Original ticket http://projects.scipy.org/scipy/ticket/892 on 2009-03-16 by @sturlamolden, assigned to unknown.

SciPy's interface to fftpack is different from NumPy's fftpack_lite. One notable difference is that SciPy's uses a cache in C, whereas NumPy uses a Python dict for the same purpose. SciPy's cache is accessed directly from the C wrappers to FFTPACK. This means that SciPy's FFTs are not threadsafe, and thus the GIL cannot be released. Thus, SciPy has to lock up the interpreter while doing FFTs.

Using a Python container with ndarrays as cache would allow the GIL to be released when calling FFTPACK (cf. ticket gh-1582 for NumPy).