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Mutual Exclusion - Message Passing

Introduction

• Semaphores and monitors are designed for systems that share memory between processes or threads.

• All of the processes/threads involved in synchronization can reference the same semaphore or monitor objects.

Message Passing

• When processes need to synchronize WITHOUT shared memory (e.g., distributed systems), message passing may be used.

  e.g., Mailbox or port is a place for a message to be queued up at a receiver. This enables a receiver to have multiple targets for messages if desired.

Message Passing Forms

Both the sender and receiver can be blocking or non-blocking. Three combinations are common, although any particular system will usually have only one or two combination implemented:

• Blocking send, blocking receive

  • Full synchronization between two entities

  • Both the sender and receiver are blocked until the message is delivered; this is sometimes referred to as a rendezvous. This combination allows for tight synchronization between processes.

• Non-blocking send, blocking receive

  • Partial synchronization

  • Although the sender may continue on, the receiver is blocked until the requested message arrives. This is probably the most useful combination. It allows a process to send one or more messages to a variety of destinations as quickly as possible. A process that must receive a message before it can do useful work needs to be blocked until such a message arrives. e.g., A server process that exists to provide a service or resource to other processes.

• Non-blocking send, non-blocking receive

  • Neither entity must wait

  • Used for information sharing but not always for synchronization

References

Stallings, W. (2018). Operating Systems: Internals and Design Principles (9th ed.). Pearson Education, Inc.