R. David Murray added the comment: To clarify the GIL issue (for davin, I guess? :): if the library you are using to interface with the FORTRAN code drops the GIL before calling the FORTRAN, then you *can* take advantage of multiple cores. It is only the python code (and some of the code interacting with the python objects) that is limited to executing on one core at a time. (As far as I know it isn't restricted to be the *same* core unless you set CPU affinity somehow, and I have no idea if it improves performance to use CPU affinity or not). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue27422> _______________________________________