len  	fft time 	time/(n*log2(n)
      64 	0.00000401 	0.000000010433
     128 	0.00000817 	0.000000009113
     256 	0.00001557 	0.000000007601
     512 	0.00003438 	0.000000007461
    1024 	0.00008035 	0.000000007847
    2048 	0.00016475 	0.000000007313
    4096 	0.00035014 	0.000000007124
    8192 	0.00098588 	0.000000009257
   16384 	0.00174352 	0.000000007601
   32768 	0.00481598 	0.000000009798
   65536 	0.01124780 	0.000000010727
  131072 	0.04224559 	0.000000018959
  262144 	0.05636219 	0.000000011945
  524288 	0.13526216 	0.000000013579
 1048576 	0.27629685 	0.000000013175
 2097152 	0.54814117 	0.000000012446