R.P. Souto, H.F. de Campos Velho, S. Stephany, E.S. Chaulhoub (2003): Performance Analysis of Radiative Transfer Algorithms for Inverse Hydrologic Optics in a Parallel Environment, International Conference on Transport Theory, 20-25 July, Rio de Janeiro, Brazil.

Abstract: Inverse problems in Hydrologic Optics can be solved by an implicit technique for parameter estimation from radiometric measurements. The associated forward problem is the solution of the radiative transfer equation (RTE), which is very processing demanding. In a typical inversion, hundreds of iterations may be required and therefore the choice of an algorithm suitable for parallelization is an important issue.

Three algorithms for solving the RTE were studied - HYDROLIGHT, PEESNA and LTSN. These algorithms correspond, respectively to invariant imbedding, analytical discrete-ordinates and LTS$_N$ methods. As a first step, the performance of each algorithm was evaluated running in the same sequential machine. Timing and profiling of the three codes was performed in order to evaluate processing times and to identify performance bottlenecks.

The three codes employ the Fourier decomposition of the radiances obtaining independent azimuthal modes. Therefore, an independent RTE can be written for each azimuthal mode and can be assigned to a different processor, in a parallel implementation. The speed-up that can be achieved increases with the fraction of time spent in the azimuthal mode, but total execution time is also an important issue.

Each algorithm was studied concerning the feasibility of its parallelization using the MPI message passing communication library and execution in a distributed memory machine, a multicomputer based on IA-32 architecture.