Shared Memory Programming

• Shared memory parallel programming is perhaps the easiest model to understand because of its similarity with operating systems programming and general multiprogramming.
• Shared memory programming is done through some extensions to existing programming languages, operating systems, and code libraries.
• Shared address space programming paradigms can vary on mechanisms for data sharing, concurrency models, and support for synchronization.
• Process based models assume that all data associated with a process is private, by default, unless otherwise specified (using UNIX system calls such as shmget and shmat).
• While this is important for ensuring protection in multiuser systems, it is not necessary when multiple concurrent aggregates are cooperating to solve the same problem.
• The overheads associated with enforcing protection domains make processes less suitable for parallel programming.
• In contrast, lightweight processes and threads assume that all memory is global. By relaxing the protection domain, lightweight processes and threads support much faster manipulation.
• Directive based programming models extend the threaded model by facilitating creation and synchronization of threads.
• In a shared memory parallel program, there must exist three main programming constructs:
  1. task creation,
  2. communication,
  3. synchronization.