G. Cowles1, C. Chen1, H. Liu1
1B. Rothschild School for Marine
Science and Technology University of Massachusetts-Dartmouth New
Bedford, MA 02744
The parallelization methodology for the FVCOM code is described.
The implementation is a Single Program Multiple Data (SPMD) approach
and uses a message-passing model to perform the necessary
interprocessor communication and synchronization. The resulting
parallelization is efficient, transparent to the user, and portable
to a variety of parallel architectures. The physical domain is
decomposed into subdomains using the METIS graph partitioning
libraries. Each subdomain is assigned to a processor for
integration of the model equations. To compute the flux at the
subdomain (interprocessor) boundaries, flow data must be exchanged
among processors. The exchange subroutines utilize non-blocking
sends and receives from the MPI (Message Passing Interface) 2.0
library. Other parallel constructs such as Gathers and Broadcasts
are performed using the functionality of the MPI library. In
addition to the parallelization, several major modifications to the
FVCOM core code have been made. These include migratio n to Fortran
95/2K, utilization of allocatable memory, conversion of i/o to
NetCDF format, and streamlining of the flux subroutines. The
parallel FVCOM code was tested primarily on the IBM Regatta
multiprocessor machine at ARSC in Fairbanks, Alaska. The model used
for testing was a prognostic simulation of the Gulf of Maine/Georges
Bank region utilizing 25000 elements and 31 sigma layers. The
efficiency of the parallel implementation can be measured in terms
of its speedup and/or scalability on a multiprocessor computer. For
the GOM/GB simulation, the code maintains 13X throughput on 16
processors, indicating the efficiency of the current parallel
implementation. Wall clock time for a one month integration of
GOM/GB has been reduced from 63 hours (desktop, serial) to 3 hours
(16 processors, parallel). The parallel implementation extends the
range of the FVCOM code to applications with increased spatial
resolution or longer integration periods.
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