Chapter 4 Processes

1
Chapter 4 Processes
2
Outline

• Process Concept
• Process Scheduling
• Operation on Processes
• Cooperating Processes
• Interprocess Communication
• Communication in Client-Server Systems

3
Process Concept
4
Overview

• OS executes a variety of programs
• Batch system jobs
• Time-shared systems user programs or tasks
• The terms job and process almost interchangeably
• Process a program in execution
• A process includes (process image)
• Text section (program code), data section (global
  variable)
• Current activities PC (Program Counter),
  registers
• Stack Temporary data (parameter, local
  variable)
• Process vs. Program active vs. passive entities

5
0
Logically contiguous memory space (Virtual
memory)
12345
But physical memory occupied by a process may
not be contiguous (Chap 9)
6
Steps for loading a process in memory

• Linker combines object modules into a single
  executable binary file (load module)
• The loader places the load module in physical
  memory

7
Loader
Process Control Block
Program Code
Program Code
Data
Data
Stack
Executable binary file (Load Module)
Process Image in Main Memory

• Program ? Process

8
OS Requirements for Processes

• OS must interleave the execution of several
  processes to maximize CPU usage while providing
  reasonable response time
• OS must allocate resources to processes (memory,
  I/O device, etc.) while avoiding deadlock
• OS must support inter-process communication,
  synchronization, and user creation of processes

9
Process State

• As a process executes, it changes state
• New The process is being created (put in job
  queue)
• Ready The process is waiting to be assigned to
  a processor (put in ready queue)
• 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 (put in waiting queues)
• Terminated The process has finished execution

10
Diagram of Process State

• Only one process can be running on any processor
  at any instant
• Many processes may be ready and waiting

11
Process State (Cont.)

• The New state
• OS has performed the necessary actions to create
  the process
• Has created a process identifier
• Has created tables needed to manage the process
• Memory table, file table, PCB
• But has not yet committed to execute the process
  (not yet admitted)
• Because resources are limited
• Process image may be put in secondary storage
  (like swapping space)

12
Process State (Cont.)

• The Terminated state
• Exit 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

13
Operating System Control Structures

• An OS maintains the following tables for managing
  processes and resources
• Memory tables
• I/O tables (DST)
• File tables
• Process tables (this chapter)

14
If Primary Process Table is allocated in advance
(like booting), there is a upper-bound for the
of processes that can exist in a system
15
PCB may not be adjacent to code, data, and stack
16
Location of the Process Image

• Each process image is in virtual memory
• May not occupy a contiguous range of physical
  addresses (relies on memory management scheme)
• But contiguous in the processs own virtual
  (logical) memory
• Both a private and shared memory address space is
  used
• The location of 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 bring into main memory

17
Process Control Block (PCB)

• Information associated with each process.
• Process identifier, user identifier, parent
  process identifier
• Process state new, ready, running, waiting
• Program counter address of the next instruction
• CPU registers accumulators, index registers
• CPU scheduling information priority
• Memory-management information base/limit
  register, page tables
• Accounting information CPU time used, time
  limits
• I/O status information list of I/O device
  allocated, list of open files
• Pointer to other PCBs

18
Process Control Block (PCB)
19
(No Transcript)
20
(No Transcript)
21
Thread

• The process model implies that a process is a
  program that performs a single thread of
  execution
• In a single-thread word-processor program, the
  user cannot simultaneously type in characters and
  run the spell checker within the same process
• If a process has multiple threads of execution
• It can perform more than one task at a time
• Chapter 5

22
Process Scheduling
23
Goal

• Multiprogramming -- have some process running at
  all times
• Maximize CPU utilization
• Time Sharing -- Let users interact with each
  program while it is running
• Minimize response time
• For a uni-processor system
• There will never be more than one running process
• The reset process will have to wait until the CPU
  is free and can be rescheduled

24
Process Scheduling Queues

• Job queue (New) set of all newly created
  processes in the system
• Ready queue (Ready) set of all processes
  residing in main memory, ready and waiting to
  execute
• Device queues (Waiting) set of processes
  waiting for an I/O device
• Process migration between the various queues
• Process Scheduling queues are implement by linked
  lists
• Link by pointers in PCBs

25
Ready Queue And Various I/O Device Queues
26
Representation of Process Scheduling
Short-term Scheduler
Job queue
Long-term Scheduler
27
Schedulers

• Long-term scheduler (or job scheduler) selects
  which processes should be brought into the ready
  queue (New to Ready)
• Short-term scheduler (or CPU scheduler) selects
  which process should be executed next and
  allocates CPU (Ready to Running)
• Medium-term scheduler removes processes from
  memory at some later time, the process can be
  reintroduced into memory and its execution can be
  continued where it left off

28
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
• Long-term scheduler controls multiprogramming
  degree
• Select a good mix of I/O- and CPU- bound
  processes
• 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.

29
CPU Switch From Process to Process (Context
Switch)
state current value of hardware PC,registers,
base/limitregisters (one CPU only has one or
fewsuch things)
Context Switch
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
• The context of a process is represented in the
  PCB of a process
• Context-switch time is overhead the system does
  no useful work while switching
• Time dependent on hardware support, memory
  management method
• Eg. Multiple sets of registers

31
Context Switch (Cont.)

• Steps for Context Switch
• Save context of processor including program
  counter and other registers
• Update the PCB of the running process with its
  new state and other associate information
• Move PCB to appropriate queue - ready, waiting
• Select another process for execution (short-term
  scheduler)
• Update PCB of the selected process
• Restore CPU context from that of the selected
  process

32
When to Switch a Process ?

• A process switch may occur whenever the OS has
  gained control of CPU
• System 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

33
Examples of Interrupts

• Clock process has expired his time slice and is
  transferred to the ready state
• I/O
• First move the processes that where waiting for
  this event to the ready state
• Then resume the running process or choose a
  process of higher priority
• Memory fault (Chapter 10)
• Memory address is in virtual memory so it must
  bring corresponding block into main memory
• Thus move this process to a blocked state
  (waiting for the I/O to complete)

34
Addition of Medium Term Scheduling
Medium-term Scheduler
Suspend
Medium-term Scheduler
Short-term Scheduler
Job queue
Long-term Scheduler
35
The need for Swapping

• So far, all the processes had to be (at least
  partly) in main memory
• Even with virtual memory, keeping too many
  processes in main memory will deteriorate the
  systems performance
• The worst case all the processes in main memory
  are blocked and CPU is idle
• The OS may need to suspend some processes, i.e.
  to swap them out to disk.
• Chapter 9

36
Operations on Processes
37
Process Creation

• When does a process get created?
• Submission of a batch job
• User logs on command interpreter
• Created by OS to provide a service to a user
  (ex printing a file)
• Spawned by an existing process
• a user program can create a number of processes

38
Process Creation (Cont.)

• Steps for process creation
• Assign a unique process identifier
• Allocate space for the process image
• Initialize process control block
• many default values (ex state is New, no I/O
  devices or files...)
• Set up appropriate linkages
• Ex add new process to linked list used for the
  scheduling queue

39
Process Creation -- Parent/Child Process

• Parent process creates 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 parents resources
  (partition)
• Parent and child share no resources
• Execution
• Parent and children execute concurrently
• Parent waits until children terminate

40
A Tree of Processes On A Typical UNIX System
41
Process Creation -- Parent/Child Process (Cont.)

• Address space
• Child duplicate of parent
• Child has a program loaded into it
• UNIX examples
• fork system call creates new process
• execlp system call used after a fork to replace
  the process memory space with a new program
• Other OS
• DEC VMS create a new process, loads a specified
  program into that process, and starts it running
• Windows NT both models

42
Fork and Execlp
fork
Parent
Child
Parent Process Image
execlp
Parent
Child
43
C Program Forking A Separate Process
44
When does a process gets terminated?

• Batch job issues Halt instruction
• User logs off
• Process executes a service request to terminate
• Error and fault conditions

45
Reasons for Process Termination

• Normal completion
• Time limit exceeded
• Memory unavailable
• Memory bounds violation
• Protection error
• example write to read-only file
• Arithmetic error
• Time overrun
• process waited longer than a specified maximum
  for an event

46
Reasons for Process Termination (Cont.)

• I/O failure
• Invalid instruction
• happens when try to execute data
• Privileged instruction
• Operating system intervention
• such as when deadlock occurs
• Parent request to terminate one offspring
• Parent terminates so child processes terminate

47
Process Termination

• Process executes last statement and asks OS to
  decide it (exit)
• Output data from child to parent (via wait)
• Process resources are de-allocated 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
• OS does not allow child to continue if its parent
  terminates
• Cascading termination

48
Cooperation Processes
49
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
• Cooperation processes require communication and
  synchronization

50
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.
• The producer and consumer must be synchronized
• Consumer does not try to consume an item that has
  not yet been produced
• Buffer implementation
• Explicitly coded by AP with the use of shared
  memory
• Interprocess communication facility (IPC)

51
Shared Bounded-Buffer Example

• Shared memory
• Shared buffer -- circular array
• in next free position
• out first full position
• BUFFER_SIZE max of items (can only use
  BUFFER_SIZE 1 items)
• Require that producers and consumers share a
  common buffer pool, and that the code for
  implementing the buffer be written explicitly by
  the application programmer

52
Shared Bounded-Buffer Example (Cont.)
Producer
Consumer
53
Interprocess Communication
54
Interprocess Communication (IPC)

• Mechanism for processes to communicate and to
  synchronize their actions (provided by OS)
• 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,
  network)
• Logical (e.g., logical properties)

55
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?

56
Logical Implementation

• Several methods for logically implementing a link
  and the send/receive operations
• Direct or indirect communication
• Symmetric or asymmetric communication
  (addressing)
• Automatic or explicit buffering
• Send by copy or send by reference
• Fixed-size or variable-sized messages

57
Direct Communication

• Processes must name each other explicitly
  symmetry
• send (P, message) send a message to process P
• receive (Q, message) receive a message from
  process Q
• Asymmetry in addressing
• send (P, message) send a message to process P
• receive (id, message) id is set to the name of
  the process with which communication has taken
  place
• 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
• Disadvantage when the name of a process changes

58
Indirect Communication

• Messages are sent to and received from mailboxes
  (also referred to as ports)
• Each mailbox has a unique id
• Processes can communicate only if they share a
  mailbox
• send(A, message), receive(A, message)
• 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

59
Indirect Communication (Cont.)

• Mailbox sharing
• P1, P2, and P3 share mailbox A.
• P1, sends P2 and P3 receive.
• Who gets the message?
• Solutions depend on the following issues
• 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

60
Indirect Communication (Cont.)

• A mailbox may be owned either by a process or by
  OS
• Owned by a process
• Owner (receiver) vs. user (sender)
• Owned by OS
• Support the following operations
• Create a new mailbox
• Send and receive messages through mailbox
• Destroy a mailbox
• Creator is the default owner
• Ownership and receiving privilege may be passed
  to other processes through appropriate system
  calls

61
Synchronization

• Blocking or non-blocking synchronous or
  asynchronous
• Blocking send sender blocked until the message
  is received by the receiver or by the mailbox
• Non-blocking send sender sends the message and
  resumes operation
• Blocking receive receiver blocks until a message
  is available
• Non-blocking receive receiver retrieves either a
  valid message or a null

Rendezvous both the send and receive are blocking
62
Buffering

• Queue of messages attached to the link
  implemented in one of three ways.
• Zero capacity 0 messages
• Sender must wait for receiver
• Bounded capacity finite length of n messages
• Sender must wait if link full
• Unbounded capacity infinite length
• Sender never waits

63
Client-Server Communication

• Especially for client-server distributed systems
• Sockets
• Remote Procedure Calls (RPC)
• Remote Method Invocation (RMI)

64
Socket

• Socket communication endpoint of a
  communication link
• Model network I/O based on conventional file I/O
• Concatenation of IP address and port
• The socket 161.25.19.81625 refers to port 1625
  on host 161.25.19.8
• Communication consists between a pair of sockets.
• Low-level form of communication
• Allow only an unstructured stream of bytes to be
  exchanged
• Responsibility of Client/Server to impose a
  structure on the data
• Berkeley socket
• WinSock standard group for Windows applications

65
(No Transcript)
66
Port
After connection is established, noneed to
mention ports in messages
Socket
Datagram
need to mention ports in every message
67
Socket Primitives
68
Connectionless Socket Communication
69
Connection-Oriented Client/Server Socket
Communication (I)
Reply
Request
Rendezvous

• Connection-oriented communication pattern using
  sockets.

70
Connection-Oriented Client/Server Socket
Communication (II)
The original port is used foraccepting
connection requestsfrom other clients
Use the new port to communicatewith each
connected client
71
Establishing a Simple Server (Stream Socket) in
Java

• Step1 Create a ServerSocket Object
• ServerSocket s new ServerSocket(port,
  queueLength)
• Step2 Create a Socket and Wait for a Connection
• Socket connect s.accept()
• Step3 Associate Input and Output Stream with the
  Socket
• connect.getInputStream
• connect.getOutputStream
• Step4 Process Connection
• Step5 Close Connection

Figure 4.10 Time-of-day server
72
Establishing a Simple Client (Stream Socket) in
Java

• Step1 Create a Socket to make connection
• Socket connect new Socket(ServerName, port)
• Step2 Associate Input and Output Stream with the
  Socket
• connect.getInputStream
• connect.getOutputStream
• Step3 Process Connection
• Step4 Close Connection

Figure 4.11 Time-of-day client
73
Establishing Datagrams in Java

• Set up a DatagramSocket
• Send socket new DatagramSocket()
• Receive socket new DatagramSocket(port)
• Set up a Packet
• Send s new DatagramPacket(.Packet Info..)
• Receive r new DatagramPacket(data, length)
• Send/Receive
• socket.receive(packet)
• socket.send(packet)

74
RPC Operations

• Remote procedure call (RPC) abstracts procedure
  calls between processes on networked systems.
• Principle of RPC between client and server (next
  slide)
• Flow of RPC
• Implementation Issues
• Parameter and result passing, and data conversion
• Binding
• Compilation (self study)
• Exception and failures handling (self study)
• Security (self study)
• Further Study of RPC Chapter 4 of Advanced OS
  Course

75
Principle of RPC Between a Client and Server
Program
Information can be transported from caller to
callee in parameters and can come back in the
procedure result
Client suspends
Receive(blocked)
Receive(blocked)
76
Flow of RPCs
77
Steps of a RPC

• Client procedure calls client stub in normal way
• Client stub builds message, calls local OS
• Client's OS sends message to remote OS
• Remote OS gives message to server stub
• Server stub unpacks parameters, calls server
• Server does work, returns result to the stub
• Server stub packs it in message, calls local OS
• Server's OS sends message to client's OS
• Client's OS gives message to client stub
• Stub unpacks result, returns to client

78
Client and Server Stubs

• Client stub contains the actual procedures that
  the client program will call
• Collect and pack parameters into outgoing message
  and then call the runtime system to send it
• Unpack the reply and return values to the client
• Server stub contains the procedures called by
  the runtime system on the server machine when an
  incoming messages arrives, and then call the
  actual server procedures that do the work

79
Parameter Passing and Data Conversion

• Parameter marshaling rules for RPC parameter
  passing and data/message conversion
• Passing reference parameters call-by-copy/restor
  e
• Ex. Array Call-by-value (entry)
  call-by-reference (exit)
• How about complex data structure like tree or
  graph?
• Data representation and type checking
• Different types of machines ? different data
  representation
• ASCII, EBCDIC
• 32-bit 2s complement or 16-bit
  sign-and-magnitude for integer
• External data representation (XDR)
  machine-independent data representation
• Transfer syntax ? rules regarding transfer of
  messages
• Message format, data representation in messages

80
Parameter Passing

• Steps involved in doing remote computation
  through RPC

2-8
Server may support manyprocedures
81
Binding a Client to a Server
Matchmaker
82
Execution of RPC
83
Exception Handling

• Exception in server procedures
• Overflow/underflow, protection violation
• Fundamental questions
• How does the server report status information to
  the client?
• How does a client send control information (e.g.
  stop) to the server?
• Mechanisms
• Put flag for exception handling in the send
  primitives
• Use a separate channel (socket connection or
  port) for exchanging exception-handling messages

84
Failure Handling

• Five classes of RPC failures
• The client is unable to locate the server
• The request message from the client to the server
  is lost
• The server crashes after receiving a request
• The reply message from the server to the client
  is lost
• The client crashes after sending a request

85
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.

86
Remote Method Invocation (Cont.)

• Difference between RPC and RMI
• RPC support procedural programming
• Only remote procedures or functions may be called
• RMI is object-based
• RMI supports invocation of methods on remote
  objects
• The parameters to remote procedures are ordinary
  data structures in RPC
• It is possible to pass objects as parameters to
  remote methods in RMI

87
Marshalling Parameters in RMI