Effect of Virtual Channels and Memory Organization on Cache-Coherent Shared-Memory Multiprocessors

In this paper, performance of wormhole routed 2-D torus network with virtual channels has been evaluated for cache-coherent shared-memory multiprocessors with execution-driven simulation using various applications. The traffic in such systems is very different from the traffic in message-passing environment and is characterized by traffic bursts, one-to-many and many-to-one traffic, and small fixed length messages. We show the impact of various network parameters, such as number of virtual channels, number of flit buffers per virtual channel, and number of internal links. We have also considered low-order and high-order interleaving of memory blocks on nodes to show its impact on the network performance. The study shows that four virtual channels per link is most efficient for 2-D torus networks. The number of flit buffers per virtual channel also has a considerable impact and 2 to 4 flit buffers are usually enough. The number of internal links also has an impact on the performance for applications, such as MP3D, that generate large contention for shared variables. Larger number of internal links are also useful in case of high-order interleaved memory to reduce hot-spots at the communication interface of favorite nodes.