A Generic Implementation of Barriers Using Optical Interconnects

Barriers have long been recognized as important performance-critical constructs in parallel applications. As a consequence, researchers have proposed fast implementations of barriers in both traditional electrical networks and in non-conventional networks such as optical NoCs. We prove in this paper that current protocols for barriers in optical NoCs are simplistic and cannot be trivially extended to accommodate for normal events that arise in regular operation such as presence of multiple applications, context switches, thread migrations, and variability in the number of active threads. We propose two generic protocols for barriers that can take all such cases into account, are fast, and try to minimize the number of messages sent over the NoC. One of these protocols is a centralized protocol (suitable for less cores), and the other is a distributed protocol, which is scalable. For a suite of standard benchmarks we found the latter to yield a mean speedup of 30.77% over a design that uses a hardware tree barrier. Our barrier implementation per se is roughly 2X and 20X faster than prior implementations that use transmission lines and electrical links respectively.