OpenMp

OpenMp

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  OpenMP By:By: Neel BhadNeel Bhad GeetBhadGeet Bhad
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  What Is OpenMP..?WhatIs OpenMP..? ● In simple word runs a user program in parallel. ● It utilize to main concepts for parallelism Multithreading Shared Memory ● It takes user application, tear it down into group of threads and runs them on a shared memory foundation
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  ● OpenMP isportable: supported by HP, IBM, Intel, SGI, SUN, and others ● It is the de facto standard for writing shared memory programs. To become an ANSI standard? ● OpenMP can be implemented incrementally, one function or even one loop at a time. ● A nice way to get a parallel program from a sequential program. Why OpenMP?
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  ● What isparallel processing ? It is ability of processing more than one job simultaneously. ● Why going parallel ? Great deal of data to be processed Time needed to calculate an engineering equation Need jobs to be done faster
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  Programming Model • SharedMemory • Parallelism by threading • Fork-Join model • Explicit Parallelism • Nested Parallelism • Dynamic Threads • Input / Output • Memory model 5
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  Shared Memory (Cont.) •Following system can be used for shared memory access • a single core chip (older PC’s, sequential execution) • a multicore chip (such as your laptop?) • multiple single core chips in a NUMA system • multiple multicore chips in a NUMA system (VT SGI system)
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  UMA Vs. NUMA •Unified Memory Access ( UMA ) 7
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  UMA Vs. NUMA(Cont.) • Non Unified Memory Access ( NUMA ) 8
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  Fork – JoinModel (Cont.) 9 F J Master Thread Thread
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  OpenMP Elements • CompilerDirectives • Runtime Libraries • Environmental Variables 10
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  The OpenMP ProgrammingModel • Based on compiler directives • Nested Parallelism Support API allows parallel constructs inside other parallel constructs • DynamicThreads API allows to dynamically change the number of threads which may used toexecute different parallel regions • Input/Output OpenMP specifies nothing about parallel I/O The programmer has to insure that I/O is conducted correctly within the context of a multi-threaded program • Memory consistency Threads can "cache" their data and are not required to maintain exact consistency with real memory all of the time When it is critical that all threads view a shared variable identically, the programmer is responsible for insuring that the variable is FLUSHed by all threads as needed
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  How Does ItWork? • OpenMP is an implementation of multithreading • A master thread "forks" a specified number of slave threads Tasks are divided among slaves • Slaves run concurrently as the runtime environment allocating threads to different processors
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  Hello World inOpenMP #include <omp.h> #include <iostream> using namespace std; int main() { #pragma omp parallel num_threads(3) { cout << "Hello Worldn”; // what if // cout << "Hello World" << endl; } }
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  X3H5 MPI PthreadsHPF OpenMP Performance  ?  Scalability    Incremental Parallelization   Portability     Support for Dusty Deck Codes  ?  High Level User API    Support Data Parallelism    Ease of Debugging     OpenMP Compared
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  ThankYou