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MPI: Difference between revisions
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== A Primer on Parallel Programming == | == A Primer on Parallel Programming == | ||
{{quote|To pull a bigger wagon it is easier to add more oxen than to find (or build) a bigger ox.|Gropp, Lusk & Skjellum|Using MPI}} | {{quote|To pull a bigger wagon it is easier to add more oxen than to find (or build) a bigger ox.|Gropp, Lusk & Skjellum|Using MPI}} | ||
To build a house as quickly as possible, we do not look for the fastest person to do all the work but instead we hire as many people as required and spread the work among them so that tasks are performed at the same time --- "in parallel". Computational problems are conceptually similar. Since there is a limit to how fast a single machine can perform, we attempt to divide up the computational problem at hand and assign work to be completed in parallel to multiple computers. This approach is important not only in speeding up computations but also in tackling problems requiring large amounts of memory. | To build a house as quickly as possible, we do not look for the fastest person to do all the work but instead we hire as many people as required and spread the work among them so that various construction tasks are performed at the same time --- "in parallel". Computational problems are conceptually similar. Since there is a limit to how fast a single machine can perform, we attempt to divide up the computational problem at hand and assign work to be completed in parallel to multiple computers. This approach is important not only in speeding up computations but also in tackling problems requiring large amounts of memory. | ||
The most significant concept to master in designing and building parallel applications is ''communication''. Complexity arises due to communication requirements. In order for multiple workers to accomplish a task in parallel, they need to be able to communicate with one another. In the context of software, we have many processes each working on part of a solution, needing values that were computed ---or are yet to be computed!--- by other processes. | The most significant concept to master in designing and building parallel applications is ''communication''. Complexity arises due to communication requirements. In order for multiple workers to accomplish a task in parallel, they need to be able to communicate with one another. In the context of software, we have many processes each working on part of a solution, needing values that were computed ---or are yet to be computed!--- by other processes. | ||
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The Message Passing Interface (MPI) is, strictly speaking, a ''standard'' describing a set of subroutines, functions, objects, ''etc.'', with which one can write parallel programs in a distributed memory environment. Many different ''implementations'' of the standard have been produced, such as Open MPI, MPICH, and MVAPICH. The standard describes how MPI should be called from Fortran, C, and C++ languages, but unofficial "bindings" can be found for several other languages. | The Message Passing Interface (MPI) is, strictly speaking, a ''standard'' describing a set of subroutines, functions, objects, ''etc.'', with which one can write parallel programs in a distributed memory environment. Many different ''implementations'' of the standard have been produced, such as Open MPI, MPICH, and MVAPICH. The standard describes how MPI should be called from Fortran, C, and C++ languages, but unofficial "bindings" can be found for several other languages. | ||
MPI is an open, non-proprietary standard | Since MPI is an open, non-proprietary standard, an MPI program can easily be ported to many different computers. Applications that use it can run on a large number of cores at once, often with good parallel efficiency (called "scalability"). Given that memory is local to each process, some aspects of debugging are simplified --- it isn't possible for one process to interfere with the memory of another, and if a program generates a segmentation fault the resulting core file can be processed by standard serial debugging tools. However, due to the need to manage communication and synchronization explicitly, MPI programs may appear more complex than programs written with tools that support implicit communication. Furthermore, in designing an MPI program one should take care to minimize communication overhead to achieve a good speed-up from the parallel computation. | ||
In the following we will highlight a few of these issues and discuss strategies to deal with them. Suggested references are presented at the end of this tutorial and the reader is encouraged to consult them for additional information. | In the following we will highlight a few of these issues and discuss strategies to deal with them. Suggested references are presented at the end of this tutorial and the reader is encouraged to consult them for additional information. | ||
== MPI Programming Basics == | == MPI Programming Basics == | ||
This tutorial will present the development of code in C and Fortran, but the concepts apply to any language for which MPI bindings exist. For simplicity our goal will be to parallelize the venerable "Hello, World!" program, which appears below for reference. | This tutorial will present the development of an MPI code in C and Fortran, but the concepts apply to any language for which MPI bindings exist. For simplicity our goal will be to parallelize the venerable "Hello, World!" program, which appears below for reference. | ||
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