Hi all, A user testing exponent 15145xxx on a P4 recently got a reproducible round off error of 0.5. As we all know that error ruins the LL result. Consequently, I've changed the FFT crossover point in version 22 from 15,160,000 down to 15,140,000 on the P4.
In addition, the above uncovered a bug whereby a reproducible roundoff error greater than 0.48 would cause an infinite loop - backing up to the previous save file in hopes that the roundoff error will eventually become less than 0.48. To eliminate the problem in the future, version 22 will do round off error checking every iteration for exponents within 2% of the maximum exponent that can be tested by the current FFT length. The current version only does roundoff error checking every 128 iterations. If a reproducible roundoff error is greater than 0.45, then we restore the previous save file and shift the data one bit - this should change the FFT data so that a more acceptable roundoff is generated. What should you do if you are running version 21 on a P4? You can do one of the following: 1) Nothing. You are unlikely to run into trouble. Just keep an eye on your results.txt looking for an unending string of roundoff errors on the same iteration. 2) Turn on Advanced/Round Off Checking. This will error check every iteration instead of every 128 iterations. Again keep an eye on results.txt for unending error messages. Roundoff checking on the P4 is very fast. 3) Upgrade to version 22 (only the Windows version has the above fix). It isn't fully tested but should be safe. The only dangerous change I've made is compiling with maximum optimization to speed up the GCD code. Sorry for the trouble, George _________________________________________________________________________ Unsubscribe & list info -- http://www.ndatech.com/mersenne/signup.htm Mersenne Prime FAQ -- http://www.tasam.com/~lrwiman/FAQ-mers
