------- Forwarded message follows ------- From: "Brian J. Beesley" <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Date sent: Fri, 22 Jun 2001 18:46:43 -0000 Subject: Re: Mersenne: Factoring on a P4 Copies to: [EMAIL PROTECTED] On 22 Jun 2001, at 13:12, [EMAIL PROTECTED] wrote: > For some reason, I am at a loss to explain, a v21 P4 1.4 GHz > factors significantely slower that a P3 v20 700MHz. Is there a > reason, and solution, for this? Good question. AFAIK George has done nothing to the factoring code. You will see a big "speed loss" if you compare factoring under 2^64 with factoring over 2^64 on _any_ system - that's simply explained by triple- precision integer arithmetic being much slower than double-precision integer arithmetic. Intel's development for the P4 was very much geared towards making SSE2 work well. Unfortunately this left less room in the silicon for some of the "ordinary" integer stuff on which the factoring code (but not the LL testing code) in Prime95 depends. If memory serves me right, the 32 bit by 32 bit integer multiply instruction was badly hit by this. Consequently the factoring throughput of a P4 would be expected to be considerably less than that of a PIII running at the same clock speed. I would expect that a PIII-700 and a P4-1400 would probably run factoring at about the same speed. Earlier I wrote: For now, those who are lucky enough to have P4 systems are probably best advised to stick to LL testing (and trial factoring - which will not be affected by any inefficiency in the P4 integer instructions), leaving trial factoring to those with slower systems. Slip of the brain, I'm afraid. I meant "LL testing (and P-1 factoring..." Incidentally ECM ought to run pretty well on the P4, though there may be some more optimization to come for the very small run lengths typically used by ECM. Regards Brian Beesley _________________________________________________________________________ Unsubscribe & list info -- http://www.scruz.net/~luke/signup.htm Mersenne Prime FAQ -- http://www.tasam.com/~lrwiman/FAQ-mers
