Monitors Message Passing - Operating Systems - Lecture Slides, Slides of Computer Science

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Operating Systems

Lecture 13:

Monitors. Message Passing.

Readers/Writers.

Roadmap

• Concurrency
• Mutual Exclusion: Hardware Support
• Semaphores
• Monitors
• Message Passing
• Readers/Writers Problem

Main characteristics:

1. Local variables are accessible only by the monitor’s procedures and not by any external procedure.
2. A process enters the monitor by invoking one of its procedures.
3. Only one process may be executing in the monitor at a time;

Structure of a Monitor

Synchronization

• Synchronisation achieved by condition variables within a monitor - only accessible by the monitor.
• Operation on condition variables:
  • Cwait(c): Suspend execution of the calling process on condition c
  • Csignal(c): Resume execution of some process blocked after a cwait on the same condition

Roadmap

• Concurrency
• Mutual Exclusion: Hardware Support
• Semaphores
• Monitors
• Message Passing
• Readers/Writers Problem

Message Passing

• The actual function of message passing is normally provided in the form of a pair of primitives:
• send (destination, message)
• receive (source, message)

General Message Format

Blocking send,

Blocking receive

• Both sender and receiver are blocked until message is delivered
• Known as a rendezvous
• Allows for tight synchronization between processes.

Non-blocking Send

• More natural for many concurrent programming tasks.
• Nonblocking send, blocking receive
  • Sender continues on
  • Receiver is blocked until the requested message arrives
• Nonblocking send, nonblocking receive
  • Neither party is required to wait

Producer/Consumer with Messages

Summary

• OS as:
  • Multiprogramming, multiprocessing, distributed processing
  • Fundamental theme: concurrency
• Mutual exclusion:
  • Condition in which only one of a set of concurrent processes is able to access a given resource or perform a given function at any time
  • One approach involves the use of special-purpose machine instructions
• Synchronization mechanisms:
  • Semaphores: signalling between processes
  • Monitors: programming language construct
  • Message passing
• Classical Inter-Process Communication (IPC) problems:
  • Producer/consumer, Barbershop, Readers/Writers

Readers/Writers Problem

• A data area is shared among many processes
  • Some processes only read the data area, some only write to the area
• Conditions to satisfy:
  1. Multiple readers may read the file at once.
  2. Only one writer at a time may write
  3. If a writer is writing to the file, no reader may read it.

• Version