Introduction to OpenMP

• A parallel programming model, an API

• Comprised of

  • Compiler directives

  • Runtime Library Routines

  • Environment Variables

• Programming Model

• All OpenMP programs begin as a single process: the master thread. The master thread executes sequentially until the first parallel region construct is encountered.

• FORK: the master thread then creates a team of parallel threads.

• JOIN: When the team threads complete the statements in the parallel region construct, they synchronize and terminate, leaving only the master thread.

• Compiler directives statements (start with#pragma omp) tell Compiler that here follows a parallel region

  • a parallel region is bounded with{ }

• Runtime Library Routines can give you some information about your environment

  • omp_get_thread_num(). Tells the thread id of the current thread.

  • omp_get_num_threads(). Tells the total thread number. If you don't tell Compiler how many threads you want to have in your program, it will be the number of your cores by default.

  • ... you can find more on the internet about this.

• Environment Variables tell Compiler that some variables are public and some variables are private.

  • As OpenMP is a shared memory model, variables are shared by threads by default.

  • You can define private variables

    • usingprivate()statement.

    • Notice that all variables defined in the parallel region are private variables.