len  	fft time 	time/(n*log2(n)
      64 	0.00000494 	0.000000012869
     128 	0.00000965 	0.000000010775
     256 	0.00001882 	0.000000009190
     512 	0.00003832 	0.000000008317
    1024 	0.00008495 	0.000000008296
    2048 	0.00018662 	0.000000008284
    4096 	0.00058010 	0.000000011802
    8192 	0.00131693 	0.000000012366
   16384 	0.00259754 	0.000000011324
   32768 	0.00596053 	0.000000012127
   65536 	0.01373275 	0.000000013097
  131072 	0.06007272 	0.000000026960
  262144 	0.35565386 	0.000000075373
  524288 	0.80610664 	0.000000080922
 1048576 	1.66848468 	0.000000079560
 2097152 	3.54296845 	0.000000080449