Shared-memory Programming参考.ppt

Parallel Programmingin C with MPI and OpenMP Michael J. Quinn Chapter 17 Shared-memory Programming Outline OpenMP Shared-memory model Parallel for loops Declaring private variables Critical sections Reductions Performance improvements More general data parallelism Functional parallelism OpenMP OpenMP: An application programming interface (API) for parallel programming on multiprocessors Compiler directives Library of support functions OpenMP works in conjunction with Fortran, C, or C++ What’s OpenMP Good For? C + OpenMP sufficient to program multiprocessors C + MPI + OpenMP a good way to program multicomputers built out of multiprocessors IBM RS/6000 SP Fujitsu AP3000 Dell High Performance Computing Cluster Shared-memory Model Fork/Join Parallelism Initially only master thread is active Master thread executes sequential code Fork: Master thread creates or awakens additional threads to execute parallel code Join: At end of parallel code created threads die or are suspended Fork/Join Parallelism Shared-memory Model vs.Message-passing Model (#1) Shared-memory model Number active threads 1 at start and finish of program, changes dynamically during execution Message-passing model All processes active throughout execution of program Incremental Parallelization Sequential program a special case of a shared-memory parallel program Parallel shared-memory programs may only have a single parallel loop Incremental parallelization: process of converting a sequential program to a parallel program a little bit at a time Shared-memory Model vs.Message-passing Model (#2) Shared-memory model Execute and profile sequential program Incrementally make it parallel Stop when further effort not warranted Message-passing model Sequential-to-parallel transformation requires major effort Transformation done in one giant step rather than many tiny steps Parallel for Loops C programs often express data-parallel operations as for loops for (i = first; i size; i += prime) marked[i] = 1;