len  	fft time 	time/(n*log2(n)
      64,	0.00000551 	0.000000014342
     128,	0.00000892 	0.000000009961
     256,	0.00001676 	0.000000008185
     512,	0.00003463 	0.000000007516
    1024,	0.00007756 	0.000000007574
    2048,	0.00018117 	0.000000008042
    4096,	0.00057088 	0.000000011615
    8192,	0.00126077 	0.000000011839
   16384,	0.00279449 	0.000000012183
   32768,	0.00597813 	0.000000012163
   65536,	0.01398614 	0.000000013338
  131072,	0.03831381 	0.000000017195
  262144,	0.50278681 	0.000000106554
  524288,	1.27478409 	0.000000127971
 1048576,	2.76363756 	0.000000131781
 2097152,	5.83232562 	0.000000132432