Changes between Initial Version and Version 1 of Workshops/IntroToHpc2015/Programming/OpenMp

10/12/15 16:01:57 (7 years ago)


  • Workshops/IntroToHpc2015/Programming/OpenMp

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     3= OpenMP =
     5* What is OpenMP?
     6   * The OpenMP is an Application Program Interface (API) that supports shared-memory parallel programming.
     7* What is the shared-memory?
     8   * The memory space that is accessible from multiple CPUs without an operating system call involvement.
     9* Limitation
     10   * The number of CPUs on a single compute node bounds the number of threads. (<->Distributed Memory Cluster)
     13==  A Programmer’s View of OpenMP ==
     14* OpenMP is a portable, threaded, shared-memory programming specification with “light” syntax
     15   * Exact behavior depends on OpenMP implementation!
     16   * Requires compiler support (C or Fortran)
     17* OpenMP will:
     18    * Allow a programmer to separate a program into serial regions and parallel regions.
     19* OpenMP will not:
     20    * Parallelize automatically
     21    * Guarantee speedup
     23== OpenMP's Execution Model ==
     24* OpenMP is built around a shared memory space and generates concurrent threads – this is how the parallelism is facilitated.
     25* CPUs that share memory are called “symmetric multi-processors”, or SMP for short.
     26* 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”.
     27* 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.
     28* These days, most major compilers do support OpenMP directives for most platforms (IBM's XL suite, Intel, even GCC ≥ 4.2).
     29* A fork is when a single thread is made into multiple, concurrently executing threads.
     30* A join is when the concurrently executing threads synchronize back into a single thread.
     31* During the overall execution, threads may communicate with one another through shared variables.
     32* OpenMP programs essentially consist of a series of forks and joins.
     35== Brief Tutorials for Programming Languages ==
     36* [[cypress/Programming/Cexamples/OpenMp|C]]
     37* [[cypress/Programming/CppExamples/OpenMpCpp|C++]]
     38* [[cypress/Programming/FortranExamples/OpenMpFortran|Fortran]]