From the eddy FSL webpage
eddy_openmp: This executable has been parallelised through OpenMP, which allows eddy to use more than one core/CPU when running.
However when I run eddy_openmp within nipype I notice that not all cores are used, even though I switch on MultiProc. I am running two subjects and have 6 cores. MultiProc assigns one core per subject but does not use the remaining 4 cores.
If I run eddy_openmp in the terminal all cores are used as expected.