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Chapter 3: Processes��https://classroom.google.com/c/Nzk1NzAxMzkyMTIy?cjc=2lo4evz7

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Process Concept

An operating system executes a variety of programs:

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

Process – a program in execution; process execution must progress in sequential fashion
Multiple parts

The program code, also called text section
Current activity including program counter, processor registers
Stack containing temporary data

Function parameters, return addresses, local variables

Data section containing global variables
Heap containing memory dynamically allocated during run time

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Process Concept (Cont.)

Program is passive entity stored on disk (executable file), process is active

Program becomes process when executable file loaded into memory

Execution of program started via GUI mouse clicks, command line entry of its name, etc
One program can be several processes

Consider multiple users executing the same program

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

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Diagram of Process State

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Process Control Block (PCB)

Information associated with each process

(also called task control block)

Process state – running, waiting, etc
Program counter – location of instruction to next execute
CPU registers – contents of all process-centric registers
CPU scheduling information- priorities, scheduling queue pointers
Memory-management information – memory allocated to the process
Accounting information – CPU used, clock time elapsed since start, time limits
I/O status information – I/O devices allocated to process, list of open files

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CPU Switch From Process to Process

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Process Scheduling

Maximize CPU use, quickly switch processes onto CPU for time sharing
Process scheduler selects among available processes for next execution on CPU
Maintains scheduling queues of processes

Job queue – set of all processes in the system
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
Processes migrate among the various queues

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Schedulers

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU

Sometimes the only scheduler in a system
Short-term scheduler is invoked frequently (milliseconds) ⇒ (must be fast)

Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue

Long-term scheduler is invoked infrequently (seconds, minutes) ⇒ (may be slow)
The long-term scheduler controls the degree of multiprogramming

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 strives for good process mix

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Addition of Medium Term Scheduling

Medium-term scheduler can be added if degree of multiple programming needs to decrease

Remove process from memory, store on disk, bring back in from disk to continue execution: swapping

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Operations on Processes

System must provide mechanisms for:

process creation,
process termination,
and so on as detailed next

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Process Creation

Parent process create children processes, which, in turn create other processes, forming a tree of processes
Generally, process identified and managed via a process identifier (pid)
Resource sharing options

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

Execution options

Parent and children execute concurrently
Parent waits until children terminate

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A Tree of Processes in Linux

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Process Creation (Cont.)

Address space

Child duplicate of parent
Child has a program loaded into it

UNIX examples

fork() system call creates new process
exec() system call used after a fork() to replace the process’ memory space with a new program

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C Program Forking Separate Process

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Process Termination

Process executes last statement and then asks the operating system to delete it using the exit() system call.

Returns status data from child to parent (via wait())
Process’ resources are deallocated by operating system

Parent may terminate the execution of children processes using the abort() system call. Some reasons for doing so:

Child has exceeded allocated resources
Task assigned to child is no longer required
The parent is exiting and the operating systems does not allow a child to continue if its parent terminates

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Process Termination

Some operating systems do not allow child to exists if its parent has terminated. If a process terminates, then all its children must also be terminated.

cascading termination. All children, grandchildren, etc. are terminated.
The termination is initiated by the operating system.

The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process

pid = wait(&status);

If no parent waiting (did not invoke wait()) process is a zombie
If parent terminated without invoking wait , process is an orphan

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Interprocess Communication

Processes within a system may be independent or cooperating
Cooperating process can affect or be affected by other processes, including sharing data
Reasons for cooperating processes:

Information sharing
Computation speedup
Modularity
Convenience

Cooperating processes need interprocess communication (IPC)
Two models of IPC

Shared memory
Message passing

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Communications Models

(a) Message passing. (b) shared memory.

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Producer-Consumer Problem

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

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

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Bounded-Buffer – Shared-Memory Solution

Shared data

#define BUFFER_SIZE 10

typedef struct {

. . .

} item;

item buffer[BUFFER_SIZE];

int in = 0;

int out = 0;

Solution is correct, but can only use BUFFER_SIZE-1 elements

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Bounded-Buffer – Producer

item next_produced;

while (true) {

/* produce an item in next produced */

while (((in + 1) % BUFFER_SIZE) == out)

; /* do nothing */

buffer[in] = next_produced;

in = (in + 1) % BUFFER_SIZE;

}

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Bounded Buffer – Consumer

item next_consumed;

while (true) {� while (in == out)

; /* do nothing */� next_consumed = buffer[out];

out = (out + 1) % BUFFER_SIZE;�

/* consume the item in next consumed */

}

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Interprocess Communication – Shared Memory

An area of memory shared among the processes that wish to communicate
The communication is under the control of the users processes not the operating system.
Major issues is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory.

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Interprocess Communication – Message Passing

Mechanism for processes to communicate and to synchronize their actions

Message system – processes communicate with each other without resorting to shared variables

IPC facility provides two operations:

send(message)
receive(message)

The message size is either fixed or variable

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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

Properties of communication link

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 bi-directional

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Indirect Communication

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

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

Properties of communication link

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

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Indirect Communication

Operations

create a new mailbox (port)
send and receive messages through mailbox
destroy a mailbox

Primitives are defined as:

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

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

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Indirect Communication

Mailbox sharing

P₁, P₂, and P₃ share mailbox A
P₁, sends; P₂ and P₃ receive
Who gets the message?

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.

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End of Chapter 3

Operating System Concepts – 9^th Edition