= OpenMP =

* What is OpenMP? 
   * The OpenMP is an Application Program Interface (API) that supports shared-memory parallel programming. 
* What is the shared-memory?
   * The memory space that is accessible from multiple CPUs without an operating system call involvement.
* Limitation 
   * The number of CPUs on a single compute node bounds the number of threads. (<->Distributed Memory Cluster)

[[Image(SMPmachine.png)]]
==  A Programmer’s View of OpenMP ==
* OpenMP is a portable, threaded, shared-memory programming specification with “light” syntax
   * Exact behavior depends on OpenMP implementation!
   * Requires compiler support (C or Fortran)
* OpenMP will:
    * Allow a programmer to separate a program into serial regions and parallel regions.
* OpenMP will not:
    * Parallelize automatically
    * Guarantee speedup

== OpenMP's Execution Model ==
* OpenMP is built around a shared memory space and generates concurrent threads – this is how the parallelism is facilitated.
* CPUs that share memory are called “symmetric multi-processors”, or SMP for short.
* Each thread is typically run on its own processor, though it is becoming common for each CPU or “core” to handle more than one thread “simultaneously”; this is called “hyper-threading”.
* Thread (process) communication is implicit, and uses variables pointing to shared memory locations; this is in contrast with MPI, which uses explicit messages passed among processes.
* These days, most major compilers do support OpenMP directives for most platforms (IBM's XL suite, Intel, even GCC ≥ 4.2).
* A fork is when a single thread is made into multiple, concurrently executing threads.
* A join is when the concurrently executing threads synchronize back into a single thread.
* During the overall execution, threads may communicate with one another through shared variables.
* OpenMP programs essentially consist of a series of forks and joins.

[[Image(OMPexecModel.png)]]
== Brief Tutorials for Programming Languages ==
* [[cypress/Programming/Cexamples/OpenMp|C]]
* [[cypress/Programming/CppExamples/OpenMpCpp|C++]]
* [[cypress/Programming/FortranExamples/OpenMpFortran|Fortran]]