20130707, 21:10  #23 
"Serge"
Mar 2008
Phi(4,2^7658614+1)/2
21646_{8} Posts 
There is one group that had approached a gnfs193 on a collegesize cluster. The reservation is still pending, >3 years later.
Don't take even a gnfs193 lightly. (...and there are no much smaller unreserved gnfs numbers in the Cunningham project. There are gnfs192 numbers: 3,664+ and 5,485+, and then there are some above.) You may or may not know that msieve has certain issues (and these are under active research) every time the size limit is advanced. It is not just a matter of huge resources and "pushing a button". The often partially quoted Donald Knuth's saying goes: "Science is what we understand well enough to explain to a computer. Art is everything else we do." 
20130708, 06:01  #24 
(loop (#_fork))
Feb 2006
Cambridge, England
2·29·109 Posts 
I have a bit of experience (25 factorisations) in the high150slow160s range; with my circumstances (proportionally better resources for linear algebra than for sieving) I have found that 31bit large primes, 14e siever, and aiming for no more than 160 million relations works significantly more quickly than 30bit at that level.
Last fiddled with by fivemack on 20130708 at 06:02 
20130708, 07:08  #25  
"Curtis"
Feb 2005
Riverside, CA
2^{3}·3^{2}·61 Posts 
Quote:
How big are the matrices your settings produce? How much do they vary for, say, a C160? Thanks for the info while I don't plan to do a C170+, I'm interested in learning where we should deviate from the defpar file for 145165 digit work. 

20130708, 08:46  #26 
(loop (#_fork))
Feb 2006
Cambridge, England
2×29×109 Posts 
I tend to run with rlim=alim, run over Q from lim/3 to lim, and do test sieving until I find a value of lim that gives enough relations  I'm not using defpar.txt at all.
lim=48M was what I used for the last C160 I did. The matrices are maybe nine millionish, they do vary a fair amount; I'm using the same computers for sieving and for matrix, saving 10% of sieving time even at the price of doubling the matrix time is a good tradeoff. Last fiddled with by fivemack on 20130708 at 08:48 
20130708, 10:04  #27  
Sep 2010
Scandinavia
3·5·41 Posts 
Quote:
Quote:
But when I tried sieving with Code:
rlim: 48000000 alim: 48000000 lpbr: 31 lpba: 31 mfbr: 62 mfba: 62 rlambda: 2.65 alambda: 2.65 Is this because of your preference for faster sieving, or is two rels per q not a good rule of thumb? This of course relates to my questions earlier in this thread; what are the consequences of changing these parameters, other than the change in yield and sieving speed that I can see? Thanks for your input! 

20130708, 11:07  #28 
(loop (#_fork))
Feb 2006
Cambridge, England
2·29·109 Posts 
I suppose I think of 'two relations per Q' more as a lower bound; if you're not getting even that many relations, probably the parameters could be better. That's in part because it tends to mean you're working at the edge of the siever's capability and so you'll risk unreasonable duplicate rates.
The minima in this world are generally reasonably flat, but increasing the largeprime bound does sometimes morethandouble the rate of relation production while lessthandoubling the number of relations needed, which is an unusually good tradeoff. 
20130708, 14:41  #29 
Just call me Henry
"David"
Sep 2007
Cambridge (GMT/BST)
2^{2}·1,433 Posts 
For some jobs it might be worth baring in mind that increasing the
lpb(r/a) without changing mfb(r/a) doesn't change the speed of the sieving. It will just be a harder filtering problem. Since filtering doesn't take at all long with smaller numbers if a number is on the border then you could hope to get a little help from an increased lpb(r/a) without increasing the complexity much(i.e. you expect 80% of the extra relations to be singletons). Working out how many relations you need could be tricky using this trick. 
20130709, 12:20  #30  
Sep 2010
Scandinavia
615_{10} Posts 
Quote:
By the way: Say I'm choosing two CPUs for my G34board, looking to maximize Msieve performance. Should I go with 6140 or 6172? (8*2.6GHz or 12*2.1GHz) Would a cooler alternative like 6166 HE be crazy? Quote:
I have figured out that if mfb is too far behind 2*lpb then the yield will be bad or terrible. 

20130709, 13:29  #31  
Just call me Henry
"David"
Sep 2007
Cambridge (GMT/BST)
2^{2}×1,433 Posts 
Quote:
You are not expecting to get the full 2x relations like you would need if you also increased mfb. For example if increasing the lpb by 1 gets you 1.2x as much yield then you can expect to need <=1.2x as many relations. This trick is based upon you needing <1.2x or making it easier to oversieve to get a better matrix. A high percentage of the extra relations will be singletons but the few that aren't could be quite helpful. If it wasn't for filtering taking longer you could run every factorization with the maximum lpb(33 in most binaries but all that is needed is a recompile). The limit basically means you throw away relations while sieving. 

20130709, 13:46  #32  
(loop (#_fork))
Feb 2006
Cambridge, England
2·29·109 Posts 
Quote:
Quote:
The motherboards and memory are quite powerintensive so the 15 watt difference in CPU power is immaterial. If just linear algebra, that's an interesting question (three CPUs per memory controller seems to saturate, so there's less point in getting six rather than four), and I actually don't know what the answer is now that msieve threading has been so dramatically improved. At present, the best parameters I have for MPI linear algebra on my quadsocket 6168 Opteron run only 10% faster than on my singleHaswell; I clearly ought to devote some effort to finding better parameters! 

20130709, 13:48  #33  
Sep 2010
Scandinavia
1147_{8} Posts 
Quote:
I don't quite get the bold part. I don't understand how mfb works. Are you saying it increases complexity and that I can sometimes get the benefits of a higher lpb without paying the price of a higher mfb? (which would be higher complexity, and I don't know what that is in this context(harder filtering? Is that what your other post was saying?)) I feel that I'm missing a few pieces, but I'm still learning. Hopefully others will benefit from these discussions. Thank you all for your patience. Quote:
Quote:
Yes, I'm considering those 6166 HE. The price is hard to beat. They should sieve slightly faster than a 6140. The question is how much worse they would be in LA. Does it make any difference that the total L2cache will be larger on the 12core ones? MPI seems tricky... I guess I'll have to look into that subject. Last fiddled with by lorgix on 20130709 at 14:26 Reason: adding response to fivemack 

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