Application-bypass reduction for large-scale clusters

Process skew is an important factor in the performance of parallel applications, especially in large-scale clusters. Reduction is a common collective operation which, by its nature, introduces implicit synchronization between the processes involved in the communication and is therefore highly susceptible to performance degradation due to process skew. A collective operation with application-bypass does not require the application to block in order for the operation to make progress. Application-bypass collective operations are therefore highly tolerant of skew. In this paper we describe the design and implementation of an application-bypass version of the reduction operation in MPICH over GM. We evaluate our implementation on a 16-node cluster. Under conditions of process skew we find a factor of improvement of up to 3.3 for our application-bypass reduction versus the default MPICH implementation. In addition, we see that this factor of improvement increases with system size, indicating that the application-bypass implementation is more scalable and skew-tolerant than the default non-application-bypass version. This framework promises design and development of high-performance and scalable collective communication libraries for next-generation large-scale clusters.