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Up Topic Rybka Support & Discussion / Rybka Discussion / Old (Knight) underpromotion bug with hours-long stall
- - By KWRegan (*) Date 2012-01-18 06:15 Edited 2012-01-18 06:44
Way back in March 2008, user "estragone" reported a bug in Rybka 2.3.2a here where White gets +2.68 for a short displayed PV in which White stands checkmated!  The following shows this bug is still operative in Rybka 4.1:



Now I have seen it combined with an hours-long stall (not a hang)---here using Rybka 3 x64 1-cpu in 50-PV mode with 512MB hash---that is reproducible from a clean start with empty hash tables.  This is a snap taken right at 20 hours:



It is easy to create other cases---Rybka displays the knight underpromotion in the PV only when it is the first move, but always gives the correct evaluation of the underpromotion as the value. Of course I'm dismayed by this bug still existing---none of Houdini, Firebird, Fire, Komodo, etc. shows it. 

What I wish to know further is whether the stalling aspect is related to this Oct. 2008 post (in German on another forum) showing an influence of the hash table size on the running time.

Thanks for all pointers.  The possible larger significance of hash-table anomalies is hinted at on an old page of mine here

---Ken Regan
Parent - - By Mark (****) Date 2012-01-18 11:10
I don't have any info on the stalling bug, but I'm still waiting for your conclusions on the analysis of the Kramnik-Grischuk game from Mexico'07.  :wink:
Parent - - By KWRegan (*) Date 2012-01-18 20:20
To complete that, I need:
1. Good wrong-color Bishop knowledge (in Rybka 4, but...)
2. Persistent Hash (...not in Rybka 4 anymore)
3. Support for Nalimov 6-piece tablebases---or at least ones with distance-to-mate.
4. No killer bugs (...like the one in this post...)
5. Good retention of hashed values upon stepping back one or two ply.

Regarding 5., I believe it has been noted on this board already that Rybka tends to "lose" hashed values upon taking a few steps back.  Alternative to 2. would be position-learning with user-specified values such as Crafty has---having DTM and e.g. storing "9.71" if in 4 moves the PV trades down to DTM 25.  This is important to avoid cycling and find shortest wins.

I've purchased and installed Houdini Aquarium 2.0 "regular", but have not yet had time to see if it meets my needs in a stable manner.  Komodo?  Critter?  maybe Stockfish?  Thanks to any for info on the above, and for the interest.
Parent - By Mark (****) Date 2012-01-18 21:18
Although I haven't used it for over a year myself, now that you have Aquarium, you could use IDeA to organize and continue the analysis.  Also, I have the full set of 6-man egtbs available if you need any of them.
Up Topic Rybka Support & Discussion / Rybka Discussion / Old (Knight) underpromotion bug with hours-long stall

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