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.
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.
Conversely, if you need a system with optimal integer performance,
you'd be much better advised to buy a system based on a fast Athlon
processor than a system based on a Pentium 4. Athlons running integer
code even run much cooler; the FPU turns itself off when it isn't
needed, leading to a big drop in current consumption.
BTW there was an "unreasonable" acceleration of trial factoring
between the P5 architecture (Pentium Classic/MMX) and the P6
architecture (Pentium Pro / PII / PIII / Celeron / Xeon), so you
can't simply assume that Intel doesn't care about integer
performance!
Regards
Brian Beesley
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