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I see that you are looking for the performance difference while running the 2D double complex FFT with a different number of cores. In this case, if you are referring to Intel® Math Kernel Library 2018 – C I would recommend referring to https://software.intel.com/en-us/mkl-developer-reference-c-getting-help-and-support for better assistance on the case.
You could also refer to https://software.intel.com/en-us/support to create an account Intel® developer zone.
Hope his help.