Open MP

OPEN MPShared Memory Multiprocessing Programming

Presenter : 백운혁windage.co.kr

[email protected]

● Agenda

• Background• Multiprocessing Programming• Preference• Programming Model• Parallel Section• Clauses• Synchronization

● Background

Parallel has no semantic impact

Concurrency has a substantial semantic impact

● Background

• Symmetric Multi Processor(SMP)– Shared Memory Multiprocessing Programming– OpenMP

• Massively Parallel Processor(MPP)– Message Passing Interface Programming– MPI

● Background

● Multiprocessing Programming

● Multiprocessing Programming

• General Threading Model– Main thread executes the program’s main function– Main thread creates subthreads, witch execute other

functions in the program.– Subthreads can also create additional threads.

● Multiprocessing Programming

• Benefits– Increased performance and better resource utilization

• Even on single processor systems – for hiding latency and increasing throughput

– IPC thread shared memory is more efficient

• Risk– Increases complexity of the application– Difficult to debug (data races, deadlocks, etc.)

● OpenMP Preference (later VS2005)

● OpenMP Preference (later GCC 4.2)

● Programming Model

• Generate multi-thread code by referencing directive code

• Compiler is required to support openMP

• To eliminate synchronization and dependency

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Programming Model

● Parallel Section (for)

#pragma omp for [clause [clause …] ] { for loop}

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for loop

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● Parallel Section (sections)

#pragma omp sections [clause [clause …] ] { [#progma omp section] structured code block}

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sections

Join

● Clauses

• private(var1, …)– to block sharing selected values with threads

• shared(var1, …)– to share selected values with threads

• schedule(type [, chunk_size])– to divide work with threads for equality

• static• dynamic

● Synchronization (critical section)

• Declaring critical section in parallel section

• Running only single thread at moment

#pragma omp critical [(name)]{ structured code block}

OPEN MPShared Memory Multiprocessing Programming

[email protected]

https://computing.llnl.gov/tutorials/openMP/