20110121, 14:03  #1 
Mar 2007
Austria
2·151 Posts 
Problem with LLR 3.8x for PRPing!
To everybody who is PRPing candidates using any 3.8 subversion of LLR:
You are in danger of getting incorrect results or even missing primes! It doesn't happen very often,but it can: One number where LLR 3.8.x fails is: 245*830^4921. See for yourself: Correct result from LLR 3.7.1,also confirmed with PFGW,phrot,and a selfwritten GMP program: Code:
245*830^4921 is not prime. RES64: FC47BD5F01792B65. OLD64: F4D7381D046B822C Time : 230.578 ms. Code:
245*830^4921 is not prime. RES64: E75504D6062020AD. OLD64: B5FF0E8212606205 Time : 113.636 ms. Don't forget that this problem also applies to PRPNet clients testing candidates with a 3.8x LLR version! LLr 3.7.1 or phrot are safe too,but they are significantly slower. 
20110121, 22:16  #2 
A Sunny Moo
Aug 2007
USA (GMT5)
14151_{8} Posts 
What version of LLR 3.8.x are you using? Some of the earlier 3.8.x versions had some issues with bad residuals, but those have now been corrected. You should be good with the latest 3.8.4, which uses the exact same core code as PFGW.
Note also that LLR sometimes produces a different (but still correct) residual on some bases because it ran the test in a different base. (The technical explanation of this is kind of long, but the upshot is, even when LLR's residuals will occasionally differ from PFGW's, LLR will always be consistent with LLR and PFGW with PFGW.) 
20110121, 22:24  #3 
Mar 2007
Austria
2·151 Posts 
This was with LLR 3.8.4.
The issue was also confirmed by Jean Penne. PFGW,contrary to LLR,detects the rounding error occuring and reruns the test with a larger FFT length. 
20110121, 22:56  #4 
A Sunny Moo
Aug 2007
USA (GMT5)
3×2,083 Posts 
Hmm...interesting. I wasn't aware of this. Does LLR not even notice the roundoff error at all, or does it detect it but fail to run the test with a larger FFT? (If the latter, then this could be a relapse of an earlier issue.)

20110122, 16:28  #5 
May 2004
FRANCE
2×3^{2}×31 Posts 
LLR Version 3.8.4 : user interface bug fixed!
Hi All,
The false residue found by nuggetprime is really due to a rounding error, and LLR is quite able to detect it, redo the iteration using a more reliable code, and, if the error persists, to rerun the test while using a larger FFT. But to have this behavior, it is necessary to set ErrorCheck=1 in the .ini file. With the Windows GUI application, it is easy to set this option, and, normally it might be also easy to do that with the Linux or cllr application, by setting oErrorCheck=1 in the command line... Unfortunately, due to a bug in the user interface, this command line option was ignored! This bug is now fixed, and I updated to day all the V 3.8.4 binaries and sources accordingly. Here is a test of your problematic example : I:\D\Jean\LLR\llr384dev>cllr a6 oVerbose=1 oErrorCheck=1 d q"245*830^4921" Base prime factor(s) taken : 83 Starting N+1 prime test of 245*830^4921 Using FFT length 448, a = 3 Iter: 2806/4779, ERROR: ROUND OFF (0.5) > 0.4 Continuing from last save file. Resuming N+1 prime test of 245*830^4921 at bit 2 [0.04%] Using FFT length 448, a = 3 Disregard last error. Result is reproducible and thus not a hardware problem. For added safety, redoing iteration using a slower, more reliable method. Continuing from last save file. Resuming N+1 prime test of 245*830^4921 at bit 2806 [58.71%] Using FFT length 448, a = 3 245*830^4921 is not prime. RES64: FC47BD5F01792B65. OLD64: F4D7381D046B822C Time : 2.292 sec. Regards, Jean 
20110122, 21:54  #6 
A Sunny Moo
Aug 2007
USA (GMT5)
3·2,083 Posts 
Thanks Jean! Just curious, does setting ErrorCheck=1 slow down the program at all? If not, then perhaps it should be set as the default. (I'm guessing there's little or no speed penalty since PFGW presumably has this error checking enabled by default.)
Last fiddled with by mdettweiler on 20110122 at 21:55 
20110122, 23:36  #7  
May 2005
2×809 Posts 
Quote:
According to LLR readme: Quote:
I am runnning several base =3 and 10 tests for some time, so I wonder if I should redo any tests now? (not to mention base =2 tests)... 

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