Intel® Math Kernel Library 11.0.2 User Guide
You may obtain a better performance of a one-dimensional Cluster FFT by enabling a low-communication Segment Of Interest FFT (SOI FFT) algorithm. To use this algorithm, you must set the MKL_CFFT_ENABLE_SOI environment variable to "1" or "yes".
While using fewer MPI communications, the SOI FFT algorithm incurs a minor loss of precision (about one decimal digit).