X10 as a Parallel Language for Scientific Computation: Practice and Experience

X10 is an emerging Partitioned Global Address Space (PGAS) language intended to increase significantly the productivity of developing scalable HPC applications. The language has now matured to a point where it is meaningful to consider writing large scale scientific application codes in X10. This paper reports our experiences writing three codes from the chemistry/material science domain: Fast Multipole Method (FMM), Particle Mesh Ewald (PME) and Hartree-Fock (HF), entirely in X10. Performance results are presented for up to 256 places on a Blue Gene/P system. During the course of this work our experiences have been shared with the X10 development team, so that application requirements could inform language design discussions as the language capabilities influenced algorithm design. This resulted in improvements in the language implementation and standard class libraries, including the design of the array API and support for complex math. Data constructs in X10 such as \emph{places} and \emph{distributed arrays}, and parallel constructs such as \emph{finish} and \emph{async}, simplify implementation of the applications in comparison with MPI. However, current implementation limitations in X10 2.1.2 make it difficult to achieve scalable performance using the most natural expressions of the algorithms. The most serious limitation is the use of point-to-point communication patterns, rather than collectives, to implement parallel constructs and array operations. This issue will be addressed in future releases of X10.