Question

Is it okay to create a single FFTSetup data structure and use it to perform multiple FFT computations concurrently? Would something like the following work?

FFTSetup fftSetup = vDSP_create_fftsetup(
                                         16,         // vDSP_Length __vDSP_log2n,
                                         kFFTRadix2  // FFTRadix __vDSP_radix
                                         );
NSAssert(fftSetup != NULL, @"vDSP_create_fftsetup() failed to allocate storage");

for (int i = 0; i < 100; i++)
{
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
  ^{
    vDSP_fft_zrip(
                  fftSetup,              // FFTSetup __vDSP_setup,
                  &(splitComplex[i]),    // DSPSplitComplex *__vDSP_ioData,
                  1,                     // vDSP_Stride __vDSP_stride,
                  16,                    // vDSP_Length __vDSP_log2n,
                  kFFTDirection_Forward  // FFTDirection __vDSP_direction
                  );
  });
}

I suppose that the answer depends on the following considerations:

1) Does vDSP_fft_zrip() only access the data within fftSetup (or the data pointed to by it) in a “read-only” fashion? Or are there perhaps some temporary buffers (scratch space) within fftSetup that is written to by vDSP_fft_zrip() in performing its FFT computations?

2) If data like that in fftSetup is being accessed in a “read-only” fashion, is it okay for multiple processes/threads/tasks/blocks to access it simultaneously? (I am thinking of the case where it is possible for more than one process to open the same file for reading, though not necessarily for writing or appending. Is this analogy appropriate?)

On a related note, just how much memory is being taken up by the FFTSetup data structure? Is there any way to find out? (It is an opaque data type.)

Answer 1

You may create one FFT setup and use it repeatedly and concurrently. This is the intended use. (I am the author of the current implementations of vDSP_fft_zrip and other FFT implementations in vDSP.)