# Usage of CUDA.CUFFT.cufftPlanMany

**URL:** <https://discourse.julialang.org/t/usage-of-cuda-cufft-cufftplanmany/86403>\
**Category:** GPU\
**Tags:** fft\
**Created:** [August 26, 2022, 8:26pm UTC](https://discourse.julialang.org/t/usage-of-cuda-cufft-cufftplanmany/86403 "2022-08-26T20:26:10Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![gerhardu](https://avatars.discourse-cdn.com/v4/letter/g/eb8c5e/32.png) [@gerhardu](https://discourse.julialang.org/u/gerhardu)\
**Post date:** [August 26, 2022, 8:26pm UTC](https://discourse.julialang.org/t/usage-of-cuda-cufft-cufftplanmany/86403/1 "2022-08-26T20:26:10Z")

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Hi,  
I need to calculate approx 600 FFT’s of 3 dimensional arrays (e.g. 128^3).

I know how to do this on CPUs and also how to do this sequentially on a GPU.  
By sequentially I mean that I copy one of the 600 arrays to the GPU, calculate the FFT and send it back to the host. Since the arrays are quite small, i guess i could gain a lot by using a batched FFT calculation.  
As far as I understand CUDA.CUFFT.cufftPlanMany does exactly this. But i could not figure out how to use it. Has anyone a working example in Julia?

Maybe some more info what I am doing, in case someone has a better way of solving this:  
A, B, C are Arrays of 3-dimensional Arrays

Pseudo code:  
for i in 1:600  
tmpA = ifft(A[i])  
tmpB = ifft(B[i])  
C[i] = fft(tmpA.\* (tmpB.^2 + tmpA.^2)

Thanks

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<div class="post-metadata">

**Author:** ![gerhardu](https://avatars.discourse-cdn.com/v4/letter/g/eb8c5e/32.png) [@gerhardu](https://discourse.julialang.org/u/gerhardu)\
**Post date:** [August 30, 2022, 6:32pm UTC](https://discourse.julialang.org/t/usage-of-cuda-cufft-cufftplanmany/86403/2 "2022-08-30T18:32:41Z")

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I found the solution:  
There is no need to invoke CUDA.CUFFT.cufftPlanMany. The functionality of batched fft’s is contained in julias AbstractFFT structure.

Eg if N ffts of size 128^3 need to be calculated, then one simply copies the data of the 128^3 arrays in an 3+1 dimensional array (extension in each dimension 128,128,128, N): the first one to newarray(:,:,:,1), the second one to newarray(:,:,:,2) and so forth up to newarray(:,:,:,N).  
Having assembled newarray, the next step is to simply performing the fft along the first 3 dimensions:  
fft(newarray,[1,2,3]). This automatically computes all the N fft’s.  
One then extracts the individual N 128^3 arrays from the returned 3+1 dimensional array like outlined above. Works as well on the GPU!