We investigate two interconnection techniques that would allow the scaling of a Beowulf class system to unprecedented sizes (hundreds or thousands of processors). The first is a commercially available high-speed backplane with fast-ethernet and gigabit ethernet switch modules. The second is a network that combines point-to-point connections and commodity switches, employing the Beowulf nodes as IP routers as well as computational engines. The first method has the drawback of a relatively high price tag - a significant but not overwhelming fraction of the cost of the system must be devoted to the switch. The second approach is inexpensive, but introduces extra latency and may restrict bandwidth depending on the topology employed. Our tests are designed to quantify these effects.
We perform three sets of tests. Low-level tests measure the hardware performance of the individual components, i.e., bandwidths and latencies through various combinations of network interface cards, node-based routers and switches. System-level tests measure the bisection bandwidth and the ability of the system to communicate and compute simultaneously, under conditions designed to generate contention and heavy loads. Finally, application benchmarks measure the performance delivered to a subset of the NAS parallel benchmark suite.
Low-level measurements indicate that latencies are only weakly affected by introduction of node-based routers, and that bandwidths are completely unaffected. Switches introduce between 19 and 30 microseconds of latency and linux-based routers introduce 75 microseconds.
System level measurements indicate that even under load, the routed network delivers a bisection bandwidth near what one would expect by simply summing the capacities of the individual components. A very large scatter is introduced into latencies, however.
Application level measurements reveal that the routed network is acceptable for some applications, but can have significant deleterious effect on others. Successful use of routed networks for very large beowulf systems will requires a deeper understanding of this issue.
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