Chapter 4 Processes

1
Chapter 4 Processes
2
Contents

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

3
a brief history

• early computer systems allowed only one program
  to be executed at a time
• modern computer systems allow multiple programs
  to be loaded into memory and to be executed
  concurrently
• firmer control and more compartmentalization of
  programs in execution

4
why concurrent programs

• multiple user programs
• more productive
• more responsive (time sharing system, TSS)
• various system tasks outside the kernel
• for kernel safety, simplicity, ease of design,
  etc.

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4.1 Process Concept

• jobs in batch system
• user programs or tasks in a time-shared system
• system programs such as memory management,
  network I/O, file systems, etc.

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4.1.1 The Process

• a process is a program in execution
• a process contains
• text section, or code area
• program counter (PC)
• stack (user stack, kernel stack), contains method
  parameters, return addresses, local variables
• stack pointer (SP)
• data section
• registers

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process .vs. program

• compare program, file, source file, object file,
  executable file
• a program is a passive entity, such as a file on
  disk
• a process is an active entity, with a program
  counter (PC) specifying the next instruction to
  execute and a set of associated resources
• many processes may be associated with the same
  program
• one process may be associated with multiple files
  (executable, DLLs)
• one process may spawns may processes

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4.1.2 Process State

• as a process executes, it changes state
• each process may be in one of the following
  states
• new being created
• running instructions are being executed
• waiting waiting for some event to occur
• ready waiting to be assigned to a processor
• terminated has finished execution
• different OS may have different terminologies
  and/or a little bit different meanings

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Diagram of process state

• new being created
• running instructions are being executed
• waiting waiting for some event to occur
• ready waiting to be assigned to a processor
• terminated has finished execution

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4.1.3 Process Control Block

• each process is represented in the OS by a
  process control block (PCB), also called a task
  control block
• the PCB contains many pieces of information
  associated with the process
• the PCB resides in the kernel space

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Contents of PCB

• Process state
• Program counter
• CPU registers
• CPU scheduling information
• Memory-management information
• Accounting information
• I/O status information

12
CPU switches from process to process
processes switch overhead
scheduling dispatch
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4.1.4 Threads

• conventional process contains a single thread of
  execution
• modern OS support processes that contain multiple
  threads of execution in a single process
• eg. a word processor
• one thread accepts user key strokes,
• another does format and pagination,
• and still another runs spell checker,
• and even another runs background printing
• and

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4.2 Process Scheduling

• objective of multiprogramming
• have some process running at all times, so as to
  maximize CPU utilization
• objective of time-sharing
• switch CPU among processes so frequently that
  users can interact with his program
• process scheduling which process should be
  running?

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4.2.1 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
• generally stored as a linked list
• device queues set of processes waiting for a
  particular I/O device, eg. a tape driver, a disk
• each device has its own device queue

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Ready Queue and Various I/O Device Queues
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Representation of Process Scheduling

• process migrates among the various queues

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4.2.2 Schedulers

• long-term scheduler (or job scheduler) selects
  which processes should be loaded for execution
• short-term scheduler (or CPU scheduler) selects
  which process should be executed next and
  allocates CPU

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Schedulers

• 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
• long-term scheduler must make a careful selection
  of processes mix

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Medium-term scheduler

• swapping remove a process from memory (swap
  out) to reduce its active contention for the CPU
  and save memory, and reintroduce the process into
  memory to continue its execution (swap in) at a
  later time

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4.2.3 Context Switch

• the context of a process is represented in the
  PCB of the process
• when CPU switches to another process, the system
  must save the state of the old process into its
  PCB and load the saved state for the new process
• context-switch time is overhead, because the
  system does no useful work while switching
• context-switch time dependent on hardware support
• memory speed, number of registers, special
  instructions support, multiple register sets,
  memory management scheme, etc.

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4.3 Operations on Processes

• creation
• scheduling dispatch
• termination

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4.3.1 Process Creation

• parent process create children processes, which,
  in turn create other processes, forming a tree of
  processes

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parent and children processes

• Resource sharing
• CPU time, memory, files, I/O devices
• parent and children share all resources
• children share subset of parents resources
• parent should partition its resources among its
  children
• parent and child share no resources
• Execution
• parent and children execute concurrently
• parent waits until children terminate
• Address space
• child is a duplicate of the parent process (UNIX)
• child has a (new) program loaded into it (DEC
  VMS)
• Windows NT support both modes

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Unix examples

• UNIX examples
• fork system call creates new process
• exec system call used after a fork to replace the
  process memory space with a new program
• execl, execle, execlp
• execv, execve, execvp

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fork()???????(????)
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C program forking a process

• include ltstdio.hgt
• void main( )
• int pid
• pid fork( ) // fork another process
• if (pid lt 0) // parent process, an error
  occurred
• printf(fork failed\n)
• exit(-1)
• else if (pid gt 0) // parent process
• wait(NULL) // wait for the child to
  complete
• printf(child completed\n)
• exit(0)
• else // pid0, child process
• execlp(/bin/ls, ls, NULL)

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4.3.2 Process Termination

• process executes last statement and asks the OS
  to delete it (via exit, explicitly or
  inexplicitly)
• output data from child to parent (via wait)
• process resources are deallocated by operating
  system (memory, files, I/O buffers, etc.)
• a process (usually the parent) may terminate
  execution of another (a child) process, for
  possibly one of the reasons (abort)
• child has exceeded resources previous allocated
• the task assigned to the child is no longer
  required
• parent is exiting, some OS do not allow child to
  continue if its parent terminates, usually
  referred to as cascading termination

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process termination example

• In UNIX system
• a process can terminate itself by calling exit( )
• a parent process may wait for childs termination
  by using the wait( ) system call
• the wait( ) returns the child process identifier,
  so that the parent can tell which of its possibly
  many children has terminated
• if the parent terminates, all its children have
  assigned as their new parent the init process,
  and continue to run (rather than being terminated)

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4.4 Cooperating Processes

• concurrent processes executing in the OS may be
  either independent or cooperating processes
• independent process cannot affect or be affected
  by the execution of another process
• a process that does not share any data with any
  other processes
• cooperating process can affect or be affected by
  the execution of another process
• a process that shares data with other processes

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Advantages of process cooperation

• Information sharing
• a shared file to be accessed by several users who
  are interested in it
• Computation speed-up
• break a particular task into subtasks and
  executes them in parallel (multiple CPUs or I/O
  channels)
• Modularity
• construct the system in a modular fashion,
  dividing the system into separate processes or
  threads
• Convenience
• an individual user may have many tasks to work at
  one time, e.g. a user may be editing, printing,
  compiling in parallel

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Producer-Consumer Problem

• Paradigm for cooperating processes a producer
  process produces information that is consumed by
  a consumer process
• e.g. a print program produces characters that are
  consumed by the printer driver, a compiler
  produce assembly code which is consumed by an
  assembler
• a buffer of items that is filled by the producer
  and emptied by the consumer
• if the buffer is empty, the consumer must wait
  until an item is produced
• unbounded-buffer places no practical limit on the
  size of the buffer
• bounded-buffer assumes that there is a fixed
  buffer size gt the producer must wait if the
  buffer is full

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unbounded-buffer (unrealistic)
typical
in
out
occupied
in6 out2

free
0 1 2 3 4 5 6 7 8 9 10
out
empty
in
in6 out6
inout

0 1 2 3 4 5 6 7 8 9 10
consumer while (outin) // wait info
bufferout out out1 // consuming
info
producer // produce something
bufferin something in in1
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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
• the shared buffer is implemented as a circular
  array, with two pointers in and out
• buffer is empty when inout
• buffer is full when ((in1)BUFFER_SIZE)out
• solution is correct
• but can only use BUFFER_SIZE-1 elements

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bounded-buffer representation
typical
1
2
empty
inout
full
(in1)out
36
bounded-buffer the producer

• item nextProduced // local variable
• while (1)
• / produce an item in nextProduced /
• while (((in 1) BUFFER_SIZE) out)
• / buffer is full, do nothing /
• bufferin nextProduced
• in (in 1) BUFFER_SIZE

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bounded-buffer the consumer

• item nextConsumed // local variable
• while (1)
• while (in out) // buffer is empty
• / do nothing /
• nextConsumed bufferout
• out (out 1) BUFFER_SIZE
• / consume the item in nextConsumed /

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4.5 Interprocess Communication

• mechanism for processes to communicate and to
  synchronize their actions
• shared-memory environment, code written
  explicitly by the application programmer
• alternatively, the OS provide IPC facility for
  cooperating processes to communicate with each
  other
• IPC provides a way to allow processes to
  communicate and to synchronize their actions
  without sharing the same address space
• particular useful in a distributed environment

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4.5.1 Message-Passing System

• message system processes communicate with each
  other without resorting to shared
  variables/data/memory
• services are provided as ordinary user processes,
  i.e. the services operate outside of the kernel
• cf. microkernels, messages passed within the
  kernel
• two basic operations
• send(message), message length fixed or variable
• receive(message)

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communication link

• if P and Q wish to communicate, they need to
• establish a communication link between them
• exchange messages via send and receive
• implementation of communication link
• physical (e.g. shared memory, hardware bus,
  network)
• logical (logical properties)
• in Linux pipes, sockets, semaphore, shared
  memory, message queue

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4.5.2 Links implementation

• direct or indirect communication
• symmetric or asymmetric communication
• automatic or explicit buffering
• send by copy or send by reference
• fixed-size or variable-size message

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4.5.2.1 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 the communication link
• a link is established automatically
• a link is associated with exactly two
  communicating processes
• exactly one link exists between each pair
• the link may be unidirectional, but is usually
  bi-directional

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symmetric .vs. asymmetric

• symmetric communication
• send (P, message)
• receive(Q, message)
• asymmetric communication
• send(P, message)
• receive(id, message), receive a message from any
  process, where id is the name of the process with
  which communication has taken place
• disadvantage both schemes have the limited
  modularity, changing the name of a process may
  necessitate examining all other process
  definitions

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4.5.2.2 Indirect Communication

• messages are sent to and received from mailboxes,
  or ports
• each mailbox has a unique identification
• processes can communicate only if they share 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

• properties of the communication link
• a link is established between two processes only
  if both share a common mailbox
• a link may be associated with more than two
  processes
• each pair of processes may share several
  communication links
• link may be unidirectional or bi-directional

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Indirect Communication

• mailbox sharing
• P1 , P2 , and P3 share mailbox A
• P1 sends, while 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, and sender is notified who the receiver
  was

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ownership of the mailbox

• owned by the process who creates it
• resides in the owners address space
• the owner can only receive message from it
• the user can only send message to it
• mailbox disappears while the owner terminates
• owned by the OS
• OS must provide a mechanism that allows a process
  to
• create a new mailbox
• send and receive messages through that mailbox
• delete a mailbox
• ownership may be passed or shared by using system
  call

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4.5.3 Synchronization

• blocking (synchronous) versusnonblocking
  (asynchronous)
• blocking send the sending process is blocked
  until the message is received by the receiving
  process or by the mailbox
• nonblocking send the sending process sends the
  message, and resumes operation
• blocking receive the receiver blocks until a
  message is available
• nonblocking receive the receiver retrieves
  either a valid message or a null, and resumes
  operation
• different combinations of send and receive are
  possible

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4.5.4 Buffering

• direct or indirect, messages reside in a
  temporary queue
• three possible queue implementations
• zero capacity the queue has maximum length 0
  the sender must block until the recipient
  receives the message
• bounded capacity the queue has finite length n
  the sender blocks when the queue is full
• unbounded capacity the sender never blocks

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Case study Mach Win 2000

• 4.5.5 Mach
• 4.5.6 Windows 2000

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4.6 client-server communication

• a process on one machine needs to access data on
  another machine
• three ways
• Sockets
• Remote Procedure Calls (RPC)
• Remote Method Invocation (Java)

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4.6.1 Sockets

• a socket is defined as an endpoint for
  communication
• a socket is a concatenation of an IP address and
  a port number
• The socket 161.25.19.81625 refers to port 1625
  on host 161.25.19.8
• communication consists between a pair of sockets
• connections are all unique

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socket connection
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socket communication

• two kind of services
• TCP, connection-oriented service
• UDP, connectionless service
• socket communication is considered as a low-level
  form of communication
• support only an unstructured byte stream
• common and efficient, anyway

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4.6.2 Remote Procedure Calls

• remote procedure call (RPC) abstracts procedure
  calls between processes on networked systems
• stub 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 performs
  the procedure on the server

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Considerations in RPC

• external data representation (XDR)
• big-endian versus small-endian
• semantics of RPC
• carried out exactly once (no lost, no duplicate)
• servers port binding
• cf. standard library call, binding takes place
  during link, load, or execution time
• predetermined, fixed port address
• determined while compiling and linking
• rendezvous mechanism, a matchmaker daemon

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Execution of RPC matchmaker
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4.6.3 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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Marshalling Parameters
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4.7 Summary

• a process is a program in execution
• as a process executes, it changes state
• ready, running, waiting,
• a process is represented by its PCB
• process queues
• ready queue, I/O request queue,
• long-term (job) and short-term (CPU) scheduling

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Summary

• concurrent processes
• independent processes
• cooperating processes
• e.g. producer consumer
• processes communication
• shared memory
• message passing

62
Homework

• paper
• 1, 4, 6
• oral
• 5
• lab
• inter-process communication mechanism on Linux