FFT Order Limitations?

[emhjohnson]

Hi,

I cannot seem to find documentation for the minimum and maximum FFT orders supported by IPP.

http://software.intel.com/en-us/articles/mkl-ipp-choosing-an-fft/suggests a maximum order (for DFT) of 2^29 (no mention of the range supported for FFT, aside from powers of 2).

http://software.intel.com/en-us/forums/showthread.php?t=60410suggests a maximum order of 2^27.

Is the range of supported FFT orders 2^1 ... 2^27?

Where can I find this information?

Thanks,

Eric

[emhjohnson]

To answer my own question, I used the lovely trial and error code below (and managed a segfault):

Code:

#include #include #include #include #include static const char * ippStatusErrStr(IppStatus status) { switch (status) { case ippStsNullPtrErr: return "IPP Null Pointer Error"; case ippStsFftOrderErr: return "IPP FFT Order Error"; case ippStsFftFlagErr: return "IPP FFT Flag Error"; case ippStsMemAllocErr: return "IPP Memory Allocate Error"; default: return "Unrecognized IPP Error"; }; } int main(int argc, char **argv) { int n; for (n = 1; n < 33; n++) { IppStatus status; IppsFFTSpec_R_32f *spec; status = ippsFFTInitAlloc_R_32f (&spec, n, IPP_FFT_DIV_INV_BY_N, ippAlgHintNone); if (status != ippStsNoErr) { printf ("Error for order %d: %s\n", n, ippStatusErrStr(status)); } else { printf ("Created order %d R_32f input FFT\n", n); ippsFFTFree_R_32f (spec); } } return 0; }

Output:

Created order 1 R_32f input FFT Created order 2 R_32f input FFT Created order 3 R_32f input FFT Created order 4 R_32f input FFT Created order 5 R_32f input FFT Created order 6 R_32f input FFT Created order 7 R_32f input FFT Created order 8 R_32f input FFT Created order 9 R_32f input FFT Created order 10 R_32f input FFT Created order 11 R_32f input FFT Created order 12 R_32f input FFT Created order 13 R_32f input FFT Created order 14 R_32f input FFT Created order 15 R_32f input FFT Created order 16 R_32f input FFT Created order 17 R_32f input FFT Created order 18 R_32f input FFT Created order 19 R_32f input FFT Created order 20 R_32f input FFT Created order 21 R_32f input FFT Created order 22 R_32f input FFT Created order 23 R_32f input FFT Created order 24 R_32f input FFT Created order 25 R_32f input FFT Created order 26 R_32f input FFT Created order 27 R_32f input FFT Error for order 28: IPP Memory Allocate Error Segmentation fault

-Eric

[Mesbah__Samy]

I had a 32 bit app that I converted to x64 and instead of windowing smaller FFTs for most data I need I can use a single FFT that is 27th order, sometimes 28th or 29th.

I am using VS2019 C++, IPP 2019 update 5, dynamically linked.

I am using the same build setup on my office PC (win 10 pro 1903, core i7-7700, 32GB RAM) and home PC (win 10 pro 1903, core i5-2500, 16GB RAM) and the same release compiles on both machines as well.

Here is the strange part:

The initial call to set up the 64f FFT: 

			status = ippsFFTGetSize_R_64f(useOrder, IPP_FFT_DIV_INV_BY_N, ippAlgHintAccurate, &specSize, &specBufferSize, &scratchBufferSize);

The lesser spec machine (core i5-2500) allows a maximum FFT order of 28, after which status is -17 (invalid FFT order)

The higher spec machine (core i7-7700) allows a maximum FFT order of 27, after which status is -17.

Is there a rationale for why a 7th gen processor would have this limitation compared to a 2nd gen one?  Also I know "ippAlgHintAccurate" is ignored for x64 since it wont ever use the FPU but I also have win32 configs of the same project which can work with smaller windows.

I am curious what determines the max FFT order, and quite frankly for x64 this shouldn't even be limited to 27/28.  On the plus side even a 28th order FFT on a core i5-2500 is extremely fast.

[Andrey_B_Intel]

Hi Samy.

I confirm your issue. The  "Core i5-2500" has AVX instruction set while "Core i7-7700" has AVX2. IPP has two branches for them and avx2 supports 2^27 only.  We will fix the bug in one on the next IPP releases to support 2^28. As a workaround, you can manually switch IPP to run AVX code on "Core i7-7700" but it may affect performance.

    Ipp64u defaultFeatures;
    ippGetCpuFeatures(&defaultFeatures, 0);
    defaultFeatures &= (ippCPUID_AVX2-1);
    status =  ippSetCpuFeatures(defaultFeatures);

Thanks for your feedback.