Inter-process Communication

Inter-process Communication An Overview of Shared Memory and Messaging

Overview • Inter-process Communication (IPC) • Shared Memory • Concept • Associated Linux Commands • Messaging • Concept • Associated Linux Commands • Summary of Concepts

Scope of Presentation • Impart General Concepts • Assumption Audience has General Programming Knowledge • Present Two of Many IPC Strategies • Recognition of Other IPC Methodologies

Overview of IPC • Definition – Ability for processes to share information and resources , as well as provide critical notices of process status to other processes. • Coordination of Processes • Integrity of Data • Protection of Order of Operations • Complex problem solving • Non-linear Problems • Linear Problems

Traditional Linear Problem Problem Solver Process Processor Master Process

Linear Problem with Answer Caching(using IPC and multiple processors) Processor 1 Master Process Problem Solver Process Processor 2 Potential Problem Answer Cache Message Queue Cache Problem Solver Processes Processor 3

Management of IPC • Operating System Level • Process Level • Two Common Techniques • Shared Memory • Messaging

Shared Memory • Allow two or more processes access to the same address space for reading and writing. • What could go wrong! • Process Level Management of Access • Protecting Access with a Semaphore

Semaphore • A semaphore is a global memory variable that processes use to manage access to memory and resources. • Analogy: Classroom Speaking Permission Card • No speaking unless card is in Student’s Possession • Student Must Get the Card and Return the Card • Simple Semaphore – Reading and Writing • Complex Semaphore – Bank of Printers

Shared Memory Pros and Cons • Pros • Good speed once access is granted. • Simple implementation • Cons • Performance degradation as user access requests increase • Limited scalability and applicability to purpose

Linux OS: Semaphore Commands(not inclusive) • semget() – Initializes or gains access to a semaphore. • semop() - Performs operations on a semaphore set. • semctl()- Changes permissions and other characteristics of existing semaphore sets.

Messaging • Allow processes to communicate via messages that are managed by the OS. • Data Requests • Processing Requests • Process Control Through Message Forwarding • Direct Messaging • Synchronous (ex. Requesting data) • Asynchronous (ex. Print job) • Indirect Messaging • Mailbox Concept • OS Controlled Queue

Messaging Pros and Cons • Pros • Strong ability to scale solutions and retain performance • Ability to queue requests and allow processes to continue • Flexibility to create complex dependent operations through message redirection • Cons • Complex implementation

Linux OS: Messaging Commands(not inclusive) • msgget() - Initializes a new message queue. • msgsnd() – Used to send messages to a message queue. • msgrcv() – Used to receive messages from a message queue.

Summary of Concepts • Inter-process Communication • Shared Memory • Messaging • Customized Operating Systems • Messaging Commands • Semaphore • ??? • Limits of IPC

Inter-process Communication