Lecture 3

Process Concepts

What is a Process?

A process is the dynamic execution context of an executing program. Several processes may run concurrently, each as a distinct entity with its own state. The process state consists of at least:

• the code for the running program• the static data for running program• a space in the heap for dynamic data• the address of next instruction• execution stack• values of the CPU registers• OS resources (e.g. open filees)• process execution state

Computer MemoryProcess Code

New

Process State

Terminated

Each process transitions from one state to the next as it shares the CPU and additional computer resources with other processes. A process changes states due to actions by the OS and other entities.

Suspend & Resume

Suspending a process indefinitely removes it from contention for time on a processor without being destroyed. Suspending a process is useful for detecting security threats and for software debugging purposes (e.g. deadlock detection and recovery). A suspension may be initiated by the process being suspended or by another process.

All processes are provided with a set of memory addresses, called a virtual address space A process’s PCB is maintained by the kernel in a protected region of memory that user processes cannot access A Linux (Unix) PCB stores:

Linux (Unix) Processes

• The contents of the processor registers

• PID

• The program counter

• The system stack

• All processes are listed in the process table

• All processes interact with the OS via system calls

• A process can spawn a child process by using the fork system call

• Child receives a copy of the parent’s resources as well

• Process priorities are integers between -20 and 19 (inclusive)

• A lower numerical priority value indicates a higher scheduling priority

• UNIX provides IPC mechanisms, such as pipes, to allow data transfer

The operating system keeps track of all active processes using a dynamic data structure called the process control block. The PCB maintains the execution state and location in memory of each process.

Process Control Block (PCB)

• process state• process number• program counter• stack pointer • CPU registers• memory addresses• owner's username• list of open files• queue pointers• scheduling info• I/O status

Process State Queues

Context Switch

A context switch occurs when a running process is returned to the ready state (or is blocked) and another process is moved into the running state.

When the OS is ready to start executing a waiting process, it loads the hardware registers (PC, SP, and other location registers) from the values stored in that process' PCB.

While a process is running, the CPU modifies the Program Counter (PC), Stack Pointer (SP), and CPU registers.

When the OS stops a process, it saves the current values ose of the registers, (PC, SP, etc.) into the PCB for that process.

In time sharing systems, many processes are running concurrently, so context switching occurs many times each second (100's or even 1000's per second).

The time required to load the PCB with the current info for one process and to transfer the contents of the PCB into active memory for another determines the cost of context switching (overhead).

Example: Context Switching

One process can initiate another to do work. The creating process is called the parent process and the created process is called the child process.

A processs may share its resources with a child process. When a parent creates a child, it can either wait for the child to complete or it can operate concurrently with the child. In a Linux (Unix) OS subprocesses are created by a call the the system routine called fork.

Creating a Process

• variable values and the program counter are copied from parent to child

• the return value of fork in the parent process is the child process ID (the ID in the child process is set to 0)

• the parent can wait for the child to terminate by executing a wait system call

• the parent can continue to execute

• the child can start a new a different program (by making a call to exec)

• when you log into a machine running Linux (Unix) you create a shell process

• every command you tyope into the shell is a child of your shell proce3ss and is an implicit fork and exec pair

• if you type an & after the command, Linu will run the process concurrently with your shell, otherwise, your next shell commandc must wait until the first one completes

fork( ) forks a new child process that is a copy of the parentexeclp( ) replaces the program of the current process with the named programsleep( ) suspends execution for at least the specified timewaitpid( ) waits for the named process to finish execution gets( ) reads a line form a file

Example Linux (Unix) Program

Process Termination

When a process terminates the OS reclaims all the resources assigned to that processIn Linux (Unix)

• a process can terminate itself using the exit system call• a process can terminate a child using the kill system call

Interrupts

Interrupts enable software to respond to signals from hardware

May be initiated by a running processInterrupt is called a trapSynchronous with the operation of the processFor example, dividing by zero or referencing protected memory

May be initiated by some event that may or may not be related to the running processAsynchronous with the operation of the processFor example, a key is pressed on a keyboard or a mouse is moved

Low overhead

Polling is an alternative approach

Processor repeatedly requests the status of each device

Increases in overhead as the complexity of the system increases

2004 Deitel & Associates, Inc.

Handling interrupts

After receiving an interrupt, the processor completes execution of the current instruction, then pauses the current process

The processor will then execute one of the kernel’s interrupt-handling functions

The interrupt handler determines how the system should respond

Interrupt handlers are stored in an array of pointers called the interrupt vector

After the interrupt handler completes, the interrupted process is restored and executed or the next process is executed

2004 Deitel & Associates, Inc.

Common Interrupt TypesIntel CPU

2004 Deitel & Associates, Inc.

Common Exception TypesIntel CPU

2004 Deitel & Associates, Inc.

Interprocess Communication

Many operating systems provide mechanisms for interprocess communication (IPC)

Processes must communicate with one another in multiprogrammed and networked environments (for example, a Web browser retrieving data from a distant server)

Essential for processes that must coordinate activities to achieve a common goal

Message-based interprocess communication

Messages can be passed in one direction at a time (sender to receiver)

Message passing can be bidirectional (either process can be

sender/receiver)

Messages can be blocking or nonblocking

Popular implementation is a pipe ( i.e. a protected communication buffer)

2004 Deitel & Associates, Inc.

Summary

• A process is the unit of execution

• Processes are specified in Process Control Blocks

• Each process has a state

• There are, at most n processes running in parallel on an n-core computer

• The OS starts and stops process by context switching

• Interrupt Handling permits software to respond to messages from hardware

• Interprocess Communication permits message-passing between processes