Getting started on bluevista
last update: 05/04/2006

Programming models

Parallel programmming has three aspects:

Using parallel threads on a node.
Using message passing between processes. Message passing is a form of interprocess communication in which processes send discrete messages to one another to exchange data.
Parallel programs are called "hybrid" when they use both threads and message passing.

Bluevista is a cluster of Symmetric Multi Processor (SMP) systems, a computer architecture that has 78 eight-CPU nodes. Multiple simultaneous processes can be run within a node, on multiple nodes, or both. The CPUs on a node share that node's memory and I/O bus (data path). Each node runs its own copy of the AIX operating system. Any idle CPU can be assigned to any task, and additional CPUs and nodes can be utilized by a job to improve performance and handle increased loads.

SMP threads

Programming strategies

We outline four basic programming strategies here. These strategies allow you to match your program's requirements to the capabilities of the platform's computing architecture. All four strategies have the same goal: to obtain accurate results for computational problems in the minimum amount of wallclock time.

The four programming strategies, in order of increasing demands on the system and the programmer, are:

One serial process

One process that spawns multiple threads

Multiple parallel processes that are single-threaded
One code acting on multiple data structures (Single Program Multiple Data -- SPMD)
Multiple codes acting on multiple data structures (Multiple Programs Multiple Data -- MPMD)

Hybrid: Multiple parallel processes (SPMD or MPMD) that use multiple threads

SMP threads

SMP is short for Symmetric Multiprocessing, and it refers to a computer architecture that provides multiple processors that complete individual processes simultaneously. The individual processes are the "threads." On NCAR cluster systems, a number of processors are placed on a shared-memory node, and SMP threads run on these nodes. The threads are under programmer control in the job code. Most commonly in Fortran, the control is in the form of OpenMP directives, although the Fortran OpenMP directives have pragma counterparts in C and C++ as well. This document provides an example of an OpenMP program.

Message passing

Message passing is a parallel programming style in which explicit messages containing user-defined data are sent between tasks. In Fortran, C, and C++ programs today, the programmer may define and control these messages by calls to a message-passing library called MPI (Message Passing Interface). This document provides an example of an MPI code. At NCAR, many of the flagship models such as CAM, CCSM, and WRF are available as MPI codes.

Hybrid

The hybrid programming model is one that uses both message passing and SMP threads. Programmers choose this model when the code benefits from both fine-scale and large-scale paralllelism. This document provides an example of a hybrid code. At NCAR, several of the flagship models such as CAM, CCSM, and WRF are available as hybrid codes, specifically as Fortran90 code involving both OpenMP directives and MPI library calls.

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