Processes

Process ConceptProcess SchedulingOperations on ProcessesCooperating ProcessesInterprocess CommunicationCommunication in Client-Server

Systems

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 almost interchangeably.

Process – a program in execution; process A process includes:

program counter stack data section

Data

Text

Heap

Stack

Process in memory Value of Program counter

Contents of processor’s registers

Temporary data:Function parameter,

return addresses and local variables

Contains global variables

Memory: that is dynamically allocated

during process run time

A process is more than the program code –text section

Relationship b/w process and program

Is process same as a program? No

Program is sequence of statements that a programmer writes.

A single program may contain more than one processes Many processes may be derived from a single program

When you perform “Copy” at the Command Prompt of Windows XX, several processes are derived from that. Copy source_File destination_File

Multiprogramming VS Uniprogramming

Some systems allow only one process called Uni-programming Systems. To invoke a new process other processes must be

finished. Most systems now a days allow more than one

process called Multiprogramming Systems. In case of uni-programming you don’t need to

store information about the process as much as you would need to store information about a process being executed in multiprogramming system.

Process State In user view processes are running simultaneously In computers view these processes are in contention for the CPU and I/O 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.

Diagram of Process State

The state of a process is defined in part by the current activity of that process

Process Control Block (PCB) 1/2Saves Information associated with each process. Remember that we studied in interrupts that the process state is saved in memory. PCB provides data structure for saving process state

Process state New, Wait, Ready etc.

Program counter Next instruction to be executed.

CPU registers EAX, EBX, SP etc.

Process Control Block (PCB)

CPU Switch From Process to Process

Process Control Block (PCB)

CPU scheduling information Process priority, pointers to scheduling queues

Memory-management information Information about base and limit registers (remember

the Memory-protection chapter 2?), page tables etc.

Accounting information Amount of CPU time, Time limits (CPU protection),

process numbers etc

I/O status information List of I/O devices being used by the process

Processes

Process ConceptProcess SchedulingOperations on ProcessesCooperating ProcessesInterprocess CommunicationCommunication in Client-Server

Systems

Process Scheduling

Objective of multi-programming is to have some processes running at all the time Maximization of CPU utilization

It is achieved by time sharing

Process Scheduler is used to control the “flow” of processes

Process Scheduling QueuesJob queue – set of all processes in the system.

Ready queue – set of all processes residing in main memory, ready and waiting to execute. –Linked List

Device queues – set of processes waiting for an I/O device.—A device queue for each device

Monitor, HDD, Floppy and other I/O devices have separate Queues.

Process migrates between the various queues.

Ready Queue And Various I/O Device Queues

Head is to tell the start of the queue where as Tail is tofacilitate the entry of new Process in the queue

Representation of Process Scheduling Queue

Resource

SchedulersMore processes are submitted than can be

executed immediately -- spoolingLong-term scheduler (or job scheduler) –

selects which processes should be brought into the ready queue. Might take Seconds to be activated

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU. Is happening in milliseconds

Long term and Short term Schedulers The long term scheduler may need to be invoked only when a

process leaves the system Degree of multiprogramming -- number of processes in memory

Long Long TermTerm

schedulerscheduler

ReadyReadyQueueQueue

ShortShortTermTerm

SchedulerSchedulerCPU

Newprocess

Schedulers cont.

I/O bound –Process that spends more of its time doing I/O than it spends doing computation

CPU bound –Process that generates I/O requests infrequently, using more of its time doing computations.

Long term scheduler should select a good process mix of I/O bound and CPU bound processes –balance in queues.

Long term scheduler may be absent or minimal on some systems –(e.g. Time sharing systems (UNIX, Microsoft windows).

Swapping – good to improve the process mix.

Addition of Medium Term Scheduling

Temporarily swaps out the process to reduce the degree of multiprogramming

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.

Long-term scheduler must strike a balance between these two types of processes

Current Trends…

Long-term scheduler is absent in almost all the latest operating systems like Windows and Unix ; Due to time-sharing we don’t need to have long-

term schedulers. Degree of multiprogramming is in user’s hands Greater the degree of multi-programming ,

slower the computer gets

Context SwitchWhen 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 is represented in the PCB of the process

Context-switch time is overhead; the system does no useful work while switching. Useful work? Useful with respect to users

point of view… Time dependent on hardware support.

Processes

Process ConceptProcess SchedulingOperations on ProcessesCooperating ProcessesInterprocess CommunicationCommunication in Client-Server

Systems

Process CreationParent process create children processes, which, in turn

create other processes, forming a tree of processes.Resource sharing in parent and child processes ( 3

Types) 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.

Process Creation (Cont.)

Address space Child duplicate of parent. (Linux) Child has a program loaded into it. (Windows)

UNIX / Linux examples fork system call creates new process exec system call used after a fork to replace the

process’ memory space with a new program. Windows 2000/XP examples

CreateProcess system call Several others also present to control the execution of

process.

Unix Process Implementation

Parent creates the process using Fork() call

Child is created, and is exact copy of parent

Child does not execute till it is issued Exec() call

As parent and child are same the distinction of being a parent or child is made by ProcessID number

UNIX FORK and EXEC functions#include <sys/types.h>#include<stdio.h>#include<unistd.h>int main(){pid_t pid;//fork another processpid = fork();If (pid < 0) { //error occurred

fprintf(stderr, “Fork failed”);exit(-1); }

else (if pid ==0) {execlp(“/bin/ls”,”ls”); } //ls is “Dir” equivalent of linux

Else {wait(NULL); //wait till child process is completeprintf(“Child Complete”);

Exit(0):}

POSIX

fork()

exec()exit()

waitparent

child

resumes

Win16 function WinExec() (Now based on CreateProcess() function of Win32)

#include “windows.h”#include “iostream.h”int main(int argc, char* argv[]){

WinExec("Calc",NULL); return 0;

}

Documentation (ref. MSDN)Creates a new process and runs it

in separate memory space

Processes are created in Win32 API using CreateProcess() function

CreateProcess() is similar to fork()

fork() is passed no parameters, CreateProcess() expects no fewer than ten

Address space difference --loading another program in child’s address space

Process Termination

Process executes last statement and asks the operating system to delete it (exit()). Following steps are performed on exit

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

Parent may terminate execution of children processes (abort ()). Why would parent do that?

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.

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

Inter Process CommunicationIndependent 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 (breaking a task into subtasks)

Modularity (Divide the system functions into separate processes or threads)

Convenience (editing, printing and compiling at the same time)

Outline Process Concept Process Scheduling Operations on Processes

Process Creation Process Termination Shared Memory Systems Message Passing Systems

Naming Synchronization Buffering

Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation

Examples of IPC Systems

Shared memory systems Communicating processes need to establish a

region of shared memory. Shared memory region resides in the address

space of process creating SM seg. producer process produces information that is

consumed by a consumer process. Compiler, assembler, loader Producer is Server and a Consumer is client

We must have a buffer of items that can be filled by the producer and emptied by the consumer.

Consumer shouldn’t try to consume that has not yet been produced

Shared memory systems (2)

unbounded-buffer places no practical limit on the size of the buffer. Consumer may have to wait; producer can always produce

bounded-buffer assumes that there is a fixed buffer size. Consumer must wait if the buffer is empty; producer must

wait if the buffer is full

Shared buffer is implemented as a circular array with two logical pointers in and out

Buffer is empty when in==out Bufffer is full when ((in+1)%BUFFER_SIZE)==out

Shared memory systems (3)

# define BUFFER_SIZE 10

typedef struct {

…..

}item;

Item buffer[BUFFER_SIZE];

int in =0;

int out=0;

item nextproduced;

while (true){

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

/*do nothing*/

buffer[in] = nextproduced;

in = (in +1)% BUFFER_SIZE;

}

item nextconsumed;

while (true) {

while(in==out)

;//do nothing

nextconsumed = buffer[out];

out=(out+1)%BUFFER_SIZE;

}

Outline Process Concept Process Scheduling Operations on Processes

Process Creation Process Termination Shared Memory Systems Message Passing Systems

Naming Synchronization Buffering

Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation

Examples of IPC Systems

Message Passing Systems

Mechanism for processes to communicate and to synchronize their actions.

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

Several methods for logically implementing send()/ receive() operations:

Direct & indirect communication Synchronous or asynchronous communication Automatic or explicit buffering

Lets do an example of Direct message passing#include <windows.h>#include <iostream.h>void main(void){

char str[100];cin >> str;

/*this sleep() is not required. Just wanted to tell you another function that delays the execution. Basically this functions puts the current thread to sleep for specified amount of time in milliseconds */Sleep(3000); //wait for 3 seconds..//Return the handler to the window with the specified caption HWND hWnd = FindWindow(NULL, str);//Send message to window with handle hWnd and ask it to closeSendMessage(hWnd,WM_CLOSE,0,0L);//Guess what would happen if SetWindowText(hWnd,"Calc2000"); is used //instead of SendMessage

}

Its simple to execute this program:Program asks for the window caption

Enter the caption e.g. CalculatorCalculator program must be runningThis program will simply close the

program

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.

In MS Windows examples of such communication enabling functions (system calls) are SendMessage(…) //Function used to send message to other window PostMessage(…) //posts a message to a queue

Hard-Coding ----disadvantage

Indirect Communication (1) 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.

Indirect Communication (2)

Operations 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 A

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

POSIX message queues use an integer value to identify a mailbox

Indirect Communication (3) 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.

Indirect Communication (4)

Mailbox may be owned by a process or OS Owner – Process

Owner –who can only receive messages through mailbox User –Who can only send messages to the mailbox No confusion about who should receive a message sent to this

mailbox

Owner –OS mailbox is independent –not attached with any particular process OS must provide a mechanism

crate a new mailbox, send and receive messages, delete ownership and receiving privilege may be passed to other

processes –multiple receivers for each mailbox.

MS Windows API for Inter process Indirect communications

MailSlots A mailslot is a mechanism for one-way interprocess

communications (IPC). A Win32-based application can store messages in a mailslot. The owner of the mailslot can retrieve messages that are stored there. Good for a process broadcasting its message to many processes.

CreateMailSlot ( … ) ReadFile( … ) WriteFile ( … ) Must be synchronized to some extent.

The creater can read or write the mail slot but the client can only send data. Not like the description of previous slide

void main(void){

HANDLE hMS; hf ; BOOL ret=1;DWORD bWritten, b, c, d, e;char a[10];//lets create mail slot (kind of mail server)hMS=CreateMailslot("\\\\.\\mailslot\\my_mailslot",0,MAILSLOT_WAIT_FOREVER,NULL); if (hMS == INVALID_HANDLE_VALUE){ cout << "Failed to create mailslot" << endl; }/*open abstract file (this file is not in hard disk but in RAM). we will read or write to this file.NOTE: Client cannot read this file, only thing client can do is to write to this file.. just like email.. sender cannot view email on your end */hf = CreateFile("\\\\.\\mailslot\\my_mailslot",GENERIC_READ | GENERIC_WRITE,

FILE_SHARE_READ | FILE_SHARE_WRITE,NULL,OPEN_EXISTING,NULL,NULL);/*dummy wait for another process to send any message. In real life a process might be involved with any other computations and still other process can post mail message to it which this process can read upon return... when you will do some visual programming (event driven) this will become more clear.*/cin >> a;//just to show you another API call //Read the documentation to be clear about it! :) GetMailslotInfo(hMS,&b,&c,&d,&e);a[0] = '\0'; //initalize the arrayret = ReadFile(hMS,&a[0],c,&bRead,(LPOVERLAPPED)NULL); cout << a;GetMailslotInfo(hMS,&b,&c,&d,&e);ret = ReadFile(hMS,&a[0],c,&bRead,(LPOVERLAPPED)NULL); cout << a;cin >> a; //dummy wait again!

}

Mailslot ServerProcess that receives messages

(Server.cpp)

#include <iostream.h>#include <windows.h>void main(void){

HANDLE hfile;hfile = CreateFile("\\\\.\\mailslot\\my_mailslot",GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE,NULL,OPEN_EXISTING,NULL,NULL);

DWORD bWritten;char a[100];cout << endl <<"Enter message to send to other process: ";cin >> a;BOOL ret = WriteFile(hfile,a,strlen(a), &bWritten,NULL); //variable lengthret = WriteFile(hfile,"OS!",3, &bWritten,NULL);//hardcoded

}

Mailslot ClientProcess that receives messages

(Client.cpp)

Create client.cpp and server.cpp. Compiler them and execute them separatelyFirst execute server.exe. Don’t do any thing with this executableExecute client.cpp and enter a message less than 10 charsThen continue the server.exe my pressing enter key!

MS Windows API for Inter process Indirect communications

Named pipes Protocol used to transfer data between

processes, usually across a network. Typically, a process creates a named pipe

(conceptually, a conduit) with a well-known name this process is the server process.

Processes that can get the name of the pipe can open the other end of the pipe and can exchange data by performing read and write operations on the pipe.

Outline Process Concept Process Scheduling Operations on Processes

Process Creation Process Termination Shared Memory Systems Message Passing Systems

Naming Synchronization Buffering

Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation

Examples of IPC Systems

Synchronization Message passing may be either blocking or non-

blocking. Blocking is considered synchronous Non-blocking is considered asynchronous Blocking send: The sender process is blocked

until the message is received by the receiving process or by the mail box.

Nonblocking send, Blocking receive, Nonblocking receive

Producer – consumer problem ?

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.

Outline Process Concept Process Scheduling Operations on Processes

Process Creation Process Termination Shared Memory Systems Message Passing Systems

Naming Synchronization Buffering

Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation

Examples of IPC Systems --Self study

Client-Server Communication

SocketsRemote Procedure CallsRemote Method Invocation (Java)

SocketsBICSE (not to be covered as it is part of your C language course)

A socket is 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. No programming practice for sockets because

you will do it in either C or in Advanced Java

Socket Communication

Following code creates a process (e.g. Calculator) and executes it as a child process.

WaitForSingleObject() halts the execution of parent process till the time child process is finished.

#include <iostream.h>#include <windows.h>void main(void){ STARTUPINFO si; PROCESS_INFORMATION pi; //create process wants this structure to be initilized ZeroMemory( &si, sizeof(si) ); //fills the process with all zero’s si.cb = sizeof(si); // Start the child process. Returns false if child process failed to

execute if( !CreateProcess( NULL, // No module name (use command line). "calc", // Command line. NULL, // Process handle not inheritable. NULL, // Thread handle not inheritable. FALSE, // Set handle inheritance to FALSE. 0, // No creation flags. NULL, // Use parent's environment block. NULL, // Use parent's starting directory. &si, // Pointer to STARTUPINFO structure. &pi ) // Pointer to PROCESS_INFORMATION structure. ) { cout << "CreateProcess failed." ; }

cout << "Child is running with parent on timeshared bases" << endl; cout <<"Now i am going to block the parent till child exits" << endl; // Wait till child process exits.

WaitForSingleObject( pi.hProcess, INFINITE );cout << "Child process exited" << endl;

}

How to create child process in Win32 using CreateProcess()Reference MSDN

Do Read Documentation of

CreateProcess()STARTUPINFOPROCESS_INFORMATIONWaitForSingleObject()

#include <Windows.h>#include <iostream.h>int main(void){ DWORD dwThreadId, dwThrdParam = 1; HANDLE hThread; hThread = CreateThread( NULL, // no security attributes 0, // use default stack size ThreadFunc, // thread function – The function that executes as a

thread &dwThrdParam, // argument to thread function 0, // use default creation flags &dwThreadId); // returns the thread identifier // Check the return value for success. if (hThread == NULL) {

cout << " Thread creation failed" << endl;return 0;

}int x=0;while ( x < 50){

cout << "i am parent ---" << x << endl;x ++;

} CloseHandle( hThread );}

Creating a Thread inMS Platforms

Thread executes as aSeparate process

DWORD WINAPI ThreadFunc( LPVOID lpParam )

{

char szMsg[80];

unsigned int x; x = 0;

while (x < 50)

{

cout << x << " --- Thread is being executed" << endl;

x++;

}

return 0;

} This function runs as a

concurrent thread.

Output: The output may vary from computer to computer

int main(){//fork another processInt pid = fork();If (pid < 0) { //error occurred

fprintf(stderr, “Fork failed”);exit(-1); }

else (if pid ==0) {execlp(“/bin/ls”,”ls”); }

Else {wait(NULL); //wait till child process is

//completeprintf(“Child Complete”);

Exit(0):}

int main(){//fork another processInt pid = fork();If (pid < 0) {

fprintf(stderr, “Fork failed”);exit(-1); }

else (if pid ==0) {execlp(“/bin/ls”,”ls”); }

Else {wait(NULL);printf(“Child Complete”);

Exit(0):}

PARENT CHILD

NOTE: Same code runs for both child and parent (Identified by pid returned by fork()

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

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.

In MS Windows examples of such communication enabling functions (system calls) are SendMessage(…) //Function used to send message to other

window PostMessage(…) //posts a message to a queue

Lets do an example of Direct message passing#include <windows.h>#include <iostream.h>void main(void){

char str[100];cin >> str;

/*this sleep() is not required. Just wanted to tell you another function that delays the execution. Basically this functions puts the current thread to sleep for specified amount of time in milliseconds */Sleep(3000); //wait for 3 seconds..//Return the handler to the window with the specified caption HWND hWnd = FindWindow(NULL, str);//Send message to window with handle hWnd and ask it to closeSendMessage(hWnd,WM_CLOSE,0,0L);//Guess what would happen if SetWindowText(hWnd,"Calc2000"); is used //instead of SendMessage

}

Its simple to execute this program:Program asks for the window caption

Enter the caption e.g. CalculatorCalculator program must be runningThis program will simply close the

program

Interesting Program

#include <windows.h>

#include <iostream.h>

void main(void)

{

FILETIME a,b,c,d;

HANDLE h =OpenProcess(PROCESS_ALL_ACCESS,TRUE,1920);

GetProcessTimes(h,&a,&b,&c,&d); //READ THE DOCUMENTATION

SYSTEMTIME aa,bb,cc,dd;

FileTimeToSystemTime(&c,&cc);

cout << cc.wMilliseconds <<endl;

}

Read the documentation carefully (ref msdn) and guess what is this program doing!A QUIZ MIGHT BE COMING YOUR WAY!!!!!!!!!!!!!!!!!!

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.

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 A

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

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.