20090225, 10:05  #1 
"Frank <^>"
Dec 2004
CDP Janesville
832_{16} Posts 
Advice on parameters for a c131 GGNFS
I am contemplating a GGNFS job on a c131. Looking at the "defnmparams" file I have (admittedly an old copy) this is the line for 131 digits:
Code:
131,1468,1,5,7.55E+019,1.73E+018,8.41E+015,5.13E011 The only data point I have in this range is a c130 that I did ~4 years ago. The poly I used (using the factlat.pl script) looked like this: Code:
# Murphy_E 7.02e011 rlim: 6000000 alim: 6000000 lpbr: 27 lpba: 27 mfbr: 52 mfba: 52 rlambda: 2.9 alambda: 2.9 
20090225, 14:38  #2 
"Ben"
Feb 2007
110100001101_{2} Posts 
For the C131 cofactor of 36389^411 I used those exact parameters and searched with a 1500 to A 2000. Found a 7.03e11 murphy_E in that range which was quite a bit better than the 6.1e11 that I expected to find.
For the poly I used: Code:
alim: 7500000 rlim: 7500000 lpbr: 27 lpba: 27 mfbr: 54 mfba: 54 alambda: 2.6 rlambda: 2.6 hope this helps,  ben. 
20090225, 14:41  #3 
(loop (#_fork))
Feb 2006
Cambridge, England
6,323 Posts 
Time the same range of 10000 Q with alim=rlim=6M, alim=rlim=7.5M, alim=rlim=9M, alim=rlim=12M and pick whichever one's fastest, but I suspect there won't be that much of a difference.

20090225, 17:23  #4 
Oct 2006
vomit_frame_pointer
164_{16} Posts 
Don't ignore norm and alpha
On a search for a gnfs C158 polynomial the past couple of weeks, I found a couple with Murphy scores above 1.8e12, 4 between 1.7e12 and 1.8e12, and several more above 1.6e12. I picked the top 8 Murphy scores, plus another 6 which had the largest absolute values for Alpha or had tantalizingly low norms.
The top Murphy score of 1.85e12 did belong to the best polynomial: it sieved faster than the others in all ranges. But the picture was less clear for the remaining polynomials: a couple of those with Murphy values of 1.62e12 or so were clearly better choices than the 1.7e12, and one with 1.64e12 turned out to be the second best polynomial of the whole bunch. It had an alpha of 7.32 and a surprisingly low norm given the size of its leading coefficient. Scan for polynomials with decent Murphy score combined with good alpha and low norm  they can beat polynomials with higher Murphy scores. 24 hours should be enough time for a C131 search. Last fiddled with by FactorEyes on 20090225 at 17:24 
20090225, 18:15  #5 
Nov 2008
2·3^{3}·43 Posts 
Why has nobody suggested using msieve for the poly search?

20090225, 21:22  #6 
Tribal Bullet
Oct 2004
3×1,163 Posts 
Al asked me about this when he started the search; I told him that the msieve code can really only handle inputs up to 155 digits, and will use the 155digit parameters for this 158digit input (i.e. will probably not find any good polynomials).
I don't trust the code to find good polynomials for such large inputs; really a lot of tuning and experimentation is needed to make msieve a reasonable alternative to pol5 at this input size. v1.40 will go a long way towards achieving that for the pol51opt side of things, but the initial stages will still need a lot of work. PS: msieve can pretty well for a C131, but starts to do worse as the input size goes up and the code explores less and less of the search space. In any case, all the same test sieving considerations apply; usually the best polynomial has a very good root score and very good size, but worse polynomials may only have one or the other, and it's not clear which polynomial would do better in that case Last fiddled with by jasonp on 20090225 at 22:16 
20090225, 22:49  #7 
(loop (#_fork))
Feb 2006
Cambridge, England
6,323 Posts 
I have tweaked the parameter choices slightly; at the 159digit level I've got a fair number of polynomials, including scores up to 1.47e15, with
bound1 = 2e24 bound2 = 2e22 score_lo = 1e15 With what I think were the default parameters (!) I found a 1.67e15 polynomial, and I should probably have been sieving with that for the past few weeks rather than continuing to hunt ... 
20090225, 23:36  #8  
"Frank <^>"
Dec 2004
CDP Janesville
832_{16} Posts 
Thanks for all the advice. What brought this job up was an aliquot sequence result I discovered on Wieb Bosma's webpage:
Quote:
A job this big would probably take ~3 weeks for me, since I've only got one machine right now that can handle something this big, although maybe I could save some hours with some more pretesting rather than running the job as fireandforget... What about you, Tom, got any spare horsepower? Last fiddled with by schickel on 20090612 at 06:42 

20090226, 02:27  #9  
Oct 2006
vomit_frame_pointer
2^{2}×89 Posts 
Quote:
Quote:
Last fiddled with by FactorEyes on 20090226 at 02:28 

20090226, 03:03  #10  
"Frank <^>"
Dec 2004
CDP Janesville
2×1,049 Posts 
Quote:
BTW, c131= Code:
10610277836580876222002792894053006972655975366663451742659789684697038347591723392834509640575057300905110715862343627308764000429 Last fiddled with by schickel on 20090226 at 03:38 Reason: Adding PS 

20090226, 03:36  #11 
"Frank <^>"
Dec 2004
CDP Janesville
2·1,049 Posts 
I wasn't sure how big a number msieve's poly search could handle. Seeing the reply from Jason later on, I see that msieve is capable of handling a c131. Maybe a dual search on both sides to see which one does better.....

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