len  	fft time 	time/(n*log2(n)
      64 	0.00000313 	0.000000008139
     128 	0.00000452 	0.000000005045
     256 	0.00000811 	0.000000003959
     512 	0.00001910 	0.000000004145
    1024 	0.00003692 	0.000000003605
    2048 	0.00009361 	0.000000004155
    4096 	0.00018834 	0.000000003832
    8192 	0.00044736 	0.000000004201
   16384 	0.00091744 	0.000000004000
   32768 	0.00215614 	0.000000004387
   65536 	0.00436145 	0.000000004159
  131072 	0.01108437 	0.000000004975
  262144 	0.04894002 	0.000000010372
  524288 	0.09970597 	0.000000010009
 1048576 	0.23121710 	0.000000011025
 2097152 	0.49206421 	0.000000011173