C3-Process [Compatibility Mode]

8/19/2019 C3-Process [Compatibility Mode]

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Processes


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Processes and threads

Processes

Threads

Scheduling

Interprocess communication

Classical IPC problems

Deadlock


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Process Concept

An operating system executes a variety of programs: Batch system – jobs

Time-shared systems – user programs or tasks

Textbook uses the terms job and process almostinterchangeably.

Process – a program in execution; process executionmust progress in sequential fashion.

A process includes:

program counter stack

data section


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What is a process?

Code, data, and stack

Usually (but not always) has its own address space

Program state

CPU registers

Program counter (current location in the code) Stack pointer

Only one process can be running in the CPU at any

given time!


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The process model

Multiprogramming of four programs

Conceptual model 4 independent processes

Processes run sequentially

Only one program active at any

instant! That instant can be very short…

A

C

D

Single PC

(CPU’s point of view)

AB

C D

Multiple PCs

(process point of view)

B

B

A

BC

D

Time


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When is a process created?

Processes can be created in two ways

System initialization: one or more processes created when

the OS starts up

Execution of a process creation system call: something

explicitly asks for a new process System calls can come from

User request to create a new process (system call executed

from user shell)

Already running processes

User programs System daemons


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Process Creation

Parent process create children processes which, in turn,

create other processes forming a tree of processes.

Resource sharing

Parent and children share all resources. Children share subset of parent’s resources.

Parent and child share no resources.

Execution

Parent and children execute concurrently.

Parent waits until children terminate.


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Process Creation (Cont.)

Address space

Child duplicate of parent (UNIX)

Child has copy of parent’s address space with program id of 0

(parent is a non-zero value)

Enables easy communication between the two The child process’ memory space is replaced with a new program which is

then executed. Parent can wait for child to complete or create more

processes

Child has a program loaded into it directly(DEC VMS)

Windows NT supports both models


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UNIX examples

fork system call creates new process

execlp system call used after a fork to replace the

process’ memory space with a new program


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Processes Tree on a UNIX System


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When do processes end?

Conditions that terminate processes can be

Voluntary

Involuntary

Voluntary

Normal exit Error exit

Involuntary

Fatal error (only sort of involuntary)

Killed by another process


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Process Termination

Process executes last statement and asks the operating

system to delete it (exit).

Return data from child to parent (via wait).

Process’ resources are deallocated by operating system.

Parent may terminate execution of children processes(abort).

Child has exceeded allocated resources.

Task assigned to child is no longer required.

Parent is exiting. Operating system does not allow child to continue if its parent terminates -

cascading termination.


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Process Termination

In UNIX, a process can be terminated via the exit system

call.

Parent can wait for termination of child by the wait

system call wait returns the process identifier of a terminated child so that

the parent can tell which child has terminated

If a parent terminates, all children are assigned the init process

as their new parent


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Process hierarchies

Parent creates a child process

Child processes can create their own children

Forms a hierarchy

UNIX calls this a “process group”

If a process exits, its children are “inherited” by theexiting process’s parent

Windows has no concept of process hierarchy

All processes are created equal


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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

process.

terminated: The process has finished execution.


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Blocked

(waiting)

Created

Exit

Ready

Running

Process states

Process in one of 5 states Created

Ready

Running

Blocked

Exit

Transitions between states1 - Process enters ready queue

2 - Scheduler picks this process

3 - Scheduler picks a different process

4 - Process waits for event (such asI/O)

5 - Event occurs

6 - Process exits

7 - Process ended by another process

1

5

4

3

2

7

76


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Process Control Block (PCB)

Information associated with each process.

Process state

Program counter

CPU registers

CPU scheduling information

Memory-management information

Accounting information

I/O status information


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Process Control Block (PCB)


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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.

Context-switch time is overhead; the system does nouseful work while switching.

Time dependent on hardware support.

Varies from 1 to 1000 microseconds


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CPU Switch From Process to Process


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Processes in the OS

Two “layers” for processes

Lowest layer of process-structured OS handles interrupts,

scheduling

Above that layer are sequential processes Processes tracked in the process table

Each process has a process table entry

Scheduler

0 1 N -2 N -1…

Processes


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What’s in a process table entry?

File managementRoot directory

Working (current) directory

File descriptors

User ID

Group ID

Memory managementPointers to text, data, stack

or

Pointer to page table

Process managementRegisters

Program counter

CPU status word

Stack pointer

Process state

Priority / scheduling parametersProcess ID

Parent process ID

Signals

Process start time

Total CPU usage

May be

stored

on stack


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What happens on a trap/interrupt?

1. Hardware saves program counter (on stack or in aspecial register)

2. Hardware loads new PC, identifies interrupt3. Assembly language routine saves registers

4. Assembly language routine sets up stack 5. Assembly language calls C to run service routine6. Service routine calls scheduler 7. Scheduler selects a process to run next (might be

the one interrupted…)

8. Assembly language routine loads PC & registersfor the selected process


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Cooperating Processes

Independent process cannot affect or be affected by theexecution of another process.

Cooperating process can affect or be affected by the

execution of another process

Advantages of process cooperation Information sharing

Computation speed-up via parallel sub-tasks

Modularity by dividing system functions into separate processes

Convenience - even an individual may want to edit, print and

compile in parallel


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Producer-Consumer Problem

Paradigm for cooperating processes, producer

process produces information that is consumed by a

consumer process.

A buffer enables the producer and consumer to run

concurrently e.g., print program produces characters that are consumed

by the print driver

unbounded-buffer places no practical limit on the size of

the buffer.

bounded-buffer assumes that there is a fixed buffer size – producer must wait if the buffer is full


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Bounded-Buffer –

Shared-Memory Solution

Shared data#define BUFFER_SIZE 10typedef struct {. . .

} item;item buffer[BUFFER_SIZE];

int in = 0; int out = 0; Shared buffer is implemented as a circular array

in points to the next free position in the buffer out points to the first full position in the buffer buffer empty when in == out

Buffer full when((in+1)%BUFFER_SIZE)== out Solution is correct, but can only use BUFFER_SIZE-1 elements


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Bounded-Buffer Processes

PRODUCER PROCESS item nextProduced; while (1){ while (((in+1)%BUFFER_SIZE) == out)

; // buffer full, do nothing buffer[in] = nextProduced;

in = (in + 1) % BUFFER_SIZE;}

CONSUMER PROCESS item nextConsumed; while (1) { while (in == out)

; // buffer empty, do nothingnextConsumed = buffer[out];out = (out + 1) % BUFFER_SIZE;

}


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A

Producer If in+1 will point to same spot out points to, then no more room in the

buffer to insert items and while loops continues

If the buffer is not full then Producer inserts item in buffer at position in andthen increases value of in by 1 mod BUFFER_SIZE

Consumer

If nothing to consume, while loop continues If there is something to consume, then it is processed and out pointer is

increased by 1 mod BUFFER_SIZE

in=0 out=0

in=1

in=2

in=3

in=4

0 1 2 3 4


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A

in 1

out 0

in 0

out 0

A B

in 2

out 0

A B C

in 3

out 0

A B C Din 4

out 0

What would happen if anE was added to index 4?


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Interprocess Communication (IPC)

Mechanism for processes to communicate and tosynchronize their actions.

Message system – processes communicate with each

other without resorting to shared variables.

Useful in a distributed environment where the communication processes may reside on different computers connected with a

network (chat program)

IPC facility provides two operations:

send (message)

receive(message)


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Interprocess Communication (IPC)

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

establish a communication link between them

exchange messages via send/receive

Implementation of communication link

physical properties shared memory

hardware bus

logical properties Direct or indirect communication

Symmetric or asymmetric communication

Automatic or explicit buffering Send by copy or send by reference

Fixed-size or variable-size messages


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Implementation Questions

How are links established? Can a link be associated with more than two

processes?

How many links can there be between every pairof 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?


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Direct Communication

Processes must name each other explicitly (adisadvantage): 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.

Weakness: changing the name of a process may requireexamining all other process definitions to change the old name tothe new name


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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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A mailbox owned by a process has a unique owner. When process terminates, mailbox disappears

Mailbox owned by the OS can do the following:

create a new mailbox

send and receive messages through mailbox

destroy a mailbox

Ownership may be passed to other processes via system calls

resulting in multiple receivers for each mailbox

Primitives are defined as:

send( A, message) – send a message to mailbox Areceive( A, message) – receive a message from mailbox A


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Mailbox sharing P 1 , P 2 , and P 3 share mailbox A.

P 1, sends; P 2 and P 3 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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Synchronization

Message passing may be either blocking or non-blocking. Blocking is considered synchronous

Send – sending process is blocked until the message is received

by the receiving process or mailbox

Receive – receiver blocks until a message is available

Non-blocking is considered asynchronous Send – sending process sends the message and resumes

operation

Receive – receiver retrieves either a valid message or a null


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Buffering

Whether the communication link is direct or indirect,messages exchanged by communicating processes resides

in a temporary queue

Queue of messages attached to the link; implemented in

one of three ways.1. Zero capacity – 0 messagesSender must wait for receiver (rendezvous).

2. Bounded capacity – finite length of n messages

Sender must wait if link full.

3. Unbounded capacity – infinite length,sender never waits.


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Client-Server Communication

Sockets Remote Procedure Calls

Remote Method Invocation (Java)


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Sockets

A pair of processes communicating over a networkemploys a pair of sockets.

A socket is defined as an endpoint for communication A socket is identified by an IP address concatenated with a port

number

When a client process initiates a request for a connection,

it is assigned a port by the host computer (arbitrary

number greater than 1024) Unique port number per connecting process allows multiples processes

to establish connections with the same server The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8


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Socket Communication


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Remote Procedure Calls

Remote procedure Call (RPC) abstracts procedure calls

between processes on networked systems.

Enables a client to invoke a procedure on a remote host as it

would invoke a procedure locally

Stubs – client-side proxy for the actual procedure on theserver.

When the client invokes a remote procedure, the RPC system

calls the appropriate stub, passing it the parameters provided

by the remote procedure


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Remote Procedure Calls

The client-side stub locates the server and marshals the parameters (packages the parameters into a form which

may be transmitted over a network)

The server-side stub receives this message, unpacks the

marshaled parameters, and performs the procedure on the

server.

Matchmaker – an OS daemon that enables dynamic

binding of the RPC name to the actual port address of the

RPC on the server


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Marshalling Parameters


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Execution of RPC

Client : User Calls kernel to send RPC message to procedure X

Client : Kernel sends message to matchmaker to find

the port number

Message: From Client to server, port: matchmaker,Re: Address for RPC X

Server : Matchmaker receives message, looks up

answer


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Execution of RPC

Server : Matchmaker replies to client with port P Message: From Server To Client, Port Kernel, Re

RPC X, Port P

Client : Kernel places port P in user RPC message

Client : Kernel sends RPC Message: From Client, To Server, Port P,

Server : Daemon listening to port P receives message


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Execution of RPC

Server : Daemon processes request and processessend output

Message : From RPC, Port P, To Client, Port Kernel,

.

Client : Kernel receives the reply and passes it to theuser.


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Remote Method Invocation

Remote Method Invocation (RMI) is a Javamechanism similar to RPCs.

RMI allows a Java program on one JVM to invoke

a method on a remote object.



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Differences between RPC and RMI:

RPC support procedural programming – only remote

procedure or functions may be called; RMI is object based

and enables invocation of methods on remote objects In RPC the parameters are ordinary data structures; with

RMI you can pass objects as parameters

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