PDL Abstract

Carpool: A Bu€erless On-Chip Network Supporting Adaptive Multicast and Hotspot Alleviation

In Proc. of the International Conference on Supercomputing (ICS), Chicago, IL, June 2017.

Xiyue Xiang* Wentao Shi^ Saugata Ghosez Lu Peng^ Onur Mutlu§† Nian-Feng Tzeng*

* University of Louisiana at Lafayette
^ Louisiana State University
† Carnegie Mellon University
§ETH Z¨urich


Modern chip multiprocessors (CMPs) employ on-chip networks to enable communication between the individual cores. Operations such as coherence and synchronization generate a significant amount of the on-chip network traffic, and often create network requests that have one-to-many (i.e., a core multicasting a message to several cores) or many-to-one (i.e., several cores sending the same message to a common hotspot destination core) flows. As the number of cores in a CMP increases, one-to-many and many-to-one flows result in greater congestion on the network. To alleviate this congestion, prior work provides hardware support for efficient one-to-many and many-to-one flows in buffered on-chip networks. Unfortunately, this hardware support cannot be used in bufferless on-chip networks, which are shown to have lower hardware complexity and higher energy efficiency than bu?ered networks, and thus are likely a good fit for large-scale CMPs.

We propose Carpool, the first bufferless on-chip network optimized for one-to-many (i.e., multicast) and many-to-one (i.e., hotspot) traffic. Carpool is based on three key ideas: it (1) adaptively forks multicast flit replicas; (2) merges hotspot flits; and (3) employs a novel parallel port allocation mechanism within its routers, which reduces the router critical path latency by 5.7% over a bufferless network router without multicast support. We evaluate Carpool using synthetic traffic workloads that emulate the range of rates at which multithreaded applications inject multicast and hotspot requests due to coherence and synchronization. Our evaluation shows that for an 8×8 mesh network, Carpool reduces the average packet latency by 43.1% and power consumption by 8.3% over a bufferless network without multicast or hotspot support. We also find that Carpool reduces the average packet latency by 26.4% and power consumption by 50.5% over a buffered network with multicast support, while consuming 63.5% less area for each router.