On the Run-Time Cost of Distributed-Memory Communications Generated Using the Polyhedral Model

The polyhedral model can be used to automatically generate distributed-memory communications for affine nested loops. Recently, new communication schemes that reduce the communication volume have been presented. In this paper we study the extra computational effort introduced at run-time by the code generated to manage the communication details across distributed processes. We focus on the most sophisticated communication scheme so far introduced (the FOP scheme). We present an asymptotic cost study of the FOP scheme in terms of two main run-time parameters: The problem size, and the number of processors. Based on this study, we identify scalability limitations in current implementations of these techniques, and propose a simple implementation alternative to eliminate one of them. Experimental results are presented, showing the potential impact on performance of these implementation limitations when using these codes in large parallel systems.