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31.6.2 Optimizing the Parallel Solver



Increasing the Report Interval


In FLUENT, you can reduce communication and improve parallel performance by increasing the report interval for residual printing/plotting or other solution monitoring reports. You can modify the value for Reporting Interval in the Iterate panel.

Solve $\rightarrow$ Iterate...

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Note that you will be unable to interrupt iterations until the end of each report interval.



Load Balancing


A dynamic load balancing capability is available in FLUENT. The principal reason for using parallel processing is to reduce the turnaround time of your simulation, ideally by a factor proportional to the collective speed of the computing resources used. If, for example, you were using four CPUs to solve your problem, then you would expect to reduce the turnaround time by a factor of four. This is of course the ideal situation, and assumes that there is very little communication needed among the CPUs, that the CPUs are all of equal speed, and that the CPUs are dedicated to your job. In practice, this is often not the case. For example, CPU speeds can vary if you are solving in parallel on a cluster that includes nodes with different clock speeds, other jobs may be competing for use of one or more of the CPUs, and network traffic either from within the parallel solver or generated from external sources may delay some of the necessary communication among the CPUs.

If you enable dynamic load balancing in FLUENT, the load across the computational and networking resources will be monitored periodically. If the load balancer determines that performance can be improved by redistributing the cells among the compute nodes, it will automatically do so. There is a time penalty associated with load balancing itself, and so it is disabled by default. If you will be using a dedicated homogeneous resource, or if you are using a heterogeneous resource but have accounted for differences in CPU speeds during partitioning by specifying a load distribution (see Section  31.5.7), then you may not need to use load balancing.

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Note that when the shell conduction model is used, you will not be able to turn on load balancing.

To enable and control FLUENT's automatic load balancing feature, use the Load Balance panel (Figure  31.6.1). Load balancing will automatically detect and analyze parallel performance, and redistribute cells between the existing compute nodes to optimize it.

Parallel $\rightarrow$ Load Balance...

Figure 31.6.1: The Load Balance Panel
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The procedure for using load balancing is as follows:

1.   Turn on the Load Balancing option.

2.   Select the bisection method to create new grid partitions in the Partition Method drop-down list. The choices are the techniques described in Section  31.5.5. As part of the automatic load balancing procedure, the grid will be repartitioned into several small partitions using the specified method. The resulting partitions will then be distributed among the compute nodes to achieve a more balanced load.

3.   Specify the desired Balance Interval. When a value of 0 is specified, FLUENT will internally determine the best value to use, initially using an interval of 25 iterations. You can override this behavior by specifying a non-zero value. FLUENT will then attempt to perform load balancing after every N iterations, where N is the specified Balance Interval. You should be careful to select an interval that is large enough to outweigh the cost of performing the load balancing operations.

Note that you can interrupt the calculation at any time, turn the load balancing feature off (or on), and then continue the calculation.

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If problems arise in your computations due to adaption, you can turn off the automatic load balancing, which occurs any time that mesh adaption is performed in parallel.

To instruct the solver to skip the load balancing step, issue the following Scheme command:

        (disable-load-balance-after-adaption)

To return to the default behavior use the following Scheme command:

        (enable-load-balance-after-adaption)


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