Allocate intermediate multidimensional arrays in Cython without acquiring the GIL

Disclaimer: Everything here is to be taken with a grain of salt. I'm more guessing that knowing. You should certainly ask the question on Cython-User. They are always friendly and fast to answer.

I agree that Cython's documentation is not very clear:

[...] memoryviews often do not need the GIL:

cpdef int sum3d(int[:, :, :] arr) nogil: ...

In particular, you do not need the GIL for memoryview indexing, slicing or transposing. Memoryviews require the GIL for the copy methods (C and Fortran contiguous copies), or when the dtype is object and an object element is read or written.

I think this means that passing a memory view parameter, or using it for slicing or transposing doesn't need Python GIL. However, creating a memoryview or copying one needs the GIL.

Another argument supporting this is that is is possible for a Cython function to return to Python a memory view.

from cython.view cimport array as cvarrayimport numpy as npdef bla():    narr = np.arange(27, dtype=np.dtype("i")).reshape((3, 3, 3))    cdef int [:, :, :] narr_view = narr    return narr_view

Gives:

>>> import hello>>> hello.bla()<MemoryView of 'ndarray' at 0x1b03380>

which means that the memoryview is allocated in Python's GC managed memory and thus needs the GIL to be created. So you can't cant create a memoryview in a nogil section

Now for what concerns the error message

Memoryview slices can only be shared in parallel sections

I think you should read it as "You can't have a thread private memoryview slices. It must be a thread shared memoryview slices".

http://docs.cython.org/src/userguide/external_C_code.html#releasing-the-gil

"""

Releasing the GIL

You can release the GIL around a section of code using the with nogil statement:

 with nogil:

<code to be executed with the GIL released> Code in the body of the statement must not manipulate Python objects in any way, and must

not call anything that manipulates Python objects without first re-acquiring the GIL. Cython currently does not check this.

"""