Lightweight MPI Communicators with Applications to Perfectly Balanced Quicksort

TL;DR

This paper introduces RBC, a library for creating range-based MPI communicators in constant time, enabling efficient, flexible parallel algorithms like Janus Quicksort with significantly improved performance and load balancing.

Contribution

The paper presents RBC, a novel MPI-based library for instant creation of range-based communicators, and proposes Janus Quicksort, a load-balanced distributed sorting algorithm utilizing RBC.

Findings

RBC reduces communicator creation time by over 400 times.

Janus Quicksort's performance improves by a factor of 15 with RBC.

RBC maintains collective operation performance despite faster communicator creation.

Abstract

MPI uses the concept of communicators to connect groups of processes. It provides nonblocking collective operations on communicators to overlap communication and computation. Flexible algorithms demand flexible communicators. E.g., a process can work on different subproblems within different process groups simultaneously, new process groups can be created, or the members of a process group can change. Depending on the number of communicators, the time for communicator creation can drastically increase the running time of the algorithm. Furthermore, a new communicator synchronizes all processes as communicator creation routines are blocking collective operations. We present RBC, a communication library based on MPI, that creates range-based communicators in constant time without communication. These RBC communicators support (non)blocking point-to-point communication as well as (non)blocking collective operations. Our experiments show that the library reduces the time to create a new communicator by a factor of more than 400 whereas the running time of collective operations remains about the same. We propose Janus Quicksort, a distributed sorting algorithm that avoids any load imbalances. We improved the performance of this algorithm by a factor of 15 for moderate inputs by using RBC communicators. Finally, we discuss different approaches to bring nonblocking (local) communicator creation of lightweight (range-based) communicators into MPI.