Document Sample
IPC Powered By Docstoc
					   Core Inter-Process
Communication Mechanisms
      (Historically Important)

                           Fred Kuhns
          (fredk@arl.wustl.edu, http://www.arl.wustl.edu/~fredk)

  Department of Computer Science and Engineering
        Washington University in St. Louis

                         WASHINGTON UNIVERSITY IN ST LOUIS
                        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

• Dangers of process cooperation
   – Data corruption, deadlocks, increased complexity
   – Requires processes to synchronize their processing

   Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts       2
                          Purposes for IPC

                      • Data Transfer
                      • Sharing Data
                      • Event notification
                      • Resource Sharing and
                      • Process Control

Fred Kuhns (11/27/2010)     CS422 – Operating Systems Concepts   3
                              IPC Mechanisms
•   Mechanisms used for communication and synchronization
     – Message Passing
        • message passing interfaces, mailboxes and message queues
        • sockets, STREAMS, pipes
     – Shared Memory: Non-message passing systems

•   Common examples of IPC
     – Synchronization using primitives such as semaphores to higher level
       mechanisms such as monitors. Implemented using either shared memoru or
       message passing.
     – Debugging
     – Event Notification - UNIX signals

•   We will defer a detailed discussion of synchronization mechanisms and
    concurrency until a later class

•   Here we want to focus on some common (and fundamental) IPC and event
    notification mechanisms

    Fred Kuhns (11/27/2010)    CS422 – Operating Systems Concepts           4
                            Message Passing
• In a Message passing system there are no shared
  variables. IPC facility provides two operations for fixed
  or variable sized message:
    – send(message)
    – receive(message)

• If processes P and Q wish to communicate, they need to:
    – establish a communication link
    – exchange messages via send and receive

• Implementation of communication link
    – physical (e.g., shared memory, hardware bus)
    – logical (e.g., syntax and semantics, abstractions)

  Fred Kuhns (11/27/2010)    CS422 – Operating Systems Concepts   5
                    Implementation Questions
•    How are links established?

•    Can a link be associated with more than two processes?

•    How are links made known to processes?

•    How many links can there be between every pair/group of communicating

•    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?

    Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts             6
                 Message Passing Systems
•Exchange messages over a communication link

•Methods for implementing the communication
 link and primitives (send/receive):
   1.Direct or Indirect communications (Naming)
   2.Symmetric or Asymmetric communications
   3.Automatic or Explicit buffering
   4.Send-by-copy or send-by-reference
   5.fixed or variable sized messages

 Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   7
Direct Communication – Internet and Sockets
• Processes must name each other explicitly:
   – Symmetric Addressing
        • send (P, message) – send to process P
        • receive(Q, message) – receive from Q
   – Asymmetric Addressing
        • send (P, message) – send to process P
        • receive(id, message) – rx from any; system sets id = sender
• Primitives:
   – send(A, message) – send a message to mailbox A
   – receive(A, message) – receive a message from mailbox A
• Properties of communication link
   – Links established automatically between pairs
   – processes must know each others ID
   – Exactly one link per pair of communicating processes
• Disadvantage: a process must know the name or ID of the
  process(es) it wishes to communicate with
  Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts          8
              Indirect Communication - Pipes
• Messages are sent to or 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 more than 2 processes.
   – Each pair of processes may share several communication links.
• Ownership:
   – process owns (i.e. mailbox is implemented in user space): only the
     owner may receive messages through this mailbox. Other
     processes may only send. When process terminates any “owned”
     mailboxes are destroyed.
   – system owns – then mechanisms provided to create, delete, send
     and receive through mailboxes. Process that creates mailbox owns
     it (and so may receive through it) but may transfer ownership to
     another process.

  Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts       9
                            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
   – Allow only one process at a time to execute a receive
   – Allow the system to select arbitrarily the receiver.
     Sender is notified who the receiver was

  Fred Kuhns (11/27/2010)       CS422 – Operating Systems Concepts   10
                 Synchronizing Message Flow

• Message passing may be either blocking or non-
    – blocking send: sender blocked until message
      received by mailbox or process
    – nonblocking send: sender resumes operation
      immediately after sending
    – blocking receive: receiver blocks until a message is
    – nonblocking receive: receiver returns immediately
      with either a valid or null message.

Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   11
• All messaging system require framework to
  temporarily buffer messages. These queues are
  implemented in one of three ways:
     1. Zero capacity – No messages may be queued within
        the link, requires sender to block until receives
        retrieves message.
     2. Bounded capacity – Link has finite number of
        message buffers. If no buffers are available then
        sender must block until one is freed up.
     3. Unbounded capacity – Link has unlimited buffer
        space, consequently send never needs to block.

Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   12
                          Lets Get Practical

        Notes to help you with the first project

Fred Kuhns (11/27/2010)     CS422 – Operating Systems Concepts   13
                          Conventional View
    Protection domains - (virtual address space)


    How can processes communicate with each
    other and the kernel?
Fred Kuhns (11/27/2010)     CS422 – Operating Systems Concepts   14
                         Universal IPC Facilities

                                            pipe                     stop
               handle event

•   Universal Facilities in UNIX
     – Signals - asynchronous or synchronous event notification.
     – Pipes - unidirectional, FIFO, unstructured data stream.
     – Process tracing - used by debuggers to control control target process

     Fred Kuhns (11/27/2010)    CS422 – Operating Systems Concepts             15
                           Signals Overview
• Divided into asynchronous (CTL-C) and
  synchronous (illegal address)
• Three phases to processing signals:
   – generation: event occurs requiring process notification
   – delivery: process recognizes and takes appropriate
   – pending: between generation and delivery
• SVR4 and 4.4BSD define 31 signals, original had
  15. Some commercial system support > 32.
• Signal to integer mappings differ between BSD
  and System V implementations

 Fred Kuhns (11/27/2010)     CS422 – Operating Systems Concepts   17
        Signals - Virtual Machine Model
                                                                   signal handler
                                       Process X
                               (Signal handles)

                              register handlers                    instruction set
dispatch to handler
                                System call interface
    kernel                {read(), write(), sigaltstack() … }
                                  (restartable system calls)

       deliver signal
        scheduler                     I/O facilities                filesystem

Fred Kuhns (11/27/2010)       CS422 – Operating Systems Concepts                    18
• Handling, default actions
    –   Abort: terminate process, generate core dump
    –   Exit: terminate without generating core dump
    –   Ignore: ignore signal
    –   Stop: suspend process
    –   Continue: resume process
• User specified actions
    – Default action,
    – Ignore signal,
    – Catch signal - invoke user specified signal handler
• User may
    – not ignore, catch or block SIGKILL and SIGSTOP
    – change action at any time
    – block signal: signal remains pending until unblocked
Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   19
                           Signal Generation
• Exceptions - kernel notifies process with signal
• Other Process - using kill or sigsend.
• Terminal interrupts - stty allows binding of
  signals to specific keys, sent to foreground
• Job control - background processes attempt to
  read/write to terminal. Process terminate or
  suspends, kernels sends signal to parent
• Quotas - exceeding limits
• Notifications - event notification (device ready)
• Alarms - process notified of alarm via signal
 Fred Kuhns (11/27/2010)     CS422 – Operating Systems Concepts   21
                       Reliable Signals - BSD

• Persistent handlers
• Masking signals
   – signals masked (blocked) temporarily
   – user can specify mask set for each signal
   – current signal is masked when handler invoked
• Interruptible sleeps
• Restartable system calls
• Allocate separate stack for handling signals
   – why is this important?

  Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   22
                   Signals - A Few Details
• Any process or interrupt can post a signal
   – set bit in pending signal bit mask
   – perform default action or setup for delivery
• Signal typically delivered in context of
  receiving process.
   – exception is sending SIGSTOP, kernel may
     perform action directly
   – Pending signals are checked before returning
     to user mode and just before/after certain
     sleep calls.
   – Produce core dump or invoke signal handler

Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   23
                              UNIX Pipes
• Unidirectional, FIFO, unstructured data stream
• Fixed maximum size
• Simple flow control
• pipe() system call creates two file descriptors. Why?
• Implemented using filesystem, sockets or STREAMS
  (bidirectional pipe).
• Named Pipes:
   – Lives in the filesystem - that is, a file is created of
     type S_IFIFO (use mknod() or mkfifo())
   – may be accessed by unrelated processes
   – persistent
   – less secure than regular Pipes. Why?
    Fred Kuhns (11/27/2010)   CS422 – Operating Systems Concepts   24
                          Process Tracing
• ptrace()
• used by debuggers such as dbx and gdb.
• Parent process controls execution of child
    – child must notify kernel that it will be traced by
• Modern systems use the /proc file system.
    – Allows process to trace unrelated processes

Fred Kuhns (11/27/2010)    CS422 – Operating Systems Concepts   25