16 FFT 非常小。您会发现小于 64 的 FFT 将是硬编码的汇编程序，没有循环以获得尽可能高的性能。这意味着它们很容易受到指令集、编译器优化甚至 64 位或 32 位字的变化的影响。
当您以 2 的幂对 16 -> 1048576 的 FFT 大小进行测试时会发生什么？我说这是因为 Linux 上特定的硬编码 asm 例程可能不是最适合您的机器的优化程序，而您可能很幸运地在 Windows 上实现了该特定大小。比较此范围内的所有大小，您可以更好地了解 Linux 与 Windows 的性能。
您是否校准过 FFTW？当第一次运行时，FFTW 猜测每台机器最快的实现，但是如果你有特殊的指令集，或者特定大小的缓存或其他处理器特性，那么这些会对执行速度产生巨大的影响。因此，执行校准将测试各种 FFT 例程的速度，并为您的特定硬件选择最快的每个大小。校准涉及重复计算计划并保存生成的 FFTW“智慧”文件。然后可以重新使用保存的校准数据（这是一个漫长的过程）。我建议在您的软件启动时执行一次并每次重新使用该文件。我注意到校准后某些尺寸的性能提高了 4-10 倍！
下面是我用来为某些尺寸校准 FFTW 的代码的 sn-p。请注意，此代码是从我工作的 DSP 库中逐字粘贴的，因此某些函数调用特定于我的库。我希望 FFTW 特定的电话会有所帮助。
// Calibration FFTW
void DSP::forceCalibration(void)
{
// Try to import FFTw Wisdom for fast plan creation
FILE *fftw_wisdom = fopen("DSPDLL.ftw", "r");
// If wisdom does not exist, ask user to calibrate
if (fftw_wisdom == 0)
{
int iStatus2 = AfxMessageBox("FFTw not calibrated on this machine."\
"Would you like to perform a one-time calibration?\n\n"\
"Note:\tMay take 40 minutes (on P4 3GHz), but speeds all subsequent FFT-based filtering & convolution by up to 100%.\n"\
"\tResults are saved to disk (DSPDLL.ftw) and need only be performed once per machine.\n\n"\
"\tMAKE SURE YOU REALLY WANT TO DO THIS, THERE IS NO WAY TO CANCEL CALIBRATION PART-WAY!",
MB_YESNO | MB_ICONSTOP, 0);
if (iStatus2 == IDYES)
{
// Perform calibration for all powers of 2 from 8 to 4194304
// (most heavily used FFTs - for signal processing)
AfxMessageBox("About to perform calibration.\n"\
"Close all programs, turn off your screensaver and do not move the mouse in this time!\n"\
"Note:\tThis program will appear to be unresponsive until the calibration ends.\n\n"
"\tA MESSAGEBOX WILL BE SHOWN ONCE THE CALIBRATION IS COMPLETE.\n");
startTimer();
// Create a whole load of FFTw Plans (wisdom accumulates automatically)
for (int i = 8; i <= 4194304; i *= 2)
{
// Create new buffers and fill
DSP::cFFTin = new fftw_complex[i];
DSP::cFFTout = new fftw_complex[i];
DSP::fconv_FULL_Real_FFT_rdat = new double[i];
DSP::fconv_FULL_Real_FFT_cdat = new fftw_complex[(i/2)+1];
for(int j = 0; j < i; j++)
{
DSP::fconv_FULL_Real_FFT_rdat[j] = j;
DSP::cFFTin[j][0] = j;
DSP::cFFTin[j][1] = j;
DSP::cFFTout[j][0] = 0.0;
DSP::cFFTout[j][1] = 0.0;
}
// Create a plan for complex FFT.
// Use the measure flag to get the best possible FFT for this size
// FFTw "remembers" which FFTs were the fastest during this test.
// at the end of the test, the results are saved to disk and re-used
// upon every initialisation of the DSP Library
DSP::pCF = fftw_plan_dft_1d
(i, DSP::cFFTin, DSP::cFFTout, FFTW_FORWARD, FFTW_MEASURE);
// Destroy the plan
fftw_destroy_plan(DSP::pCF);
// Create a plan for real forward FFT
DSP::pCF = fftw_plan_dft_r2c_1d
(i, fconv_FULL_Real_FFT_rdat, fconv_FULL_Real_FFT_cdat, FFTW_MEASURE);
// Destroy the plan
fftw_destroy_plan(DSP::pCF);
// Create a plan for real inverse FFT
DSP::pCF = fftw_plan_dft_c2r_1d
(i, fconv_FULL_Real_FFT_cdat, fconv_FULL_Real_FFT_rdat, FFTW_MEASURE);
// Destroy the plan
fftw_destroy_plan(DSP::pCF);
// Destroy the buffers. Repeat for each size
delete [] DSP::cFFTin;
delete [] DSP::cFFTout;
delete [] DSP::fconv_FULL_Real_FFT_rdat;
delete [] DSP::fconv_FULL_Real_FFT_cdat;
}
double time = stopTimer();
char * strOutput;
strOutput = (char*) malloc (100);
sprintf(strOutput, "DSP.DLL Calibration complete in %d minutes, %d seconds\n"\
"Please keep a copy of the DSPDLL.ftw file in the root directory of your application\n"\
"to avoid re-calibration in the future\n", (int)time/(int)60, (int)time%(int)60);
AfxMessageBox(strOutput);
isCalibrated = 1;
// Save accumulated wisdom
char * strWisdom = fftw_export_wisdom_to_string();
FILE *fftw_wisdomsave = fopen("DSPDLL.ftw", "w");
fprintf(fftw_wisdomsave, "%s", strWisdom);
fclose(fftw_wisdomsave);
DSP::pCF = NULL;
DSP::cFFTin = NULL;
DSP::cFFTout = NULL;
fconv_FULL_Real_FFT_cdat = NULL;
fconv_FULL_Real_FFT_rdat = NULL;
free(strOutput);
}
}
else
{
// obtain file size.
fseek (fftw_wisdom , 0 , SEEK_END);
long lSize = ftell (fftw_wisdom);
rewind (fftw_wisdom);
// allocate memory to contain the whole file.
char * strWisdom = (char*) malloc (lSize);
// copy the file into the buffer.
fread (strWisdom,1,lSize,fftw_wisdom);
// import the buffer to fftw wisdom
fftw_import_wisdom_from_string(strWisdom);
fclose(fftw_wisdom);
free(strWisdom);
isCalibrated = 1;
return;
}
}
秘诀是使用 FFTW_MEASURE 标志创建计划，该标志专门测量数百个例程，以便为您的特定 FFT 类型（真实、复杂、1D、2D）和大小找到最快的：
DSP::pCF = fftw_plan_dft_1d (i, DSP::cFFTin, DSP::cFFTout,
FFTW_FORWARD, FFTW_MEASURE);
最后，所有基准测试还应在执行之外的单个 FFT 计划阶段执行，从在发布模式下编译的代码中调用，并优化并与调试器分离。基准测试应该在具有数千（甚至数百万）次迭代的循环中执行，然后取平均运行时间来计算结果。正如您可能知道的那样，计划阶段需要大量时间，并且执行旨在通过一个计划执行多次。