Tuesday, August 14, 2012

OpenFOAM 2.1.1 GGI / AMI Parallel Efficiency Test, Part II

These figures are pretty self-explanatory. Again, these are on single computers. My i7-2600 only has four cores, and the other data set represents a computer with 8 cores (2 x Opteron 2378). It is clear there is a significant parallelizability bottleneck (serial component) in the AMI. However, it seems this bottleneck may be controllable by minimizing AMI interface path face count, if the interface patch interpolation is a serial component.

Sunday, August 12, 2012

Connecting Two Posts: AMI Parallelizability and Hyperthreading Investiagtion



The most recent post shows that parallel efficiency stays constant with any more than one core with AMI. In an older post testing the effect of hyperthreading, efficiency went down, instead of staying constant. These results are still consistent, because the two tests were fundamentally different, one obviously using hyperthreading and the other using true parallelism. Hyperthreading makes more virtual cores, but with less physical resources per core (makes sense; you cannot get something for nothing). Since we hypothesized that the lower efficiency is due to serial component, having weaker cores will lead to a slower overall simulation, since the serial component (which can only be run on one processor) will have less resources. Thus the lower performance using hyperthreading is due to the serial component running on a weaker single core. Also, the software could have made a difference as that test was run with GGI on OpenFOAM 1.5-dev.

This has practical implications for building / choosing a computer to use with AMI simulations. One would want to choose a processor with the strongest single-core performance. Choosing a processor with larger number of cores, but weaker single cores would have a much more detrimental effect in AMI parallel simulations than one would think without considering this post. This is actually applicable to all simulations as there is always some serial component, but the lower the parallel efficiency, the more important the aforementioned guideline is.

Saturday, August 11, 2012

OpenFOAM 2.1.1 GGI / AMI Parallel Efficiency Test

A 2D incompressible turbulent transient simulation featuring rotating impellers was performed on a varying number of processors to test the parallel capability of the AMI feature in OpenFOAM. The size of the case was small, about 180,000 cells total. This ensures that memory would not be a bottleneck. The exact same case was run on different numbers of processors. The computer used had 2 x AMD Opteron 2378 (2.4 MHz), for a total of 8 cores. I am not sure what CPU specs are relevant in this case, so you can look it up. Let me know what other specs are relevant and why in the comments.
The following are the results:

 The efficiency is the most revealing metric here. There seems to be a serial component to the AMI bottle necking the performance. I will post some more results done on a different computer shortly.