OpenMP - Pros and Cons

Pros and Cons

Pros

  • Portable multithreading code (in C/C++ and other languages, one typically has to call platform-specific primitives in order to get multithreading)
  • Simple: need not deal with message passing as MPI does
  • Data layout and decomposition is handled automatically by directives.
  • Incremental parallelism: can work on one part of the program at one time, no dramatic change to code is needed.
  • Unified code for both serial and parallel applications: OpenMP constructs are treated as comments when sequential compilers are used.
  • Original (serial) code statements need not, in general, be modified when parallelized with OpenMP. This reduces the chance of inadvertently introducing bugs.
  • Both coarse-grained and fine-grained parallelism are possible
  • In irregular multi-physics applications which do not adhere solely to the SPMD mode of computation, as encountered in tightly coupled fluid-particulate systems, the flexibility of OpenMP can have a big performance advantage over MPI.

Cons

  • Risk of introducing difficult to debug synchronization bugs and race conditions.
  • Currently only runs efficiently in shared-memory multiprocessor platforms (see however Intel's Cluster OpenMP and other distributed shared memory platforms).
  • Requires a compiler that supports OpenMP.
  • Scalability is limited by memory architecture.
  • no support for compare-and-swap
  • Reliable error handling is missing.
  • Lacks fine-grained mechanisms to control thread-processor mapping.
  • Can't be used on GPU
  • High chance of accidentally writing false sharing code
  • Multithreaded Executables often incur longer startup times so, they can actually run much slower than if compiled single-threaded, so, there needs to be a benefit to being multithreaded.
  • Often multithreading is used when there is no benefit yet the downsides still exist.

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