[Numpy-discussion] numpy ndarray questions

David Cournapeau david@ar.media.kyoto-u.ac...
Mon Jan 26 22:54:23 CST 2009

Jochen wrote:
> On Tue, 2009-01-27 at 13:28 +0900, David Cournapeau wrote:
>> Jochen wrote:
>>> On Tue, 2009-01-27 at 12:49 +0900, David Cournapeau wrote:
>>>> Jochen wrote:
>>>>> Hi all,
>>>>> I just wrote ctypes bindings to fftw3 (see
>>>>> http://projects.scipy.org/pipermail/scipy-user/2009-January/019557.html
>>>>> for the post to scipy). 
>>>>> Now I have a couple of numpy related questions:
>>>>> In order to be able to use simd instructions I 
>>>>> create an ndarray subclass, which uses fftw_malloc to allocate the
>>>>> memory and fftw_free to free the memory when the array is deleted. This
>>>>> works fine for inplace operations however if someone does something like
>>>>> this:
>>>>> a = fftw3.AlignedArray(1024,complex)
>>>>> a = a+1
>>>>> a.ctypes.data points to a different memory location (this is actually an
>>>>> even bigger problem when executing fftw plans), however 
>>>>> type(a) still gives me <class 'fftw3.planning.AlignedArray'>.
>>>> I can't comment about subclassing ndarrays, but I can give you a hint
>>>> about aligned allocator problem: you could maintain two list of cached
>>>> plans, automatically detect whether your arrays are aligned or not, and
>>>> use the appropriate list of plans; one list is for aligned arrays, one
>>>> for unaligned. Before removing support for fftw, I played with some C++
>>>> code to do exactly that. You can tell fftw to create plans for unaligned
>>>> arrays by using FFTW_UNALIGNED flag:
>>>> http://projects.scipy.org/scipy/scipy/browser/branches/refactor_fft/scipy/fftpack/backends/fftw3/src/zfft.cxx
>>>> cheers,
>>>> David
>>> Hi David,
>>> I have actually kept more closely to the fftw way of doing things, i.e.
>>> I create a plan python object from two arrays, it also stores the two
>>> arrays to prevent someone to delete the original arrays and then
>>> executing the plan.
>> I am not sure I follow you when you say the "fftw way": you can avoid
>> having to store the arrays altogether while still using fftw plans,
>> there is nothing "unfftw" about using plans that way. I think trying to
>> guarantee that your arrays data buffers won't change is more complicated.
>> David
> Sorry maybe I wasn't quite clear, what I mean by the "fftw way" is
> creating a plan and executing the plan, instead of doing x=fft(y).

I guess I was not very clear, because my suggestion has nothing to do
with the above.

>  As
> you say the problem comes when you execute a plan on arrays which don't
> exist anymore, which causes python to segfault (I'm talking about using
> fftw_execute() not fftw_execute_dft). So yes essentially my problem is
> trying to ensure that the buffer does not change. 

I agree that if you just use fftw_execute, you will have segfaults: I am
just not sure that your problem is to ensure that the buffer does not
change :) You can instead handle relatively easily the case where the
buffers are not aligned, while still using fftw API. The fftw_execute is
just not an appropriate API for python code, IMHO.

Another solution would be to work on numpy itself, so that it use
aligned buffers, but that's obviously much longer term - that's just one
of those things on my TODO list that I never took the time to do,


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