1 Process Description and Control Chapter 3 All multiprogramming OS are built around the concept of processes. A process is also called a task or a job.

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Process Description and Control

Chapter 3

All multiprogramming OS are built around the concept of processes.

A process is also called a task or a job.

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Roadmap

How are processes represented and controlled by the OS?

Process states and state transitions which characterize the behaviour of processes.

Data structures used to manage processes. Ways in which the OS uses these data structures

to control process execution. Overview of process management in UNIX

SVR4.

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OS Requirements for Multiprogramming Process Management

Process management: fundamental OS task OS must:

interleave the execution of several processes to maximize CPU usage while providing reasonable response time

allocate resources to processes while protecting processes enabling synchronization of processes avoiding deadlock

support inter-process communications to share data and exchange information, and user creation of processes

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Concepts

Computer platforms consists of a collection of hardware resources

Computer applications are developed to perform some task

OS provides an interface for applications to use hardware resources

OS provides a representation of resources that can be requested and accessed by application

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The OS Manages Execution of Applications

Resources are made available to multiple applications

The processor or CPU is switched among multiple applications

The processor and I/O devices can be used efficiently

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What is a “process”?

A program in execution An instance of a program running on a

computer The entity that can be assigned to and

executed on a processor A unit of activity characterized by the

execution of a sequence of instructions, a current state, and an associated set of system instructions/data

More than just source program and data

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Process Image

A process is comprised of: Executable program (possibly shared) A set of data A number of attributes describing the context of

the process: process-specific context Process state Processor status Other resource data

Stacks

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Process Elements

While a process is running, it has a number of elements, including: Identifier State Priority Program counter Memory pointers Context data I/O status information Accounting information (?) ...

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Process Control Block (PCB)

Contains the process elements

Created and managed by the operating system

Allows support for multiple processes Process switching Interrupt handling for

process switching

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Dispatcher (short-term scheduler): Process Control and Scheduling

Is part of the OS that moves the processor from one process to another

It prevents a single process from monopolizing processor time

It decides which goes next and when according to a scheduling algorithm (to be discussed later)

The CPU will always execute instructions from the dispatcher while switching from process A to process B

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Quiz 3-1 True or False

A process consists of Source program Data/stacks Context/attributes

Give 2 attributes of a process What are stacks for? A PCB (Process Control Block) is allocated for

Each process Multiple processes that are running at the same time A few processes that shared the same program and

data

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Roadmap

How are processes represented and controlled by the OS.

Process states and state transitions which characterize the behaviour of processes.

Data structures used to manage processes. Ways in which the OS uses these data structures

to control process execution. Discuss process management in UNIX SVR4.

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Exercise

What are some of the states for a Student Registrar System?

What are some valid state transitions for a Student Registrar System?

What are the similarities and differences between a Student Registrar System and an OS?

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Two-State Process Model: A Simple Model

Process may be in one of two states Running Not-running

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Process Queue and Dispatcher

… processes moved by the dispatcher of the OS to the CPU then back to the queue until the task is competed

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Process Birth and Death

Creation Termination New batch job Normal CompletionInteractive Login Memory unavailableCreated by OS to provide a service

Protection error

Spawned by existing process

Operator or OS Intervention

See tables 3.1 and 3.2 for more

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Process Creation

The OS builds a data structure to manage the process

Traditionally, the OS creates all processes But it can be useful to let a running process to

create another one fork() in Unix/Linux (in Lab 2 and assignment 1)

This action is called process spawning Parent Process is the original, creating process Child Process is the new process

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Process Creation

Assign a unique process identifier Allocate space for the process image Initialize process control block

many default values (ex: state is New, inherited values from Parent, …)

Set up appropriate linkages Ex: add new process to linked list used for the

scheduling queue Create or expand other data structures

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Process Termination

There must be some way that a process can indicate completion.

This indication may be: A HALT instruction generating an interrupt alert to

the OS. A user action (e.g. log off, quitting an application) A fault or error Parent process terminating

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Process States

Let us expand to these states: The Running state

The process that gets executed The Not Running state

The Ready state• any process that is ready to be executed

The Blocked state• when a process cannot execute until some event occurs

(ex: the completion of an I/O)

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Other Useful States

The New state OS has performed the necessary initialization

actions to create the process has created a process identifier has created data structures or tables needed to

manage the process but has not yet committed to execute the

process (not yet admitted) because resources are not allocated or limited

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Other Useful States

The Exit state Termination moves the process to this state It is no longer eligible for execution Tables and other info are temporarily preserved

for auxiliary program Ex: accounting program that cumulates resource

usage for billing the users The process (and its tables) gets deleted

when the data is no more needed

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Process Transitions

Ready --> Running When it is time, the dispatcher (scheduler)

selects a new process to run Running --> Ready

the running process has expired his time slot the running process gets interrupted because a

higher priority process is in the ready state How about I/O operations?

Is the process still Ready? What is the new state?

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Process Transitions

Running --> Blocked When a process requests something for which

it must wait a service that the OS is not ready to perform an access to a resource not yet available initiates I/O and must wait for the result waiting for a process to provide input (IPC)

Blocked --> Ready When the event for which it was waiting occurs

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A Five-state Process Model

Ready to exit: A parent may terminate a child process

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Quiz 3

What are the states when a process is not in the Running state?

Describe the state transitions: Ready Running Running Ready Ready Blocked Running Blocked Blocked Running

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Suspended Processes – The Need for Swapping

Processor is faster than I/O, so all processes could be waiting for I/O Swap these processes to disk to free up more

memory and use processor on more processes Ready/Blocked state becomes suspend state

when swapped to disk Two new states

Blocked Suspend Ready Suspend

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New state transitions (mid-term scheduling)

Blocked --> Blocked Suspend When all processes are blocked, the OS will

make room to bring a ready process in memory Blocked Suspend --> Ready Suspend

When the event for which it as been waiting occurs (state info is available to OS)

Ready Suspend --> Ready when “less” ready processes in main memory

Ready--> Ready Suspend (unlikely) When there are no blocked processes and

must free memory for adequate performance

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A Seven-state Process Model

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Reason for Process Suspension

Reason Comment

Swapping The OS needs to release sufficient main memory to bring in a process that is ready to execute.

Other OS Reason OS suspects process of causing a problem.

Interactive User Request

e.g. debugging or in connection with the use of a resource.

Timing A process may be executed periodically (e.g., an accounting or system monitoring process) and may be suspended while waiting for the next time.

Parent Process Request

A parent process may wish to suspend execution of a descendent to examine or modify the suspended process, or to coordinate the activity of various descendants.

Table 3.3 Reasons for Process Suspension

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Quiz 3-2

Identify the 7 possible process states Describe the following state transitions

Blocked Blocked Suspend Blocked Suspend Ready Suspend Ready Suspend Running

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Roadmap

How are processes represented and controlled by the OS.

Process states and transitions which characterize the behaviour of processes.

Data structures used to manage processes. Ways in which the OS uses these data structures

to control process execution. Discuss process management in UNIX SVR4.

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Operating System Control Structures

For the OS is to manage processes and resources, it must have information about the current status of each process and resource.

Tables are constructed for each entity the operating system manages Memory tables (to be discussed later) I/O tables (to be discussed later) File tables (to be discussed later) Process tables: For OS to manage processes, OS

needs to know details: Current state Process ID Location in memory etc

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OS Control Tables

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Process Image

User program User data Stack(s)

for procedure calls and parameter passing Process Control Block (execution context)

Data needed (process attributes) by the OS to control the process. This includes:

Process identification information Processor state information Process control information

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Location of the Process Image Each process image is in virtual memory

may not occupy a contiguous range of addresses (depends on the memory management scheme used)

both a private and shared memory address spaces are used

The location if each process image is pointed to by an entry in the Primary Process Table

For the OS to manage the process, at least part of its image must be loaded into main memory

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Process Identification (in the PCB)

Each process is assigned a unique numeric identifier.

Many of the other tables controlled by the OS may use process identifiers to cross-reference process tables

A few other numeric identifiers are also used, e.g., User identifier

the user who is responsible for the job Identifier of the process that created this process

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Processor State Information (in PCB)

Contents of processor registers User-visible registers Control and status registers Stack pointers

Program status word (PSW) contains status information Example: the EFLAGS register on Pentium

machines

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Pentium II EFLAGS Register

Also see Table 3.6

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Process Control Information (in PCB) The most important data structure in an

OS. It defines the state of the OS: interprocess communication

may hold flags and signals for IPC process privileges

Ex: access to certain memory locations... memory management

pointers to segment/page tables assigned to this process

resource ownership and utilization resource in use: open files, I/O devices...

history of usage (of CPU time, I/O...)

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Process Control Information (in PCB)

Scheduling and state information Process state (i.e., running, ready, blocked...) Priority of the process Event for which the process is waiting (if

blocked) Data structure information

may hold pointers to other PCBs for process queues, parent-child relationships and other structures

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Queues as linked lists of PCBs

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Roadmap

How are processes represented and controlled by the OS.

Process states which characterize the behaviour of processes.

Data structures used to manage processes. Ways in which the OS uses these data structures

to control process execution. Discuss process management in UNIX SVR4.

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Modes of Execution To provide protection to PCBs (and other

OS data) most processors support at least 2 execution modes: Privileged mode (a.k.a. system mode, kernel

mode, supervisor mode, control mode ) manipulating control registers, primitive I/O

instructions, memory management... User mode

For this the CPU provides a (or a few) mode bit which may only be set by an interrupt or trap or OS call

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Switching Processes

Key for multiprogramming Several design issues are raised regarding

process switching What events trigger a process switch? We must distinguish between mode switching

and process switching. What must the OS do to the various data

structures under its control to achieve a process switch?

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When to Switch a Process ?

A process switch may occur whenever the OS has gained control of CPU, i.e., when: Supervisor Call

explicit request by the program (ex: file open). The process will probably be blocked

Trap An error resulted from the last instruction. It may

cause the process to be moved to the Exit state Interrupt

the cause is external to the execution of the current instruction. Control is transferred to IH

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When to Switch Processes?

Mechanism Cause Use

Interrupt External to the execution of the current instruction

Reaction to an asynchronousexternal event

Trap Associated with the execution of the current instruction

Handling of an error or anexception condition

Supervisor call Explicit request Call to an operating systemfunction

Table 3.8 Mechanisms for Interrupting the Execution of a Process

Summary

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Mode Switching It may happen that an interrupt does not

produce a process switch The control can just return to the interrupted

program Then only the processor state information

needs to be saved on stack This is called mode switching (user to kernel

mode when going into IH) What is the benefit?

Less overhead: no need to update the PCB like for process switching

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Steps in Process (Context) Switching Save context of processor including program

counter and other registers Update the PCB of the running process with its new

state and other associate info Move PCB to appropriate queue - ready, blocked Select another process for execution Update PCB of the selected process Update memory-management data structures Restore CPU context from that of the selected

process

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Quiz 3-4 Identify the causes for process switching Processing switching: Reorder the tasks:

Update PCB of the selected process Save context of processor including program counter

and other registers Move PCB to appropriate queue - ready, blocked Select another process for execution Update the PCB of the running process with its new

state and other associate info Restore CPU context from that of the selected process Update memory-management data structures

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Execution of the Operating System – Is the OS a Process?

Up to now, by process we were referring to “user process”

If the OS is just like any other collection of programs, is the OS a process?

If so, how it is controlled?

The answer depends on the OS design.

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Execution of the Operating System

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Non-process Kernel

The concept of process applies only to user programs

OS code is executed as a separate entity in privileged mode

OS code never gets executed within a process

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Execution within User Processes

Most OS code gets executed within the context of a user process

On Interrupts, Traps, System calls: the CPU switch to kernel mode to execute OS routine within the context of user process (mode switch)

Control passes to process switching functions (outside processes) only when needed

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Execution within User Processes

OS code and data are in the shared address space and are shared by all user processes

Separate kernel stack for calls/returns when the process is in kernel mode

Within a user process, both user and OS programs may execute (more than 1)

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Process-based Operating System

The OS is a collection of system processes Major kernel functions are separate processes Small amount of process switching functions is executed

outside of any process Design that easily makes use of multiprocessors

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Roadmap

How are processes represented and controlled by the OS.

Process states which characterize the behaviour of processes.

Data structures used to manage processes. Ways in which the OS uses these data structures

to control process execution. Discuss process management in UNIX SVR4.

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Unix SVR4 System V Release 4

Uses the model of fig3.15b where most of the OS executes in the user process

System Processes - Kernel mode only User Processes

User mode to execute user programs and utilities Kernel mode to execute instructions that belong to

the kernel.

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UNIX Process State Transition Diagram

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UNIX Process States

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A Unix Process

A process in UNIX is a set of data structures that provide the OS with all of the information necessary to manage and dispatch processes.

See Table 3.10 which organizes the elements into three parts: user-level context, register context, and system-level context.

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Process Creation

Process creation is by means of the kernel system call - fork( ).

This causes the OS, in Kernel Mode, to:1. Allocate a slot in the process table for the new

process.

2. Assign a unique process ID to the child process.

3. Copy of process image of the parent, with the exception of data and stacks.

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Process Creation (cont’d)

4. Increment the counters for any files owned by the parent, to reflect that an additional process now also owns those files.

5. Assign the child process to the Ready to Run state.

6. Returns the ID number of the child to the parent process, and a 0 value to the child process.

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After Creation

After creating the process the Kernel can do one of the following, as part of the dispatcher routine: Stay in the parent process. Transfer control to the child process Transfer control to another process.

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Review The key component for process representation and

control? PCB

Identify the elements that characterize the behaviour of processes.

States and state transitions Data structures used to manage processes.

Control structure and various tables Ways in which the OS uses these data structures to

control process execution. Process switching and different process models

Discuss process management in UNIX SVR4. Assignment 1