Chapel Aggregation Library (CAL)

Fine-grained communication is a fundamental prin- ciple of the Partitioned Global Address Space (PGAS), which serves to simplify creating and reasoning about programs in the distributed context. However, per-message overheads of communication rapidly accumulate in programs that generate a high volume of small messages, limiting the effective bandwidth and potentially increasing latency if the messages are generated at a much higher rate than the effective network bandwidth. One way to reduce such fine-grained communication is by coarsening the granularity by aggregating data, or by buffering the smaller communications together in a way that they can be handled in bulk. Once these communications are buffered, the multiple units of the aggregated data can be combined into fewer units in an optimization called coalescing. The Chapel Aggregation Library (CAL) provides a straight- forward approach to handling both aggregation and coalescing of data in Chapel and aims to be as generic and minimal as possible to maximize code reuse and minimize its increase in com- plexity on user applications. CAL provides a high-performance, distributed, and parallel-safe solution that is entirely written as a Chapel module. In addition to being easy to use, CAL improves the performance of some benchmarks by one to two orders of magnitude over naive implementations at 32 compute-nodes on a Cray XC50.