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Old 2013-10-14, 20:00   #11
VBCurtis
 
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"Curtis"
Feb 2005
Riverside, CA

22·23·47 Posts
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The estimates for # of relations in the factmsieve script are not very accurate- it uses an exponential with size of number as input, while the actual situation is more like a step function with the steps located where lpbr jumps bits. Serge (Batalov) posted a patch to fix this in some thread- I'll see if I can find and link it.

Actual rels needed is roughly 21M for 28-bit projects, 40M for 29-bit projects, and (I am told) the low 80s for 30-bit projects. Some polys produce more or fewer duplicate relations, and sometimes a matrix builds with fewer-than-typical relations, so there is a fair amount of variety from project to project.

My vague grasp of the "hiccup" mentioned above is that skew alters the area sieved for each special-Q, with higher skews associated with smaller areas. Very high skews may require more special-Q to be searched, and that requirement can lead to using special-Q higher than a poly's efficient sieve range. So it's not that reaching 120% of the script's expected rels is a hiccup- it's that the last few rels might be found at a rate half (or worse?) of the sec/rel you achieved during most of the sieving. If you read through threads of large forum-team-factorizations, you'll see discussions amounting to "we've run out of good Q, now what?". High-skew polys run out of good Q more often than low-skew polys.

At single-user-size projects, we can preempt this problem by choosing the next-higher siever version for a project we're even slightly nervous about. We might choose to use 14e instead of 13e for projects a few digits lower than the script's cutoff; in fact, I edited my factmsieve code to shift the cutoff a bit lower. As Mr Womack points out, this makes factorizations in the 135 to 150 digit level more fire-and-forget at the expense of a few hours of sieving. I have not yet attempted a 30-bit project myself to know how much of this extends upward.
-Curtis

Last fiddled with by VBCurtis on 2013-10-14 at 20:02 Reason: added info in first paragraph
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