Recomposing an Irregular Algorithm Using a Novel Low-Level PGAS Model

This paper presents analysis and simulation results for a toolkit of parallel graph traversal primitives which were built using a novel, low-level partitioned global address space (PGAS) programming model. Unlike high-level HEC PGAS languages (UPC, Chapel, Fortress), this mobile-subjective (MoS) model does not hide parallelization or communication overhead in the compiler or runtime. Unlike other low-level HEC languages (C/MPI) this model provides 1) facilities for fine-grain synchronization, 2) PGAS view of memory, and 3) object encapsulation. This paper shows how this programming model facilitated the transformation of the well-studied minimum spanning forest (MSF) algorithm into a new MSF algorithm which allowed for million way well-behaved parallelism on a novel multithreaded architecture. We provide analysis to show why naive formulations of MSF are not scalable for certain input graphs. We then provide analysis of the MoS reformulation to show how scalability is achieved by ensuring a good distribution of data and computation for arbitrary input graphs.