Chapter 4: Processes

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Chapter 4 Processes

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

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Process Concept

• OS executes a variety of programs
• Batch system jobs
• Time-shared systems user programs or tasks
• interchangeable terms job and process
• Process a program in execution
• process execution must progress sequentially
• A process includes
• Text section (program code), data section (global
  variable)
• Current activities PC (Program Counter),
  registers
• Stack Temporary data (parameter, local
  variable)

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Process State

• 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 process.
• terminated The process has finished execution.

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Diagram of Process State
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Process Control Block (PCB)

• Information associated with each process.
• Process state
• Program counter
• CPU registers
• CPU scheduling information
• Memory-management information
• Accounting information
• I/O status information

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Process Control Block (PCB)
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CPU Switch From Process to Process
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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.
• Process migration between the various queues.

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Ready Queue And Various I/O Device Queues
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Representation of Process Scheduling
Short-term Scheduler
Long-term Scheduler
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Schedulers

• Long-term scheduler
• job scheduler
• selects which processes should be brought into
  the ready queue.
• Short-term scheduler
• CPU scheduler
• selects which process should be executed next and
  allocates CPU.

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Addition of Medium Term Scheduling
Medium-term Scheduler
Medium-term Scheduler
Suspend
Short-term Scheduler
Long-term Scheduler
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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 (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.

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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 of a process represented in PCB
• Context-switch time is overhead the system does
  no useful work while switching.
• Time dependent on hardware support.
• Multi-sets of registers

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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

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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

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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

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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 parents resources.
• Why? No overloading system
• Parent and child share no resources.
• Execution
• Parent and children execute concurrently.
• Parent waits until children terminate.

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Process Creation (Cont.)

• Address space
• Child duplicate of parent.
• Child has a program loaded into it.
• UNIX examples
• fork system call creates new process
• Both continue, return values differ
• exec system call used after a fork to replace the
  process memory space with a new program.
• Fig. 4.8

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Fork and Execlp
fork
Parent
Child
Parent Process Image
execlp
Parent
Child
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Processes Tree on a UNIX System
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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

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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

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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

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Process Termination

• Process executes last statement and asks the
  operating system to delete it (exit).
• Output data from child to parent (via wait,
  return pid).
• Process resources are deallocated 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.
• Operating system does not allow child to continue
  if its parent terminates.
• Cascading termination.

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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
• Given multiple processing elements
• Modularity
• Convenience

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Producer-Consumer Problem

• Paradigm for cooperating processes, producer
  process produces information that is consumed by
  a consumer process.
• (Print prg, prn), (compiler, assembler),
• unbounded-buffer places no practical limit on the
  size of the buffer.
• bounded-buffer assumes that there is a fixed
  buffer size.

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Bounded-Buffer Shared-Memory Solution

• Shared data
• define BUFFER_SIZE 10
• Typedef struct
• . . .
• item
• item bufferBUFFER_SIZE
• int in 0
• int out 0
• Solution is correct, but can only use
  BUFFER_SIZE-1 elements

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Bounded-Buffer Producer Process

• item nextProduced
• while (1)
• while (((in 1) BUFFER_SIZE) out)
• / do nothing /
• bufferin nextProduced
• in (in 1) BUFFER_SIZE

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Bounded-Buffer Consumer Process

• item nextConsumed
• while (1)
• while (in out)
• / do nothing /
• nextConsumed bufferout
• out (out 1) BUFFER_SIZE

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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
• Implementation of communication link
• physical (e.g., shared memory, hardware bus)
• logical (e.g., logical properties)

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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?

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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

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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.
• Asymmetric send (p, msg), receive(id, msg)
• Limited modularity (change name!)

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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.

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Indirect Communication

• 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

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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.

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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

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Synchronization

• Message passing may be either blocking or
  non-blocking.
• Blocking is considered synchronous
• Non-blocking is considered asynchronous
• send and receive primitives may be either
  blocking or non-blocking.
• Rendezvous blocking send/receive

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Buffering

• Queue of messages attached to the link
  implemented in one of three ways.
• 1. Zero capacity 0 messagesSender 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.

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Client-Server Communication

• Sockets
• Remote Procedure Calls
• Remote Method Invocation (Java)

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Sockets

• A socket is defined as an endpoint for
  communication.
• Concatenation of IP address and port
• The socket 161.25.19.81625 refers to port 1625
  on host 161.25.19.8
• Port lt 1024std service
• 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

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Socket Communication
pp. 118, 119, 120
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Port
After connection is established, noneed to
mention ports in messages
Socket
Datagram
need to mention ports in every message
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Socket Primitives
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Connectionless Socket Communication
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Connection-Oriented Client/Server Socket
Communication (I)
Reply
Request
Rendezvous

• Connection-oriented communication pattern using
  sockets.

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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
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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)

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Remote Procedure Calls

• Remote procedure call (RPC) abstracts procedure
  calls between processes on networked systems.
• Stubs client-side proxy for the actual
  procedure on the server.
• The client-side stub locates the server and
  marshalls the parameters.
• The server-side stub receives this message,
  unpacks the marshalled parameters, and peforms
  the procedure on the server.

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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)
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Parameter Passing and Data Conversion

• Parameter marshalling 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)
• 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

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Parameter Passing

• Steps involved in doing remote computation
  through RPC

2-8
Server may support manyprocedures
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Binding a Client to a Server
Matchmaker
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Execution of RPC
XDR
p. 123
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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

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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

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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.

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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

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Marshalling Parameters
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0
Logically contiguous memory space (Virtual
memory)
12345
But physical memory occupied by a process may
not be contiguous (Chap 9)
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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

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

• Program ? Process

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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

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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)

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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

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Process Scheduling 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