Processes

Process Concept

An operating system executes a variety of programs:

For batch system: jobs
For time-shared systems: user programs or tasks

In textbook, the terms job and process are used almost interchangeably.

Process is a program in execution, process execution must progress in sequential fashion. A process includes:

Program counter
Stack
Data section

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
ready: The process is waiting to be assigned to a processor
terminated: The process has finished execution

Process Control Block

Information associated with each process:

Process state
Program counter
CPU registers
CPU scheduling information
Memory management information
Accounting information
I/O status information

Process Scheduling

There are four queues in process scheduling:

Job queue: set of all processes in the system
Ready queue: set of all processes residing in the main memory, ready and wait to execute
Device queue: set of processes waiting for an I/O device
Processes migration between the various queues

And there are two schedulers:

Long-term scheduler(job scheduler): selects processes should be brought into the ready queue
Short-term scheduler(CPU scheduler): selects which process should executed next and allocate CPU

The short-term scheduler is invoked very frequently and must be fast, the long-tern scheduler is invoked very infrequently so it may be slow. The long-term scheduler controls he degree of multiprogramming.

Operations on Processes

Context Switch

When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process. The context switch depend on hardware switching.

Context-switch time is a waste; the system does no useful work while switching.

Process Creation

Parent process create children processes, which, in turn create other processes, forming a tree of processes.

There exist three situations in resource sharing of parent and children processes:

Parent and children processes share all resources
Children share subset of parent's resources
Parent and child share no resources

The execution of the processes:

Parent and children execute concurrently
Parent waits until children terminate

The address space of processes:

Child duplicate of parent
Child has a program loaded into it

Process Termination

Process executes last statement and asks the operating system to decide it:

Output data from child to parent
Process's resource are deallocated by operating system

Parent process can terminate execution of children processes.

Cooperating Processes

Independent process cannot affect or be affected by the execution of another process.

Cooperating process can affect or be affected by the execution of another process.

The advantages of process cooperation:

Information sharing
Computation speed-up
Modularity
Convenience

Producer-Consumer Problem

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

So there need to be a shared memory as a buffer between the producer and consumer processes. And there are two kinds of buffer:

unbounded buffer place no practical limit on the size of the buffer
bounded buffer assumes that there is a fixed buffer size

Interprocess Communication

Interprocess communication is mechanism for processes to communicate and to synchronize their actions. Message system is a system used for processes to communicate with each other without resorting to shared variables.

IPC facility provides two operations:

send(message)
receive(message)

The message size can be fixed or variable.

The implementation of IPC must to answer there questions:

How are links established?
Can a link be associated with more than two processes?
How many links can there be between every pair of communicating processes?
What is the capacity of a link?
Is the size of a message that the link can accommodate fixed or variable?
Is a link unidirectional or bi-directional?

Direct Communication

Processes must name each other explicitly:

send(P, message): send a message to process P
receive(Q, message): receive a message from process Q

Indirect Communication

Messages are directed and received from mailboxes (ports).

Each mailbox has a unique id
Processes can communicate only if they share a mailbox

The indirect communication support operations:

create a new mailbox
send and receive messages through mailbox
destroy a mailbox

Synchronization

Message passing may be either blocking or nonblocking. The blocking is considered synchronous and nonblocking is considered asynchronous.

Send and Receive primitives may be either blocking or non-blocking.

Buffering

Queue of messages attached to the link, can be implemented in three ways:

Zero capacity: sender must wait for receiver
Bounded capacity: finite length of messages; sender must wait if link is full
Unbounded capacity: infinite length and sender never waits

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