Required buffer for cuFFT

Question

This question is about the buffer required by cuFFT. In the User Guide it is documented that

In the worst case, the CUFFT Library allocates space for 8*batch*n[0]*..*n[rank-1] cufftComplex or cufftDoubleComplex elements (where batch denotes the number of transforms that will be executed in parallel, rank is the number of dimensions of the input data (see Multidimensional transforms) and n[] is the array of transform dimensions) for single and doubleprecision transforms respectively.

What does "array of transform dimensions" mean? How much buffer does cuFFT need? What I understand with the above is that it needs at least 8x the size of the array being FFTed but this does not make sense to me

Thanks in advance

Daniel

Answer 1

The "array of transform dimensions" is the array containing the problem size in each dimension, see the section on multidimensional transforms for more information.

cuFFT is allocating temporary space to be able to accommodate the intermediate data, the part of the doc you quoted says this is "the worst case", so it's not "at least 8x", it's at most. The doc goes on to say:

Depending on the configuration of the plan, less memory may be used. In some specific cases, the temporary space allocations can be as low as 1*batch*n[0]*..*n[rank-1] cufftComplex or cufftDoubleComplex elements.

So for a NxM 2D single precision transform:

1*N*M*sizeof(cufftComplex) <= space for tmp data <= 8*N*M*sizeof(cufftComplex)

Answer 2

Use cufftGetSize1d and cufftEstimate1d to give you the amount of memory allocated for the buffer. The documentation says cufftPlan1d gives an estimation of the maximum amount and cufftGetSize1d provide a more precise estimation.

In my case I use both 64 and 8192 point FFTs. I get the same issue, the buffer size allocate only 1*batch*n[0] elements.I've made the test with different amount of data and different FFT size and I get this same value.

To conclude, if you need to determine the memory used by a FFT, the CuFFT library provide a fonction to do this.