Multi-processing CTH: Porting legacy FORTRAN code to MP hardware

CTH is a family of codes developed at Sandia National Laboratories for use in modeling complex multi-dimensional, multi-material problems that are characterized by large deformations and/or strong shocks. A two-step, second-order accurate Eulerian solution algorithm is used to solve the mass, momentum, and energy conservation equations. CTH has historically been run on systems where the data are directly accessible to the cpu, such as workstations and vector supercomputers. Multiple cpus can be used if all data are accessible to all cpus. This is accomplished by placing compiler directives or subroutine calls within the source code. The CTH team has implemented this scheme for Cray shared memory machines under the Unicos operating system. This technique is effective, but difficult to port to other (similar) shared memory architectures because each vendor has a different format of directives or subroutine calls. A different model of high performance computing is one where many (> 1,000) cpus work on a portion of the entire problem and communicate by passing messages that contain boundary data. Most, if not all, codes that run effectively on parallel hardware were written with a parallel computing paradigm in mind. Modifying an existing code written for serial nodes poses a significantly different set of challenges that will be discussed. CTH, a legacy FORTRAN code, has been modified to allow for solutions on distributed memory parallel computers such as the IBM SP2, the Intel Paragon, Cray T3D, or a network of workstations. The message passing version of CTH will be discussed and example calculations will be presented along with performance data. Current timing studies indicate that CTH is 2--3 times faster than equivalent C++ code written specifically for parallel hardware. CTH on the Intel Paragon exhibits linear speed up with problems that are scaled (constant problem size per node) for the number of parallel nodes.