Guaranteed in-order packet delivery using Exclusive Dynamic Virtual Channel Allocation

Abstract

In-order packet delivery, a critical abstraction for many higher-level protocols, can severely limit the performance potential in low-latency networks (common, for example, in network-on-chip designs with many cores). While basic variants of dimension-order routing guarantee in-order delivery, improving performance by adding multiple dynamically allocated virtual channels or using other routing schemes compromises this guarantee. Although this can be addressed by reordering out-of-order packets at the destination core, such schemes incur significant overheads, and, in the worst case, raise the specter of deadlock or require expensive retransmission. We present Exclusive Dynamic VCA, an oblivious virtual channel allocation scheme which combines the performance advantages of dynamic virtual allocation with in-network, deadlock-free in-order delivery. At the same time, our scheme reduces head-of-line blocking, often significantly improving throughput compared to equivalent baseline (out-of-order) dimension-order routing when multiple virtual channels are used, and so may be desirable even when in-order delivery is not required. Implementation requires only minor, inexpensive changes to traditional oblivious dimension-order router architectures, more than offset by the removal of packet reorder buffers and logic.