Multithreaded Programming and Memory Visibility
See also: Memory model (programming)Multithreaded programs usually use synchronization primitives provided by a high-level programming environment, such as Java and .NET Framework, or an application programming interface (API) such as POSIX Threads or Windows API. Primitives such as mutexes and semaphores are provided to synchronize access to resources from parallel threads of execution. These primitives are usually implemented with the memory barriers required to provide the expected memory visibility semantics. In such environments explicit use of memory barriers is not generally necessary.
Each API or programming environment in principle has its own high-level memory model that defines its memory visibility semantics. Although programmers do not usually need to use memory barriers in such high level environments, it is important to understand their memory visibility semantics, to the extent possible. Such understanding is not necessarily easy to achieve because memory visibility semantics are not always consistently specified or documented.
Just as programming language semantics are defined at a different level of abstraction than machine language opcodes, a programming environment's memory model is defined at a different level of abstraction than that of a hardware memory model. It is important to understand this distinction and realize that there is not always a simple mapping between low-level hardware memory barrier semantics and the high-level memory visibility semantics of a particular programming environment. As a result, a particular platform's implementation of (say) POSIX Threads may employ stronger barriers than required by the specification. Programs which take advantage of memory visibility as-implemented rather than as-specified may not be portable.
Read more about this topic: Memory Barrier
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