I'm curious, where does the roundoff checking every iteration come in? This explains how the program chooses the FFT size, but what are the conditions for when roundoff checking for every iteration is used? Based on my understanding, it seems to me that if the soft crossover calculation determines that an exponent can't be run at the smaller FFT size, then maybe it should just do the exponent at that FFT size with roundoff checking every iteration instead of moving to a larger FFT size. Does the program currently do this?
Thanks. At 17:55 14/08/02 -0400, George Woltman wrote: >Now the gotcha. In v22.8, FFT crossovers are flexible. If you test an >exponent within 0.2% of a crossover point, then 1000 sample iterations >are performed using the smaller FFT size and the average roundoff >error calculated. If the average is less than 0.241 for a 256K FFT or >0.243 for a 4M FFT, then the smaller FFT size is used. > >Brian Beesley has been a great help in investigating revised crossover >points and analyzing the distribution of round off errors. We noticed >that consecutive exponents can have a pretty big difference in average >roundoff error (e.g. one exponent could be 0.236 and the next 0.247). >This is why I elected to try the flexible approach described above. The >0.241 to 0.243 average was chosen hoping for about 1 iteration in a >million generating a roundoff error above 0.4. We might change the 0.241 >to 0.243 constants with more data - it is hard to get enough data points >to accurately measure 1 in a million occurrences. > >One downside is the server does not know which FFT size is used and >will credit you based on the v21 x87 crossovers. Thus, if you are a lucky >person, you might get "bonus" CPU credit where you test the exponent >at a smaller FFT size and the server credits you based on the larger >FFT's timing. Nick Glover [EMAIL PROTECTED] Computer Science, Clemson University "It's good to be open-minded, but not so open that your brains fall out." - Jacob Needleman _________________________________________________________________________ Unsubscribe & list info -- http://www.ndatech.com/mersenne/signup.htm Mersenne Prime FAQ -- http://www.tasam.com/~lrwiman/FAQ-mers
