Chapter 3: Processes
Operating System Concepts – 9th Edition
Silberschatz, Galvin and Gagne ©2013
Chapter 3: Processes
 Concept
 Scheduling
 Operations
 Interprocess Communication (IPC)
 Examples of IPC Systems
 Communication

Client-Server Systems
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Objectives
 To introduce the notion of a process
 To describe the various features of processes

scheduling

creation

termination, and

communication
 To explore interprocess communication

shared memory

message passing
 To describe communication in client-server systems
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1. Process Concept
 Process

a program in execution

job
 Multiple parts in a Process

Code section

Program counter, processor registers

Stack

Data section

Heap

containing memory dynamically allocated during run time
 Program and Process

Passive and active

One to one or one to many
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Process in Memory
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Process State
 Changes during execution

new


running


The process is waiting for some event to occur
ready


Instructions are being executed
waiting


The process is being created
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)
 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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Threads
 Each process may have one or more threads
 See next chapter
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Process Representation in Linux

Represented by the C structure task_struct
pid t pid; /* process identifier */
long state; /* state of the process */
unsigned int time slice /* scheduling information */
struct task struct *parent; /* this process’s parent */
struct list head children; /* this process’s children */
struct files struct *files; /* list of open files */
struct mm struct *mm; /* address space of this process */
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2. Process Scheduling
 Purpose

Maximize CPU usage
 Process scheduler

selects among available processes for next execution on CPU

Maintains scheduling queues of processes

Job queue
–

Ready queue
–

set of all processes in the system
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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Ready Queue And Various
I/O Device Queues
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Representation of Process Scheduling

Queuing diagram represents queues, resources, flows
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Schedulers
 Long-term scheduler

Job scheduler

Determine process to be brought into the ready queue

Invoked in seconds/minutes

Controls the degree of multiprogramming
 Short-term scheduler

CPU scheduler

Determines the process to be executed next

Sometimes the only scheduler in a system

Invoked in milliseconds-very fast
 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
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Addition of Medium Term Scheduling
 Medium-term scheduler

Applicable only 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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Multitasking in Mobile Systems
 Some systems / early systems

allow only one process to run, others suspended
 Android allows multitasking

Run foreground and background, with fewer limits

Background process uses a service to perform tasks

Service can keep running even if background process is suspended

Service has no user interface, small memory use
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Context Switch
 CPU switch to another process

Save the current state

Load the saved state

Context

represented in the PCB
 Context-switch time

Overhead  the system does no useful work while switching

The more complex the OS and the PCB  longer the context switch

Dependent on hardware support

multiple sets of registers per CPU  multiple contexts loaded at once
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3. Operations on Processes
 System must provide mechanisms

process creation

Termination

….
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Process Creation
 Process creation can cause a tree of processes
 PID is used to identify and manage the Process.
 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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A Tree of Processes in Linux
init
pid = 1
login
pid = 8415
khelper
pid = 6
bash
pid = 8416
ps
pid = 9298
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sshd
pid = 3028
kthreadd
pid = 2
pdflush
pid = 200
sshd
pid = 3610
tcsch
pid = 4005
emacs
pid = 9204
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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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Creating a Separate Process via Windows API
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Process Termination
 Child Process exit()

Output data from child to parent (via wait())

Process’ resources are deallocated by OS

Zombie


Orphan


a process that has completed execution but still has an entry in the process table
Parent process has finished or terminated
Daemon

Background process
 Parent may abort() children processes

Child has exceeded allocated resources

Task assigned to child is no longer required

If parent is exiting


Some OS do not allow child to continue if its parent terminates

All children terminated - cascading termination
Wait for termination, returning the pid:
pid t pid; int status;
pid = wait(&status);
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Multi-process Browser
 Regular browsers

Ran as a single process

One web site crashes  crash entire browser
 Google Chrome

Multi-process browser with 3 categories

Browser process



manages user interface, disk and network I/O
Renderer process

renders web pages, deals with HTML, Javascript, new one for each website opened

Runs in sandbox restricting disk and network I/O, minimizing effect of security exploits
Plug-in process

for each type of plug-in
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4. Interprocess Communication
 Processes within a system

independent

cooperating
 Cooperating processes

Can affect or be affected by other processes, including sharing data

Need inter-process communication (IPC)

Shared memory

Message passing
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Communications Models
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Cooperating Processes
 Advantages

Information sharing

Computation speed-up

Modularity

Convenience
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Example of Cooperating Process
 Producer-Consumer Problem

producer process


consumer process


Consumes the information
unbounded-buffer


produces information
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;
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Producer process
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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Consumer process
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 – Message Passing
 Mechanism

Processes communication and synchronization

Via a communication link (implementation)
 Physical:
 Logical:

shared memory, hardware bus
direct or indirect; synchronous or asynchronous
If P and Q wish to communicate
 Set
up a communication link first
 Exchange
messages via Send/Receive
 Operations

send(message)

receive(message)
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Implement a communication link
 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?
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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

Exchanged through mailbox

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

send and receive messages through mailbox

destroy a mailbox
 Primitives

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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Synchronization
 Message passing

either blocking

or non-blocking
 Blocking

Synchronous

Blocking send


the sender block until the message is received
Blocking receive

the receiver block until a message is available
 Non-blocking

Asynchronous

Non-blocking send


the sender send the message and continue
Non-blocking receive

the receiver receive a valid message or null
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Synchronization (Cont.)
 rendezvous

both send and receive are blocking
 Producer-consumer becomes trivial
message next_produced;
while (true) {
/* produce an item in next_produced */
send(next_produced);
}
message next_consumed;
while (true) {
receive(next_consumed);
/* consume the item in next_consumed */
}
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Buffering
 Queue of messages attached to the link
 Implemented in one of three ways



Zero capacity

0 messages

Sender must wait for receiver (rendezvous)
Bounded capacity

finite length of n messages

Sender must wait if link is full
Unbounded capacity

infinite length

Sender never waits
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5. Examples of IPC Systems - POSIX
 POSIX Shared Memory


Process first creates shared memory segment

shm_fd = shm_open(name, O_CREAT | O_RDRW, 0666);

Also used to open an existing segment to share it
Set the size of the object


Map it using mmap()


ftruncate(shm_fd, 4096);
Get the pointer to the memory
Now the process could write to the shared memory

sprintf(shared memory pointer, "Writing to shared memory");
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IPC POSIX Producer
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IPC POSIX Consumer
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Examples of IPC Systems - Mach
 Mach

communication is message based

system calls are messages

Each task gets two mailboxes at creation- Kernel and Notify

Only three system calls needed for message transfer


Mailboxes needed


msg_send(), msg_receive(), msg_rpc()
for communication, created via port_allocate()
Send and receive are flexible, for example four options if mailbox full:

Wait indefinitely

Wait at most n milliseconds

Return immediately

Temporarily cache a message
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Examples of IPC Systems – Windows
 Message-passing

centric via advanced local procedure call (LPC) facility

Only works between processes on the same system

Uses ports (like mailboxes) to establish and maintain communication channels

Communication works as follows:

The client opens a handle to the subsystem’s connection port object.

The client sends a connection request.

The server creates two private communication ports and returns the handle to one of them to the
client.

The client and server use the corresponding port handle to send messages or callbacks and to listen
for replies.
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Local Procedure Calls in Windows XP
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6. Communications in Client-Server Systems (Optional)
 Sockets
 Remote Procedure Calls
 Pipes
 Remote Method Invocation (Java)
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Sockets
 defined as an endpoint for communication
 Concatenation of IP address and port
 The socket 161.25.19.8:1625

refers to port 1625 on host 161.25.19.8
 Communication consists between a pair of sockets
 All ports below 1024 are well known

used for standard services
 Special IP address 127.0.0.1 (loopback)

refer to system on which process is running
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Socket Communication
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Sockets in Java


Three types of sockets

Connection-oriented (TCP)

Connectionless (UDP)

MulticastSocket class– data can
be sent to multiple recipients
Consider this “Date” server:
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Remote Procedure Calls


Remote procedure call (RPC)

abstracts procedure calls between processes on networked systems

Again uses ports for service differentiation
Stubs


The client-side stub





receives this message

unpacks the marshalled parameters, and

performs the procedure on the server
On Windows
stub code compile from specification written in Microsoft Interface Definition Language (MIDL)
Data representation

handled via External Data Representation (XDL) format to account for different architectures

Big-endian and little-endian
Remote communication has more failure scenarios than local


locates the server and marshalls the parameters
The server-side stub


client-side proxy for the actual procedure on the server
Messages can be delivered exactly once rather than at most once
OS typically provides a rendezvous (or matchmaker) service to connect client and server
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Execution of RPC
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Pipes
 Acts as a conduit
 allow two processes to communicate
 Issues

Is communication unidirectional or bidirectional?

In the case of two-way communication, is it half or full-duplex?

Must there exist a relationship (i.e. parent-child) between the communicating
processes?

Can the pipes be used over a network?
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Ordinary Pipes

Ordinary Pipes allow communication in standard producer-consumer style

Producer writes to one end (the write-end of the pipe)

Consumer reads from the other end (the read-end of the pipe)

Ordinary pipes are therefore unidirectional

Require parent-child relationship between communicating processes

Windows calls these anonymous pipes

See Unix and Windows code samples in textbook
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Named Pipes
 Named Pipes are more powerful than ordinary pipes
 Communication is bidirectional
 No parent-child relationship is necessary between the communicating processes
 Several processes can use the named pipe for communication
 Provided on both UNIX and Windows systems
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End of Chapter 3
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