os4 1

Chapter 4 Processes

os4 2

OutlineProcess ConceptProcess SchedulingOperations on ProcessesCooperating ProcessesInterprocess CommunicationCommunication in Client-Server Systems

os4 3

Process Concept (1)Process (job): a program in execution (informally). the unit of work in most systems A system consists of a collection of processes (system

processes and user processes) executing concurrently, with CPU multiplexed among them

A process is more than the program code (known as the text section). It also includes: (more formal definition) current activity (program counter, content of registers) stack containing temporary data (e.g., parameters) data section containing global variables a set of associated resources.

os4 4

Process Concept (2)program counter: next instruction to be executed A program is a passive entity but a process is an active entityTwo processes may be associated with the same program, they are considered as separate execution sequences. They have the same text section, but their data

section will vary.e.g., several users may running the copies of the mail/editor

program.

A process may spawn many processes as it runs.

os4 5

Process state new: the process is being created running: instructions are being executed waiting: the process is waiting for some event

to occur (such as an I/O completion or reception of a signal)

ready: The process is waiting to be assigned to a processor

terminated: The process has finished execution

Only one process is running on any processor at any instant. However, many processes may be ready or waiting.

Process State

os4 6

new

ready running

terminated

waiting

admitted interrupt

exit

scheduler dispatchI/O or event wait I/O or

event completion

Diagram of Process State

os4 7

Process Control Block (PCB)

Each process control block (task control block) represents a process. It includes

process state (N, R, T, W, Run) program counter CPU registers CPU scheduling information

(e.g., priority) memory-management

information(e.g., base/limit register)

accounting information I/O status information

process number

program counter

registers

memory limits

list of open files

pointerprocessstate

...

to next PCB

os4 8

CPU Switching

......................

........

......

save state into PCB0

reload state from PCB1

...

save state into PCB1

reload state from PCB0

...

idle

idle

idle

executing

executing

executing

interrupt or system call

process P0 operating system process P1

interrupt or system call

os4 9

Thread Sometimes is called a lightweight process: is

a basic unit of CPU utilization; it comprises a thread ID, a program counter, a register set, and a stack.

It shares with other threads belonging to the same process its code section, data section, and other OS resources, such as open files and signals.

A web browser might have one thread display images or text while another thread retrieves data from the network.

Modern OS has extend the process concept to allow a process to have multiple threads of execution.

Threads

os4 10

Process Scheduling

Objective of multiprogramming: CPU runs process at all times to maximize CPU utilization.Objective of time sharing: switch CPU frequently such that users can interactinteract with each program while it is running.For a uni-processor system, there is only one running process. The rest should wait until CPU free and is rescheduled.

os4 11

Scheduling QueuesScheduling queues job queue: containing all processes ready queue

usually, stored as a linked list device queues: each containing

processes waiting for it

Queuing diagram: a common representation for process scheduling rectangle: queue circle: server for a queue arrow: flow

os4 12

headtail

readyqueue registers registers

PCB7 PCB2

headtail

disk unit 0

registers registers

PCB3 PCB14

registers

PCB8

headtail

terminal unit

0registers

PCB6

headtail

mag tape unit

1

Ready queue and various I/O device queues

os4 13

Queuing-diagram Representation of Process Scheduling

ready queue

I/O queue I/O requestI/O

CPU

time slice expired

fork a child

wait for an INT

childexecutes

Childterminates

interrupt occurs

each device has one

os4 14

Schedulers (1)Long-term Scheduler (job Scheduler): selecting processes from the job pool and loads them into memory for execution. Execute less frequently (e.g., once several

minutes) Control the degree of multiprogramming Select a good process mix of I/O-bound

processes and CPU-bound processes Some system, such as time-sharing system,

do not have. Their multiprogramming degree are controlled by hardware limitation (e.g., # of terminals) or on the self-adjusting nature of users.

os4 15

Schedulers (2): Short-term scheduler (CPU scheduler)selecting one of the ready processes, and allocates the CPU to it. Execute quite frequently (e.g., once per

100ms) must be very fast

e.g., if it takes 10 ms, then 10/(100+10) percent of CPU is wasted.

ready queue

I/O queue(s)I/O

CPUshort-termlong-term end

os4 16

Schedulers (3): Medium-term Scheduler (Swapping) swap out: removing processes from memory to

reduce the degree of multiprogramming. swap in: reintroducing swap-out processes into

memory for improving process mix or for memory not enough

ready queue

I/O waitingI/O

CPU

partially executedswapped-out processes

swap in swap out

medium-term

os4 17

Schedulers (4)context switch: saving the state of the old process and loading the saved state of the new process. This is a pure overhead switch time (about 1~1000 ms) depends on

memory speed number of registers existence of special instructions

e.g., a single instruction to save/load all registers

hardware support e.g., multiple sets of registers

os4 18

Operation on Processes: Creation and Termination

Process creation A parent process creates children

processes via a system call (such as fork in UNIX)

A child process may obtain resource by1. ask new ones from OS, or2. use a subset of its parent’s

: prevent system overloading

(by too many sub-processes)

P1

P4

P3P2

P4 P5

process graph (a tree)

os4 19

Two possibilities of execution parent execute concurrently parent waits until some/all children terminate.

Two possibilities of the address space of the new process The child process is a duplicate of the parent

process. (communication via sharing variables) The child process has a program loaded into it.

(communication via message passing)e.g., UNIX : both (fork, execve)

DEC VMS: loadWindows/NT: both

os4 20

Process termination two termination system calls

exit (parent can use wait to get the output)abort: usually only the parent can abort a

child (except OS). The reasons may be: the child has exceeded its usage of some

resources the assigned task is no longer required the parent is exiting, and OS does not allow its

children to continuecascading termination (initiated by OS):

a terminated process causes all its children (and their children in turn) to terminate.

os4 21

Cooperating ProcessesTwo type of processes independent process: not affect or be

affected by others cooperating processes

reasons: information sharing (a shared file) computation speedup (CPUs, I/O channels) modularity (construct a system in a modular

fashion) convenience (A user can performs several tasks at

one time (editing, printing, compiling))

need communication/synchronization mechanisms

os4 22

An Example: The producer-consumer problem producer produces items (into buffer) for

consumere.g., print program printer driver

compiler assembler loader

unbounded-buffer problem producer: no wait consumer: wait when buffer is empty

bounded-buffer problem producer: wait when buffer is full consumer: wait when buffer is empty

os4 23

A shared-memory solution for synchronization

implement the buffer as a circular array

var buffer: array[1..n] of item;

in, out: 0..n-1; (initialized to 0)

next free: in first full: out empty: in=out at most n-1 items

in

out

os4 24

Shared-memory solution for the producer-consumer problem

Import java.util.*;Public class BoundedBuffer{ public BoundedBuffer () {

// Buffer in initially emptycount = 0; in = 0; out = 0;buffer = new Object[BUFFER_SIZE];

}// producer calls this methodpublic void enter (Object item) { // in next slice} // consumer calls this methodpublic Object remove() { // in next slice} public static final int NAP_TIME = 5;private static final int BUFFER_SIZE = 5;

private volatile int count;private volatile int in; //points to the next free positionprivate volatile int out; //points to the next free positionprivate Object [] buffer;

os4 25

public void enter(Object item) {

while (count == BUFFER_SIZE); // do nothing

// add an item to the buffer++count;buffer[in] = item;in = (in + 1) % BUFFER_SIZE;

if (count == BUFFER_SIZE)

system.out.println(“Producer Entered “ + item + “ Buffer FULL”);

else

system.out.println(“Producer Entered “ + item + “ Buffer Size” + count);

}

public Object remove () {Object item;

while (count == 0); // do nothing

// remove an item from the buffer

- -count;item == buffer[out];out = (out + 1) %

BUFFER_SIZE;

if (count == 0)

system.out.println(“Consumer Consumed “ + item + “ Buffer EMPTY”);

else

system.out.println(“Consumer Consumed “ + item + “ Buffer Size” + count);

return item;}

enter()remove()

os4 26

Inter-process Communication (IPC)

Two ways for communication shared-memory (e.g., thread facility) IPC (e.g., message-passing system)

An IPC provides a mechanism to allow processes to communicate and to synchronize their actions without sharing the same address space.An IPC is particularly useful in a distributed environment where communicating processes may reside on different computers connected with a network. chat on WWW.An IPC facility provides at least two operations:

send(message)receive(message)

os4 27

Several methods for logically implementing a link and the 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

Naming

Buffering

os4 28

Naming: direct communicationUse process identities (symmetry in

addressing)send(P, message)receive(Q, message)

Asymmetry (receiving from any process)send(P, message)receive(id, message)

Properties A link is established automatically between

every pair of processes. A link is associated with exactly two

processes. Between each pair of processes, there

exists exactly one link.

identity

from whom

os4 29

The link may be unidirectional, but is usually bidirectional.

Example: a solution to the producer-consumer problem

: limited modularity: if the name of a process is changed, all old names should be found. It is not easy for separate compilation.

repeat

...

produce an item in nextp

...

send(consumer,nextp);

until false;

repeat

receive(producer,nextc);

...

consume the item in nextc

...

until false;

The producer process The consumer process

os4 30

Use mailboxes (or called ports, each has a unique id).

send(A, message)receive(A, message)

Properties of such a link: A link is established between every pair of

processes only if they have a shared mailbox. A link may be associated with more than two

processes. Between each pair of communicating

processes, there may be a number of different links, each corresponding to one mailbox.

Naming: indirect communication

os4 31

The link may be either unidirectional or bidirectional.

One Problem: Which process, P2 or P3, will receive the message sent by P1?

Solutions: Allow a link to be associated with at most two

processes. At most one process at a time to execute a

receive operation. The OS arbitrarily selects one (not both).

mailbox AP1

P2

P3

sendreceive

receive(1) (2)

os4 32

SynchronizationMessage passing may be either blocking or nonblocking – and also as synchronous and asynchronous. Blocking send: SP is blocked until the

message is received by RP or by the mailbox. Nonblocking send: SP sends the message

and resumes operation. Blocking receive: RP is blocked until the

message is available. Nonblocking receive: RP receives a valid

message or a null.

os4 33

BufferingCapacity of a link: the number of messages that can reside (queued) temporarily.Three ways for implementation: zero capacity: queue length = 0.

The sender must wait until the recipient receives the message.

This synchronization is called a rendezvous. bounded capacity: queue length = n.

Sender should wait if queue is full. unbounded capacity.

The sender is never delayed.

No buffering

Automatic buffering

os4 34

In nonzero-capacity cases, two processes communicate asynchronously. For asynchronous communication, one way to make sure (or to wait) that the receiver has received the sent message is as follows.

process P (sender)send(Q, message) receive (Q, message)

process Q (receiver)

receive(P, message) send(P,acknowledgement")

os4 35

Production-Consumer Example – Mailbox for message passingThe buffer is implemented using the java.util.Vector class (unbounded capacity)Note: both the send() and receive() are nonblockingA multi-thread version is discussed in Chapter 5.

import java.util.*;public class MessageQueue { public MessageQueue() {

queue = new Vector (); }//This implements a nonblocking send public void send(Object item) {

queue.addElement(item); }//This implements a nonblocking receive public void receive() {

object item;if (queue.size() == 0) return null;else { item =

queue.firstElelemt();queue.removeElementAt(0);return item; }

}private Vector queue;

}

os4 36

MessageQueue mailBox;

While (true) {Date message = new Date();mailBox.send (message);

}

The producer process

MessageQueue mailBox;

While (true) {Date message = (Date) mailBox.receive ();if (message != null)

//consume the message

}

The consumer process

os4 37

An Example: MachDeveloped at Carnegie Mellon UniversityMost communications are carried out by messages. Message are sent to and received from mailboxes, called ports.Even system calls are made by messages. When each task is created, two special mailboxes – the Kernel mailbox and the Notify mailbox – are also created. The Kernel mailbox is used by the kernel to

communication with the task. The kernel sends notification of event occurrences to the Notify port.

os4 38

Three system calls using message transfer msg_send: sends a message to a mailbox msg_receive msg_rpc: Remote Procedure Calls (PRCs)

port_allocate: creates a new mailbox and allocates space for its queue of messages. The task that creates the mailbox is that mailbox’s owner. Messages are copied into mailbox. Multiple messages from the same sender are queued in FIFO order.If the mailbox is full, the sending thread has 4 options:

Wait indefinitely until there is a room in the mailbox. Wait at most n milliseconds. Do not wait at all, but rather return immediately. Temporarily cache a message.

os4 39

The receive operation must specify from which mailbox or mailbox set to receive a message.port_status: returns # of messages in a given mailbox.If no message is waiting to be received, the receiving thread may wait, wait at most n milliseconds, or not wait.The Mach message system attempts to avoid double copy operations by using virtual-memory management techniques. Mach maps the address space containing the sender’s message into the receiver’s address space.

os4 40

os4 41

Process Control Block (PCB)

Information associated with each process.Process stateProgram counterCPU registersCPU scheduling informationMemory-management informationAccounting informationI/O status information

os4 42

Process Control Block (PCB)

os4 43

CPU Switch From Process to Process

os4 44

Process Scheduling Queues

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.Process migration between the various queues.

os4 45

Ready Queue And Various I/O Device Queues

os4 46

Representation of Process Scheduling

os4 47

Schedulers

Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

os4 48

Addition of Medium Term Scheduling

os4 49

Schedulers (Cont.)

Short-term scheduler is invoked very frequently (milliseconds) (must be fast).Long-term scheduler is invoked very 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.

os4 50

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 no useful work while switching.Time dependent on hardware support.

os4 51

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.

os4 52

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.

os4 53

Processes Tree on a UNIX System

os4 54

Process Termination

Process executes last statement and asks the operating system to decide it (exit). Output 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.

os4 55

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 processAdvantages of process cooperation Information sharing Computation speed-up Modularity Convenience

os4 56

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.

os4 57

Bounded-Buffer – Shared-Memory Solution

Shared data#define BUFFER_SIZE 10Typedef struct {

. . .} item;item buffer[BUFFER_SIZE];int in = 0;int out = 0;

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

os4 58

Bounded-Buffer – Producer Process

item nextProduced;

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

out); /* do nothing */

buffer[in] = nextProduced;in = (in + 1) % BUFFER_SIZE;

}

os4 59

Bounded-Buffer – Consumer Process

item nextConsumed;

while (1) {while (in == out); /* do nothing */nextConsumed = buffer[out];out = (out + 1) % BUFFER_SIZE;

}

os4 60

Interprocess Communication (IPC)

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) – 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

Implementation of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties)

os4 61

Implementation 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?

os4 62

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.

os4 63

Indirect CommunicationMessages 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.

os4 64

Indirect CommunicationOperations create a new mailbox send and receive messages through

mailbox destroy a mailbox

Primitives are defined as:send(A, message) – send a message to mailbox Areceive(A, message) – receive a message from mailbox A

os4 65

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.

os4 66

Synchronization

Message passing may be either blocking or non-blocking.Blocking is considered synchronousNon-blocking is considered asynchronoussend and receive primitives may be either blocking or non-blocking.

os4 67

Buffering

Queue of messages attached to the link; implemented in one of three ways.1. Zero capacity – 0 messages

Sender must wait for receiver (rendezvous).

2. Bounded capacity – finite length of n messagesSender must wait if link full.

3. Unbounded capacity – infinite length Sender never waits.

os4 68

Client-Server Communication

SocketsRemote Procedure CallsRemote Method Invocation (Java)

os4 69

Sockets

A socket is defined as an endpoint for communication.Concatenation of IP address and portThe socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8Communication consists between a pair of sockets.

os4 70

Socket Communication

os4 71

Remote Procedure Calls

Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.Stubs – client-side proxy for the actual procedure on the server.The client-side stub locates the server and marshalls the parameters.The server-side stub receives this message, unpacks the marshalled parameters, and peforms the procedure on the server.

os4 72

Execution of RPC

os4 73

Remote Method Invocation

Remote Method Invocation (RMI) is a Java mechanism similar to RPCs.RMI allows a Java program on one machine to invoke a method on a remote object.

os4 74

Marshalling Parameters