Brian Beesley wrote: > On Tuesday 07 November 2006 12:18, david eddy wrote: > > > Why "rare" instead of "optimal"? Well, I don't want to be the one to > > > tell someone > > > he missed a prime because his run had a not-unexpected round off error. > > > > OTOH he (or she!) was more likely to miss finding a prime because excessive > > time was wasted using the larger FFT. > > Umm. If you get the wrong residue you're wasting all your time.
This is a calculated gamble as discussed previously. ? Also if you're > cutting it too fine, you will start getting a significant rate of roundoff > errors detected, which may be recovered but "waste" time re-computing from > the last savefile. Implementing full roundoff error checking costs about the > same in terms of time "wastage" as going to the next run length. In any case > a detected roundoff error of 0.5 cannot be reliably recovered from as it's > impossible to guess accurately whether the correct integer is the "floor" or > the "ceiling". I don't think it is worthshile to perform this check at all. The chance of the longest elements (20 bits?) overflowing can surely be found accurately by theoretical and/or empirical means. The data can surely be treated as random. Just leave the rest up to God! > > > > It amounts to paying unnecessarily high insurance. > > How about informing testers of exponents at the bottom of one of > > your ranges what the risk of using the smaller FFT is, and allowing > > them to take it if they want to? > > It may be appear to be "optimal" to guess a residue and report that without > bothering to run any of the test! After all there's a 1 in 2^64 chance you > will get it right... The server wouldn't react well to this if everyone tried > it, though. You know I was not advocating this:-) > > > > BTW I replaced Prime95v24.14 with v23.18 mid run, achieving a 14% > > speed up on my PII. Any danger I have sabotaged the test? > > Should be OK, the most likely problem with a reverse upgrade is compatibility > of the savefile. George thinks I'm OK. More worryingly, I stopped the run a couple of times when it seemed a reboot was called for, and it said "error writing intermediate file rHXXXXXX". It did not go back to an earlier iteration so I am crossing my fingers and hoping this error is not fatal. Any ideas? Regards David Eddy _________________________________________________________________ Be one of the first to try Windows Live Mail. http://ideas.live.com/programpage.aspx?versionId=5d21c51a-b161-4314-9b0e-4911fb2b2e6d _______________________________________________ Prime mailing list [email protected] http://hogranch.com/mailman/listinfo/prime
