1 Lecture 3: Processes Operating System Fall 2006

1

Lecture 3: Processes

Operating SystemFall 2006

2

Major Requirements of anOperating System Interleave the execution of several

processes to maximize processor utilization while providing reasonable response time

Allocate resources to processes in conformance with a specific policy while at the same time avoiding deadlock

Support interprocess communication and user creation of processes, both of which may aid in the structuring of applications

3

Contents Process Definition Process States Process Scheduling Process Description Process Control and Operations Interprocess Communication

4


5

Processes - Definition Also called a job Execution of an individual program Process components:

An executable program – text section The associated data needed by the program

Stack – temporary data (e.g. function parameters, return addresses, and local variables)

Data section – global variables Heap – memory which is dynamically allocated during

process run time The execution context of the program

All information the operating system needs to manage the process

Including the value of program counter and the contents of the processor’s registers

6

Process in memory

7

Process Trace Processes can be traced

For a program to be executed, a process is created for that program.

We can characterize the behavior of an individual process by listing the sequence of instructions that execute for that process.

Such a listing is called a trace of the process.

We can characterizing behavior of the processor by showing how the traces of the various processes are interleaved.

8

Example for processes tracing

9Process AOSProcess BOSProcess COSProcess AOSProcess C

time

10

Example for processes tracing (cont.)

11


12

Process State - Two-State Process Model

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

13

Not-Running Process in a Queue

14

Process State

Not-running ready to execute

Waiting (also called blocked) waiting for I/O

Dispatcher cannot just select the process that has been the longest in the queue because it may be blocked

15

Process State - A Five-State Model New: The process is being created Running: Instructions are being

executed Waiting (blocked): 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

16

Process State - A Five-State Model

17

Queueing-Diagram Representation of Five-State Model

18

Queueing-Diagram Representation of Five-State Model

19

Suspended Process – The Need for Swapping

The three principal states just described (Ready, Running, Waiting/Blocked) provide a systematic way of modeling the behavior of processes and guide the implementation of the OS.

However, there is good justification for adding other states to the model – the need for swapping

20

Suspended Process – 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 Waiting(Blocked) state becomes

suspend state when swapped to disk Two new states

Waiting(Blocked), suspend Ready, suspend

21

One Suspend State

22

Two Suspend States

23

Reasons for Process Suspension

24

Contents Process Definition Process Trace Process States Process Scheduling Process Description Process Control and Operations Interprocess Communication

25

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

Processes migrate among the various queues

26

Representation of Process Scheduling (Five-State Model)

27

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

Medium-term scheduler – corresponds to suspended state (swapping out of the memory)

28

Addition of Medium Term Scheduling

29

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

30

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

31


32

What information does the OS need to control processes and manage resources for them?

33

Memory Tables Used to keep track of both main(real) and

secondary memory. Must include the following information:

Allocation of main memory to processes. Allocation of secondary memory to processes. Protection attributes of blocks of main or virtual

memory, such as which processes may access certain shared memory regions.

Information needed to manage virtual memory.

34

I/O Tables

Used by the OS to manage the I/O devices.

May include the following information: I/O device is available or assigned Status of I/O operation Location in main memory being used as

the source or destination of the I/O transfer

35

File Tables

Provide information about the existence of files: Existence of files Location on secondary memory Current Status Attributes Sometimes this information is

maintained by a file-management system

36

Process Tables Used to manage processes Include the following information:

Where process is located Depend on the memory management scheme

being used. In the simplest case, the process image is

maintained as a contiguous block of memory. This block is maintained in secondary memory, usually disk.

Attributes necessary for its management Process ID Process state Location in memory

37

Typically Elements of a Process Image

User Data The modifiable part of the user space. May include program data, a user stack area, and

programs that may be modified. User Program

The program to be executed System Stack

Each process has one or more system stacks associated with it.

A stack is used to store parameters and calling addresses for procedure and system calls.

Process Control Block Data needed by the OS to control the process.

38

Process Control Block

Process Identification Processor State Information Process Control Information

39



40

Process identification

Numeric identifiers that may be stored with the process control block include Identifier of this process Identifier of the process that created

this process (parent process) User identifier

41



42

Processor State Information

User-Visible Registers Control and Status Registers

PC PSW

Stack Pointers Each process has one or more system

stacks associated with it.

43



44

Process Control Information

Scheduling and State Information: Process State (e.g. running, ready,

waiting, etc.) Priority Scheduling-related information

Depend on the scheduling algorithm used. e.g. amount of time it has already run,

how long it has waited Event

Identity of event the process is awaiting

45

Process Control Information (cont.)

Data Structuring A process may be linked to other processes,

e.g to its parent Interprocess Communication

Various flags, signals, and messages may be associated with communication between two independent processes.

Process Privilege Processes are granted privileges in terms of

the memory that may be accessed and the types of instructions that may be executed.

46

Process Control Information (cont.)

Memory Management Including pointers to segment and/or page

tables that describe the virtual memory assigned to this process.

Resource Ownership and Utilization Resources controlled by the process may be

indicated, such as opened files. A history of utilization of the processor or

other resources may also be included; this information may be needed by the scheduler.

47

48


49

Process Creation When a new process is to be added to

those currently being managed, the OS builds the date structures that are used to manage the process and allocates address space in main memory to the process. These actions constitute the creation of a new process.

50

Reasons for Process Creation Submission of a new batch job

The OS is provided with a batch job control stream usually a tape or disk

Interactive logon A user at a terminal logs on to the system

Created by OS to provide a service such as printing

The OS can create a process to perform a function on behalf of a user program

Spawned by existing process For purposes of modularity or to exploit parallalism, a

user program can dictate the creation of a number of processes.

When one process spawns another process, the former is called the parent and the spawned process as the child.

51

Procedure for Process Creation Once the OS decides to create a new

process, it can proceed as follows:(a) Assign a unique process identifier to the

new process(b) Allocate space for the process.(c) Initialize the process control block(d) Set the appropriate linkage

e.g. if the OS maintains each scheduling queue as a linked list, then the new process must be put in the ready or ready/suspend list

(e) Create or expand other data structures e.g. the OS may maintain an accounting file on

each process to be used subsequently for billing and/or performance assessment purpose.

52

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

Address space Child duplicate of parent Child has a program loaded into it

53

Process Creation Example for UNIXUNIX examples:

fork system call creates new process exec system call used after a fork to replace the process’ memory space with a new program

54

Process Creation Example for UNIX -C Program Forking Separate Process

int main(){Pid_t pid;

/* fork another process */pid = fork();if (pid < 0) { /* error occurred */

fprintf(stderr, "Fork Failed");exit(-1);

}else if (pid == 0) { /* child process */

execlp("/bin/ls", "ls", NULL);}else { /* parent process */

/* parent will wait for the child to complete */wait (NULL);printf ("Child Complete");exit(0);

}}

55

A tree of processes on a typical Solaris

56

Process Termination A batch job should include a Halt

instruction or an explicit OS service call for termination.

User logs off For an interactive application, there

are commands to terminate a process. Control C will terminate a process.

Error and fault conditions

57

Reasons for Process Termination Normal completion

The process executes an OS service call to indicate that it has completed running.

Time limit exceeded The process has run longer than the specified total

time limit. Memory unavailable

The process requires more memory than the system can provide.

Bounds violation The process tries to access a memory location that it

is not allowed to access.

58

Reasons for Process Termination (cont.) Protection error

The process attempts to use a resource or a file that it is not allowed to use, or tries to use it in an improper fashion.

Example: write to read-only file Arithmetic error

The process tries a prohibited computation, such as division by zero, or arithmetic overflow.

Time overrun process waited longer than a specified maximum for an

event I/O failure

An error occurs during input or output. Privileged instruction

The process attempts to use an instruction reserved for the OS

59

Reasons for Process Termination (cont.) Invalid instruction

The process attempts to execute a nonexistent instruction (often as a result of branching into a data area and attempting to execute the data)

Data misuse Operating system intervention

such as when deadlock occurs Parent terminates so child processes terminate

When a parent process terminates, the OS may automatically terminate all of the child processes.

Parent request A parent process may terminate any of its child

process.

60

Process Switching

A running process is interrupted and the OS assigns another process to the Running state and turns control over to that process

61

When to Switch a Process Interrupt

Clock interrupt process has executed for the maximum allowable time

slice I/O interrupt Memory fault

memory address is in virtual memory so it must be brought into main memory

Trap error occurred may cause process to be moved to Exit state

Supervisor call such as file open

62

Comparison between interrupt, trap and supervisor call

Mechanism Cause Use

Interrupt External to the execution of the current instruction

Reaction to an asynchronous external event

Trap Associated with the execution of the current instruction

Handling of an error or an exception condition

Supervisor call

Explicit request Call to an OS function

63

Change of Process State Save context of processor including program

counter and other registers Update the process control block of the

process that is currently in the running state Change the state of the process to one of the other

states (Ready, waiting, or Exit, etc.) Other relevant fields must also be updated, including

the reason for leaving the Running state and accounting information

Move process control block to appropriate queue - Ready, Waiting, etc.

64

Change of Process State (cont.)

Select another process for execution Update the process control block of the

process selected Change the state of this process to Running

Update memory-management data structures This may be required, depending on how address

translation is done Restore context of the selected process

65


66

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 process Advantages of process cooperation

Information sharing Computation speed-up Modularity Convenience

67

Two IPC Mechanisms Shared mamory

A region of memory that is shared by cooperating processes is established.

Processes can then exchange information by reading and writing date to the shared region.

Message passing Communication takes place by means of

messages exchanged between the cooperating processes.

68

Communications Models

69

Example for Shared mamory: 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

70

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

71

Bounded-Buffer – Insert() Method

while (true) { /* Produce an item */

while (((in = (in + 1) % BUFFER SIZE count) == out) ; /* do nothing -- no free buffers */ buffer[in] = item; in = (in + 1) % BUFFER SIZE;

}

72

Bounded Buffer – Remove() Method

while (true) { while (in == out) ; // do nothing -- nothing to consume

// remove an item from the buffer item = buffer[out]; out = (out + 1) % BUFFER SIZE;return item;

}

73

Message-Passing Mechanism for processes to communicate and

to synchronize their actions Message system – processes communicate with

each other without resorting to shared variables providing 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)

74

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

75

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

76

Indirect Communication (cont.)

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

77

Indirect Communication (cont.)

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.

78

Synchronization Message passing may be either blocking or

non-blocking Blocking is considered synchronous

Blocking send has the sender block until the message is received

Blocking receive has the receiver block until a message is available

Non-blocking is considered asynchronous Non-blocking send has the sender send the

message and continue Non-blocking receive has the receiver receive a

valid message or null

79

Buffering

Queue of messages attached to the link; implemented in one of three ways1. 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

80

End of lecture 3

Thank you!

Documents

1 Lecture 3: Processes Operating System Fall 2006