Author: Robert Hyatt
Date: 13:34:47 03/07/99
Go up one level in this thread
On March 07, 1999 at 04:50:13, Dave Gomboc wrote: >On March 02, 1999 at 17:25:08, Will Singleton wrote: > >> 5R2/8/8/4rp2/3r4/5k2/2B1p2P/2K1R3 w - - >> White to move and draw. >> >>Well, that one turned out to be more interesting than I anticipated. We found >>that a couple of programs could see the draw w/o tablebases (Ferret, and perhaps >>Lambchop), and Crafty gave the tablebase draw result. Gives the rest of us >>something to think about. >> >>Aside from that, there was pretty much agreement on Rxf5. One program chose >>Bxf5 with a larger negative score than the concensus on Rxf5. That might bear >>looking into. >> >>Zchess was the only one not to consider Rg8. The passed pawns were scored so >>highly that the idea of bringing the rooks down to defend wasn't even an option. >> >>And for some reason, it took my program a ton of nodes to resolve the fail-low, >>and then a ton more to find the better move. Something for me to worry about. >>(Maybe time to put in some endgame knowledge, but I think there's something else >>going on.) >> >>Will > >How are you trying to resolve the fail-low at the PV? Have you tried completely >restarting the search (with the TT intact! :) My understanding is that this is >the preferred method nowadays... but certainly, YMMV. > >I am interested to hear from those who do restart on PV fail-lows, and those who >tried it but found it lacking. > >Dave Gomboc I have done this two ways: (1) first root move fails low, continue searching, hoping that another move will not fail low and you get something useful. If you search all moves and the score == alpha, you have to relax alpha and search again. (2) first root move fails low, relax alpha immediately and search it again to get the true score for the first move. I do (2) at present, for a couple of reasons: (a) I want to know how far the score dropped so I have some idea of how much trouble I am in and how much extra time I might want to allocate; (b) It is not uncommon to 'win' a pawn in early iterations only to see later that you can't hold on to it. So the old best move is still best, but with a much lower score. It is more efficient to re- search it immediately than to try all the other moves (which will also fail low) and then try again. Which is better is a good question. I have seen cases where I wished I would just continue searching, and others where I am glad I re-search after the first move fails low.
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