Top 5 Developments in Network Hardware of Cluster Computing
Larger systems can be formed with a hierarchy of switches to form a tree structure. The scale of tree-based clusters is limited by the bi-section bandwidth of the root node of the tree topology. More complex network structures permit the implementation of larger systems. Among them is the CLOS network (also referred to as the fat-tree) that overcomes the deficiency of the tree topology by providing multiple channels in parallel, balanced to keep the cross-section bandwidth equal at each level in the tree. Mesh and toroidal topologies provide scalable bandwidth and locality of interconnect with fixed degree nodes but may experience relatively high latency across the system diameter.
Variations on these and other network topologies are possible and depend on requirements of a given system. A few of the most widely used network technologies used in commodity clusters are described next.
is the most widely used network for clusters, even today, although devised as a LAN and originated in the late 1970s. Its success is due in part to its repeated reinvention, which takes advantage of technology advances while meeting expanding requirements. The 10-Mbps Ethernet that was first used in Beowulf clusters in the early 1990s superceded early Ethernet at 3 Mbps. Fast Ethernet provided 100 Mbps and with low-cost switches is the mainstay of small low-cost Beowulf-class systems.
Gigabit Ethernet, as the name implies, provides a peak bandwidth of approximately 1 Gbps. But its per-node cost remains high and it suffers from the relatively long latencies of its predecessors.
was one of the first networks to be developed expressly for the SAN and cluster market. With a cost of approximately $1600 per node, Myrinet was initially reserved for the more expensive workstation clusters. But with its superior latency properties of 20μsec or less, it permitted some classes of more tightly coupled applications to run efficiently that would perform poorly on Ethernet-based clusters. More recently, reduced pricing has expanded its suitability to lower cost systems and has proven very popular.
is a recent advance in cluster network technology involving improvements in both hardware and software to further reduce data communication latency. Typically, message packets are copied from the user application space into the operating system space or vice versa. VIA (virtual interface architecture) employs a zero-copy protocol, avoiding the O/S intermediate stage and moving the packets directly between the network transport layer and the application. Giganet’s cLAN and Compaq’s Server net II both implement the VIA standard, delivering best case latencies well below 10μsec.
was perhaps the first SAN to achieve IEEE standardization and has very good bandwidth and latency characteristics. Existing implementations provide between 3.2- and 8-Gbps peak bandwidth with best latencies below 4μsec. The SCI standard includes protocol for support of distributed shared memory operation. However, most clusters employing SCI use PCI-compatible network control cards (e.g., Dolphin) that cannot support cross-node cache coherence. Nonetheless, even in distributed memory clusters, it provides an effective network infrastructure.
is the next-generation interconnection technology to extend the capabilities of SANs. Although not yet available, an industrial consortium of major computer technology (hardware and software) manufacturers has developed and released an extensive specification that will lead first to reference implementations, and eventually to widely distributed products. Bandwidths up to 12 Gbps (employing optical channels) and latencies approaching 1μsec will become possible with Infiniband, which replaces previous I/O buses (e.g., PCI) and migrates the network interconnect closer to the memory bus of the compute node.
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