Lecture 02 Process Management

Real-Time Operating Systems by Koliya Pulasinghe B.Sc.Eng.(Hons.), Ph.D., MIET, MIEEE

Process Management

Objectives

To discuss process concept and the need for process scheduling.

To show different operation on processes and how processes cooperate with each other, and inter-process communication.

Process Concept

Early computers allowed only one program to be executed at a time. (the program has complete access to all system resources.)

Current-day computer systems allow multiple programs to be executed concurrently.

These needs resulted in the notion of a process , which is a program in execution

All these processes can execute concurrently ,with the CPU is multiplexed among them.

Process Concept

In batch systems, we call the program as jobs ; In time-shared systems, we call them user programs or tasks ;

Even on a single user system , a user may be able to run several programs at one time: a word processor, web browser, etc.

In many respects all these activities are similar, so we call all of them processes.

A process is a Program in execution.

Process execution must progress in a sequential fashion.

The process

A process is more than just the program code.

A process includes:

Program counter ; containing process’s current activity and it points to the next instruction to execute.

Contents of processor registers .

Stack ; containing temporary data : subroutine parameters, return addresses, temporary variables.

Data section ; containing global variables.

Program vs. Process

Is a program process?

No, Program is a passive entity , such as the contents of a file stored on disk,

whereas a process is an active entity , with a program counter specifying the next instruction to execute and a set of associated resources.

Two processes may be associated with the same program; however they are considered two separate execution sequences.

Process State

As a process executes, it changes state.

Each process may be in one of the following process 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.

Diagram of Process State

Process Control Block(PCB)

Information associated with each process is stored in a PCB (Process Control Block)

Each process is represented by its PCB.

Each PCB contains:

Process number (pid) .

Process state : new, ready, running, waiting, etc.

Program counter : shows the address of next instruction to be executed.

CPU registers : the registers vary in number and type, depending on the computer architecture.

Process Control Block (PCB)

Each PCB contains:

CPU scheduling information : process priority, pointers to scheduling queues, etc.

Memory management information : base and limit register values, page tables, etc.

Accounting information : amount of CPU and time used,etc.

I/O status information : the list of I/O devices allocated to this process, a list of open files, etc.

Process Control Block (PCB)

CPU Switch From Process to Process

Process Scheduling

The objective of time-sharing is to switch the CPU among processes so frequently that users can interact with each program while it is running.

A uniprocessor system can run only one process and other processes can wait until the CPU is free and can be rescheduled.

Process Scheduling Queues

Job queue – All processes entering the system are put in a job queue.

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

Device queues – set of processes waiting for an I/O device;each device has its own device queue.

Process migrates between the various queues.

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Schedulers

A process migrates between various scheduling queues.

Selection on which process to migrate is done by the appropriate scheduler.

Long-term scheduler (or job scheduler )

selects which processes should be brought into the ready queue.

Is invoked very infrequently (seconds, minutes) . It may be slow; time sharing system often have no long-term scheduler.

Controls the degree of multiprogramming.

Schedulers…..

Short-term scheduler (or CPU scheduler )

selects which process should be executed next and allocated CPU.

is invoked very frequently (milliseconds). Therefore it must be fast.

Schedulers (contd.) Long-term short-term

Processes can be described as either:

I/O-bound process – spends more time doing I/O than computations; many short CPU bursts.

CPU-bound process – spends more time doing computations; few very long CPU bursts.

Long-term scheduler needs to select a good mix of I/O bound and CPU bound processes .

Schedulers

Context Switch

When CPU switches to another process, the system must save the state of the old process and load the saved state for new process.

Context-switch time is overhead; the system does no useful work while switching. This leads to a performance bottleneck.

Context-switch-time is dependent on hardware support; typically from 1 to 1000 microseconds.

Context Switch

When do we switch CPU?

Job voluntarily waits (system calls).

Interrupt: Higher priority event/job needs attention.

Interrupt: Timer.

Operations on processes

Process Creation

Process Termination

Operating System must provide a mechanism for process creation and deletion

Process Creation

Parent process creates children processes, which, in turn create other processes, forming a tree of processes; in Unix use system call fork( ).

A process needs resources (CPU time, memory, files, I/O).

When a process creates a sub-process:

For resources following possibilities exist

Parent and children may share all resources.

Children may share subset of parent’s resources.

Parent and children may share no resources.

For execution following possibilities exist

Parent and children may execute concurrently.

Parent may wait until children terminate.

For address space following possibilities exist

Child process address space is duplicate of parent’s.

Child has a program loaded into it. -> in UNIX: use of execlp().

Process termination

A process terminates when it executes last statement and asks the OS to delete it ( exit ); at that point, the process may output data from child to parent (via wait ), and process’ resources are deallocated by the OS.

Parent may terminate execution of children processes (abort); Reasons for termination:

Child has exceeded allocated resources.

Task assigned to child is no longer required.

Parent is exiting, and OS does not allow child to continue if its parent terminates.

Cooperating Processes

The concurrent processes executing in OS may be either independent or cooperating processes.

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.

Advantages of process cooperation:

Information sharing

several users may need the same piece of information.

Computation speed-up

break a task into subtasks and execute in parallel

Modularity

dividing the system function into separate processes.

Convenience

A user may want to do editing, printing in parallel.

Example of concurrent processes:

Producer-consumer problem

Paradigm for cooperating processes; producer process produces information that is consumed by a consumer process .

To allow producer and consumer processes to run concurrently, there is a buffer of items that can be filled by the producer, and emptied by the consumer.

Unbounded-buffer places no practical limit on the size of the buffer.

Bounded-buffer assumes that there is a fixed buffer size.

The buffer can be in a form of shared memory, or provided by the OS through the use of interprocess-communication (IPC).

Interprocess Communication (IPC)

Cooperating processes can communicate:

via a shared-memory environment .

via an interprocess-communication facility . (means provided by OS for processes to communicate; best provided by a message passing system.)

The function of a message system

processes communicate with each other without resorting to shared variables.

IPC

IPC facility provides at least two operations:

Send (message) – message size fixed or variable.

Receive (message).

If P and Q wish to communicate, they need to:

Establish a communication link between them.

Exchange messages via send/receive.

IPC

Several methods for logically implementing a link and send / receive operations:

Direct or indirect communication.

Symmetric or asymmetric communication.

Automatic or explicit buffering.

Send by copy or send by reference.

Fixed-sized or variable-sized messages.

Direct communication

In direct-communication, each process must explicitly name the recipient or sender of the communication.

Primitives used:

Send (P, message) – send a message to process P.

Receive (Q, message) – receive a message from process Q.

Direct communication …..

Properties of communication link for this scheme:

Links are established automatically.

A link is associated with exactly one pair of communicating processes.

Between each pair there exists exactly one link.

The link may be unidirectional, but is usually bidirectional.

Indirect communication

Messages are sent to and received from mailboxes (also referred to as ports).

Each mailbox has a unique id.

Processes can communicate only if they share a mailbox.

Primitives used:

Send (A, message) – send a message to mailbox A.

Receive (A, message) – receive a message from mailbox A.

Indirect communication

Properties of communication link for this scheme:

Link established only if processes share a common mailbox.

A link may be associated with many processes.

Each pair of processes may share several communication links.

Link may be unidirectional or bi-directional.

Operations:

Create a new mailbox.

Send and receive messages through mailbox.

Destroy a mailbox.

Indirect communication ….

Mailbox sharing If processes P1, P2, and P3 share a mailbox A, and process P1 sends a message to mailbox A while processes P2 and P3 execute a receive from A, which process will receive the message sent by P1?

Three possible solutions:

Allow a link to be associated with at most two processes.

Allow only one process at a time to execute a receive operation.

Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.

Synchronization

Message passing can be either blocking ( synchronous ) or nonblocking (asynchronous).

Blocking send: The sender is blocked until the message is received.

Nonblocking send: the sender resumes operation after sending the message.

Blocking receive: the receiver blocks until message is available.

Nonblocking receive: receiver retrieves either a valid message or a NULL.

Buffering

Whether the communication is direct or indirect, messages exchanged by communicating processes reside in a temporary queue. Such a queue can be implemented in three ways

zero capacity

bounded capacity

unbounded capacity

For the non-zero capacity case, the sender does not know if a message has arrived at the destination (may use asynchronous communication; the receiver sends back an acknowledgement to the sender.)

Exception conditions – error recovery