2002 - cesnet

In version 6.0, there was some news that we wished to also test in the extensivedistributed environment of MetaCentrum PC clusters. This includes the possibilityof using what is known as “dynamic load balancing”, which will allow forreasonable utilization of the available sources in case of an nonhomogeneouscomputational environment (as regards the performance).Tasks are loaded by FLUENT on individual computing nodes, each node isallocated a part of the task of an identical volume. The problematic situation(typical for MetaCentrum clusters) is when the performance of allocated computingnodes differs. The entire computation waits after each iteration in thecomputation convergence process for the weakest node and the computationis therefore delayed.With the use of the dynamic load balancing, FLUENT ensures a relative capacityof individual nodes and reorganizes task allocation, so that the computingtime is identical within individual nodes. Unfortunately, we found out that thismethod is not functional within Fluent version 6.0, notwithstanding the fact thatit requires the use of a graphic user interface, which is inconvenient for practicalcomputations of extensive tasks, administered by the batch system.Problems were also encountered with testing tasks with approx. 5 million cells,which we made out with what are known as nonconforming network interfaces.For parallel automatic loading to individual nodes, the interfaces should notcause any problems in a case where no network adaptation is carried out. Wefound out that such automatic task distribution was completed only for onemetacomputer configuration, failing elsewhere. This is why no further testingwith this task was carried out. This does not mean that it would be impossibleto compute the given type of task on PC clusters, it is only necessary to dividethis task into a particular number of computing nodes in advance. This is convenientfor the computational engineers; however, not for our testing of applicationperformance scaling.Within a single part of clusters (up to 32 processors) FLUENT reports a relativelygood scaling (linear acceleration), even for extensive tasks. After exceeding thislimit, computing times shorten slightly (for 40, 48, 56 processors); however, withhigher numbers the time increases again and with all the processors used (158),the computing time is worse than with 32 processors on one part of the cluster.The nonhomogeneity of a cluster may also prove to be disadvantageous for themetacomputer configurations, as we used computers with Intel Pentium II processors,700 MHz, as well as ADM Athlon 1.9 GHz (minos, PC cluster at ZČU).The use of high-speed networks (Myrinet, Gigabit Ethernet) is distinctive, particularlyfor the start-up of FLUENT and task loading. When we used the standardnetwork communicator (sockets), the FLUENT start-up period for a highernumber of linked computing nodes in a metacomputer (over 40 processors) isalarming. In some cases, it reached several hours (approximately 7 hours with104 High-speed National Research Network and its New Applications 2002