multiprogramming

• Model

Two-State Model
Five-State Model

• Scheduling

Role of the Operating System(OS)

1. OS provides a convenient, feature rich, secure, and consistent interface of application to use.
2. OS provides a uniform. abstract representation of resources that can be requested and accessed by application.

What is a process

program: A program is simply an exact formulation of an algorithm. Related Reference:
process: A process is a program in execution.Relative Reference
multiprogramming: CPU switches
execution of several programs. Relative Reference

Questions

How is multiprogramming seen by the CPU?

1) CPU executes machine instructions as dictated by the content of the program counter
2) Program counter can point to code of different programs without
making any difference

How is multiprogramming seen by an individual program?

1) Program is executed as normal and consists of a sequence of machine instructions (trace)
2) Each process uses its own logical program counter to not interfere with execution progress of other processes

At each point in time, the logical program
counter of exactly one process is loaded into the real (and only once available) program counter of the CPU

Trace of a Process:

1. Sequence of instruction that execute for a process
2. Dispatcher switches the processor from one process to another

Two essential elements of a process

Program code
PCB(Process Control Block)

PCB(Process Control Block)

Include:

The data elements contained in a PCB fall into three general categories

Process Identification

PID(Process Identifier)
Identifier of the parent process
User Identifier

Processor State Information

User-Visible Register
Register Status

Program counter
Condition codes
Status information

Stack Pointers

Subroutine Calling
Exception or Error Handling
Stack Pointer

Process Control Information

Scheduling and States Information
Data Structuring
Process Privileges
…..

Advantages of Multiprogramming

Increasing utilization of components of a computer system.

CPU load
memory utilization
hard disk load
…….

Model

Two-State Model
Five-State Model

Two-State Model

A process can be specific two states:

Running
Not Running

Five-State Model

Actual not all processes only have two states: Running and Not-Running. Often the process need to wait for a critical resource and be blocked state.
Therefore:
New(A new process is created)
Read(Process is waiting for CPU)
Running(Process is executing on CPU)
Blocked(Process is waiting for event)
Exit(Process is finished and being destroyed)

Scheduling

Operating systems with a simultaneous
execution of multiple processes must decide on the order and on the time of accessing resources by processes
When a computer is multiprogrammed it
frequently has multiple processes competing for the CPU at the same time

•A a choice has to be made which process to run next
•the part of the operating system that makes this decision is called the scheduler
•the algorithm it uses is called the scheduling algorithm
•scheduling may involve both processes and threads

When to schedule

• a new process is created
- select the new one or keep the current one running
• a process terminates
- select and run another process, if any
• a process blocks (semaphore, I/O)
- dependencies btw processes may improve scheduling
• I/O interrupt
- run a waiting process
• hardware clock
- run the scheduler each clock interrupt or every k-th clock interrupt

Scheduling: common goals

• fairness
- comparable processes should get comparable service (CPU time)
• policy enforcement
- different categories of processes may be treated differently
• balance
- try to keep all the part of the system busy when possib

Scheduling can be divided:

• non preemptive
- picks a process to run
- lets it run until it blocks, terminates or voluntary releases the CPU
- after clock interrupt, resume the process that was running before
• preemptive
- picks a process to run
- after a maximum amount of some fixed time suspends it (if still running)
- picks another process to run (if any available) requires clock

Scheduling: specific goals

• batch systems
- throughput
- turnaround time
- CPU utilization
• interactive systems
- response time/proportionality
• real-time systems
- meeting deadlines
- predictability

Scheduling in Batch Systems

• First-Come First-Served
• Shortest Job First
• Shortest Remaining Time Next

Scheduling in Interactive Systems

• Round Robin
• issues with Round Robin
• Priority Scheduling
……

Scheduling in Real Time Systems

• hard-real time
• soft-real time

Reference: William Stallings, “Operating Systems - Internals and Design Principles”, Fifth Edition, Pearson Prentice Hall, 2005
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