We have a CFD application compiled on Windows with Intel FORTRAN compiler version 220.127.116.11.
Running on Windows 10 workstation with 2 Xeon E5-2630 v3 processors @2.4 GHz, 8 physical cores each - 16 in total. 64 GB of RAM. Not hyper-threaded.
The application is OpenMP parallel, with mostly static scheduling. It is running on 16 processors utilizing them almost 100% (CPU_time/(nprocs*clock_time). While running this application, we initiate another instance of it, also running on all 16 processors. The two instances (simulations) of the application are completely independent of each other. Naturally, both use about 50% of the CPU power,
When the second simulation starts, the first one speeds up by a factor of 2 and even 2.5. As soon as the second simulation stops, the first one goes back to the original speed.
We are struggling to understand this behavior. There are no special affinity settings used in either run, using the default settings.