OpenMP

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OpenMP

• An API for shared memory multiprocessing.

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What is OpenMP?

• OpenMP is a portable, shared memory,
  multiprocessing API.
• Not a new language.
• Specifies a set of subroutines in an existing
  language (FORTRAN, C) for parallel programming on
  a shred memory machine.

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• Where is OpenMP used?
• Used on shared memory machines as opposed to
  distributed memory machines.
• Shared memory vs. distributed memory.
• Why is OpenMP popular?
• Vendor involvement
• Allows us to use existing code
• Incremental parallelization

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OpenMP Architecture
OpenMP is made up of the following components

• Directives and Pragmas
• Runtime Library Routines
• Environment variables

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OpenMP Execution Model
fork
Slave threads / processes
synchronize
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OpenMP Directives

• Control Structures
• Work Sharing
• Synchronization
• Data Scope Attributes
• Orphaning

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Parallel Region

• !OMP PARALLEL Clause
• code block
• !OMP END PARALLEL
• The block of code between the PARALLEL and END
  PARALLEL directives is xecuted by multiple
  threads, in parallel.

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Work Sharing Constructs

• A work sharing construct divides the execution of
  enclosed code region among participating
  processes
• The DO directive
• The SECTIONS directive
• The SINGLE directive

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Synchronization
Synchronization directives provide for process
synchronization and mutual exclusion

• The MASTER directive
• The BARRIER directive
• The CRITICAL directive
• The ORDERED directive
• The ATOMIC directive

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Data Scoping Directives
Clauses qualify and scope the variables in a
block of code

• PRIVATE
• SHARED
• DEFAULT (PRIVATE SHARED NONE)
• FIRSTPRIVATE
• LASTPRIVATE
• COPYIN
• REDUCTION

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Orphaning

• Directives that do not appear in the lexical
  extent of the parallel construct, but lie in the
  dynamic extent are called orphaned directives.
• Directives in routines called from within
  parallel constructs.

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Runtime Library Routines

• OMP_SET_NUM_THREADS (int)
• OMP_GET_NUM_THREADS( )
• OMP_GET_MAX_THREADS( )
• OMP_GET_THREAD( )
• OMP_GET_NUM_PROCS( )
• OMP_IN_PARALLEL( )
• OMP_SET_DYNAMIC(bool)

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Lock Routines

• OMP_INIT_LOCK(var)
• OMP_DESTROY_LOCK(var)
• OMP_SET_LOCK(var)
• OMP_UNSET_LOCK(var)
• OMP_TEST_LOCK(var)

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Environment Variables

• OMP_SCHEDULE
• OMP_NUM_THREADS
• OMP_DYNAMIC
• OMP_NESTED