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Chapter 3: Process-Concept

Operating System Concepts – 8^th Edition,

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Chapter 3: Process-Concept

Process Concept
Process Scheduling
Interprocess Communication

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Objectives

To introduce the notion of a process -- a program in execution, which forms the basis of all computation
To describe the various features of processes, including scheduling, creation and termination, and communication

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Process Concept

An operating system executes a variety of programs:

In Batch system – jobs
In Time-shared systems – user programs or tasks

Textbook uses the terms job and process almost interchangeably (job = process)
Process – is a program loaded into memory and executing.
System consists of a collection of processes:

Operating system processes executing system code
User processes executing user code.

Process execution must progress in sequential fashion

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The Process

Process includes:

Text section : which is program code
Stack- which contains temporary data (such as function parameters, return addresses, and local variables)
Data section: which contains global variables.
Program counter: the value of the program counter & contents of the processor’s registers.
Heap (some times): which is memory that is dynamically allocated during process run time.

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Process VS Program

What is the difference between a process and program??

A program by itself is not a process
A program is a passive entity, such as a file containing a list of instructions stored on disk (executable file).
A process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources.
A program becomes a process when an executable file is loaded into memory.

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Process State

As a process executes, it changes state

New: the process is being created
Running: instructions are being executed
Waiting: the process is waiting for some event to occur (e.g. An I/O completion).
Ready: the process is waiting to be assigned to a processor
Terminated: the process has finished execution

Only one process can be running on any processor at any instant. Many processes may be ready and waiting.

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Diagram of Process State

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Process Control Block (PCB)

Each process is represented in the operating system by a process control block (PCB)
PCB contains many information including :

Process state
Program counter
CPU registers
CPU scheduling information

e.g. a process priority

Memory-management information
Accounting information:

e.g . when the process was last run, how much CPU time it has used

I/O status information

It includes the list of I/O devices allocated to the process, a list of open files….etc .

Values of program counter

& CPU registers must be

saved when an interrupt occurs

to allow the process to be

continued correctly afterward

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Process Control Block (PCB)

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CPU Switch From Process to Process

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Process Scheduling

Multiprogramming aims to have some process running at all times to maximize CPU utilization.
Time sharing aims to switch the CPU among processes so frequently user interact with each program while it is running.
Process scheduler selects an process (from a set of several available processes) for execution on the CPU.

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Process Scheduling Queues

Job queue – set of all processes in the system
Ready queue – set of all processes residing in main memory, ready and waiting to execute

This queue is generally stored as a linked list.

Device queues – set of processes waiting for an I/O device

Each device has its own device queue

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Process Scheduling Queues (cont.)

Processes migrate among the various queues:

A new process is initially put in the ready queue. It waits there until it is selected for execution. Once the process is allocated the CPU and is executing, one of several events could occur:

The process could issue an I/O request and then be placed in an I/O queue.
The process could create a new sub-process and wait for the sub-process’s termination.
The process could be removed forcibly from the CPU as a result of an interrupt, and be put back in the ready queue.

A process continues this cycle until it terminates, at which time it is removed from all queues and has its PCB and resources deallocated.

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Representation of Process Scheduling

Queuing diagram

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Schedulers

The operating system must select processes scheduling these queues. The selection process is carried out by the appropriate scheduler.
Long-term scheduler (or job scheduler) – selects which processes should be brought from mass-storage into RAM (to the ready queue )
Short-term scheduler (or CPU scheduler) – selects which process (from ready queue) should be executed next and allocates CPU

Short-term scheduler is invoked very frequently (milliseconds) ⇒ (must be fast)
Long-term scheduler is invoked very infrequently (seconds, minutes) ⇒ (may be slow)

The long-term scheduler controls the degree of multiprogramming

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Schedulers (Cont)

Processes can be described as either:

I/O-bound process – spends more time doing I/O than computations, many short CPU intervals
CPU-bound process – spends more time doing computations; few very long CPU intervals.

The long-term scheduler must select a good process mix of I/O-bound and CPU-bound processes to improve total performance.

If all processes are I/O bound >>> the ready queue will almost always be empty, and the short-term scheduler will have little to do….(minimize CPU utilization)
If all processes are CPU bound >>> the I/O waiting queue will almost always be empty, devices will go unused….(minimize devices utilization)

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Medium Term Scheduling

Medium Term Scheduling idea is :

Sometimes it can be advantageous to remove processes from RAM …….(swap out)
Later, the process can be reintroduced into RAM …….(swap in), and its execution can be continued where it left off.
This scheme is called swapping. The process is swapped out, and is later swapped in, by the medium-term scheduler.

Swapping may be necessary to improve the process mix or because a change in memory requirements has exceeded available memory, requiring memory to be freed up.

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Addition of Medium Term Scheduling

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Interprocess Communication

Processes within a system may be independent or cooperating

Independent processes

Cannot affect or be affected by the execution of another process.

Cooperating processes

Can affect or be affected by the execution of another process

Reasons (/advantages) for cooperating processes:

Information sharing : (e.g. shared file)
Computation speedup:

To make a particular task run faster, we must break it into subtasks, each of which will be executing in parallel with the others.
This can be achieved only if the computer has multiple processing elements (e.g. CPUs).

Modularity:

We may want to construct the system in a modular fashion (i.e. dividing the system functions into separate processes)

Convenience

An individual user may work on many tasks at the same time.

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Interprocess Communication (Cont.)

Cooperating processes need interprocess communication (IPC)
IPC: mechanism that will allow cooperating processes to exchange data and information.
Two models of IPC:

Shared memory:

A region of memory that is shared by cooperating processes is established.
Processes can then exchange information by reading and writing data to the shared region.

Message passing

Communication takes place by means of messages exchanged between the cooperating processes..

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Communications Models

Message passing

model

Shared memory model

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Message passing VS Shared memory

Both models are common in operating systems, and many systems implement both.
Message passing:

Useful for exchanging smaller amounts of data, because no conflicts need be avoided.
Easier to implement than shared memory .

Shared memory

Shared memory is faster than message passing …..Because message passing systems requires more system calls (interrupts) .

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End of Chapter 3

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Some Helpful Notes

Some Terminologies :

Batch system :

System which Jobs are set up so they can be run to completion without human (/user) interaction.
This operating environment is termed as "batch processing" because the input data are collected into batches or sets of records and each batch is processed as a unit.
A program takes a set of data files as input, processes the data, and produces a set of output data files.
An example of a batch system is when a computer system processes telephone bills to a number of customers and then sends them out all at once rather than individually.

Program counter: is a register in a computer processor that contains the address (location) of the next instruction to be executed

Heap : is a partially sorted binary tree….(data structure)

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Some Helpful Notes

Modular programming is the process of subdividing a computer program into separate sub-programs.

Each sub-program accomplishes one function and contains all needed source code and variables.
Modular programming is a solution to the problem of very large programs that are difficult to debug and maintain.

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