Efficient unicast and multicast support for CMPs

Abstract

Beyond a certain number of cores, multi-core processing chips will require a network-on-chip (NoC) to interconnect the cores and overcome the limitations of a bus. NoCs must be carefully designed to meet constraints like power consumption, area, and ultra low latencies. Although 2D meshes with DOR (Dimension-Order-Routing) meet these constraints, the need for partitioning (e.g. virtual machines, coherency domains) and traffic isolation may prevent the use of DOR routing. Also, core heterogeneity and manufacturing and run-time faults may lead to partially irregular topologies. Routing in these topologies is complex, and previously proposed solutions required routing tables, which drastically increase power consumption, area, and latency. The exception is LBDR (Logic-Based Distributed Routing), a flexible routing method for irregular topologies that removes the need for using routing tables (both at end-nodes and switches), thus achieving large savings in chip area and power consumption. But LBDR lacks support for multicast and broadcast, which are required to efficiently support cache coherence protocols both for single and multiple coherence domains.In this paper we propose bLBDR, an efficient multicast and broadcast mechanism built on top of LBDR. bLBDR performs multicast operations using a logic-based broadcast within a domain (a region with bounds). This allows us to isolate the traffic into different domains, thus enabling the concept of virtualization at the NoC level. Also, bLBDR extends the concept of routing regions in LBDR by providing a mechanism that allows the flexible definition of multiple domains, sets of network resources.bLBDR fulfills all the practical requirements, including not only low latency and power and area efficiency, but also support for virtualization, partitionability, fault-tolerance, traffic isolation and broadcast across the entire network as well as constrained to coherency domains or regions. All this is achieved by a small and power efficient routing logic (7x area savings and 17x power reduction when compared to a routing table in an 8 × 8 mesh network).

Recursive partitioning multicast: A bandwidth-efficient routing for Networks-on-Chip

“…However, supporting multicast in NOCs has different requirements, because current NOCs have power and area constraints with high performance requirement. Most recent work on multicast routing in NOCs is Virtual Circuit Tree Multicasting [11] and bLBDR [17]. The work in [11] proposes an efficient multicast and broadcast mechanism.…”

“…This makes VCTM not scalable to large networks. bLBDR [17] enables the concept of virtualization at the NOC level and isolates the traffic into different domains. However, multicasting in bLBDR is based on broadcasting in a small domain.…”

“…In fact, cache coherence protocols heavily rely on multicast or broadcast communication characteristics to maintain ordering amongst requests [14] or to invalidate shared data spread on different caches using directory. Motivated by the importance of multicast and broadcast support, recent work proposed these functions in the design of the routers [11,17]. The key problem is to decide when and where to replicate multicast packets.…”

Efficient multicast schemes for 3-D Networks-on-Chip

“…Unfortunately, up to date, very few NoC router designs actually support multicasting [5,6,10], and even fewer for 3-D NoC-based many-core systems [11].…”

“…We call the tiles allocated to one application as subnetwork throughout the paper. Applying virtualization to 3-D NoCs faces the challenge of traffic isolation [10], i.e., communication between nodes in a virtualized sub-network shall be limited to the sub-network only. These irregular sub-networks with traffic isolation request negate the application of multicast algorithms designed for regular network topologies.…”