On maximum achievable speeds for field solvers

Prompted by the empirical evidence that achievable flow solver speeds for large problems are limited by what appears to be a time of the order of O(0.1) sec/timestep regardless of the number of cores used, the purpose of this paper is to identify why this phenomenon occurs.

A series of timing studies, as well as in-depth analysis of memory and inter-processors transfer requirements were carried out for a typical field solver. The results were analyzed and compared to the expected performance.

The analysis shows that at present flow speeds per core are already limited by the achievable transfer rate to RAM. For smaller domains/larger number of processors, the limiting speed of CFD solvers is given by the MPI communication network.

This implies that at present, there is a “limiting useful size” for domains, and that there is a lower limit for the time it takes to update a flowfield.

For practical calculations this implies that the time required for running large-scale problems will not decrease markedly once these applications migrate to machines with hundreds of thousands of cores.

This is the first time such a finding has been reported in this context.