Meso-Scale Computing with an Inexpensive Super Computer

Thomas F. Gross
NOS/ Coast Survey/ Coast Survey Developement Laboratory/ Marine Modeling and Analysis Program

Numerical hydrodynamic circulation models are limited by raw CPU computing speed. Every model could benefit from increased resolution and smaller time stepping, but practical constraints limit operational tidal prediction models to simulating 24 hours within 30 minutes of CPU time. Longer execution times
invalidate the usefulness of forecasts to the mariner. The NOS/Coast Survey Development Laboratory is experimenting with a 32 node Beowulf cluster computer built with inexpensive desktop computers, and an eight processor SGI Octane 2000. The model code for the cluster computer architecture must be hand coded with Message Passing Interface,(MPI) to minimize inter-node communication costs, but at a dollar per performance ratio of more than 50:1
the cost may be worth while. In addition the finite element, FEM, modeling technique used by the hydrodynamic model does not lend itself to automatic compilation on the SGI and requires MPI coding even in that environment.

The FEM-MPI technique uses domain splitting, solving the equations of motion in discrete geographic domains and communicating boundary conditions only between adjacent domains. The present model consists of 11,000 horizontal
nodes, 20,000 elements, 11 vertical levels with 5760 time steps per 24 hours. The 16 node 24 hour calculations is finished in 30 minutes. Linear speed up is achieved using 2, 4, and 8 node domains while increased communication costs slow up the 16 and 32 node calculations.

The cluster is made up of 33 Census PCs with 450 MHz Pentium CPUs with 64M RAM and two 24 node 10/100Mbs switches. The RedHat Linux operating system allows free communications using RSH and NIS within the cluster. The FEM code is compiled with gcc and using LAM-MPI library. All the software is open-source to allow maximum technological innovation.

Modeled tidal water level floats above the bathymetry near the
mouth of the Chesapeake Bay.

BIO -Thomas F. Gross 1984, PhD. University of Washington, School of Oceanography Post Doc Woods Hole Oceanographic Institute Faculty Skidaway Inst. Oceanography, Savannah, GA NOAA since 1998.

 

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