Author: Uri Blass
Date: 08:54:37 10/28/05
Go up one level in this thread
On October 28, 2005 at 10:49:21, Vincent Diepeveen wrote: >On October 26, 2005 at 12:11:27, Bas Hamstra wrote: > >>On October 26, 2005 at 06:58:41, Tord Romstad wrote: >> >>>On October 26, 2005 at 05:59:30, Svein Bjørnar Myrvang wrote: >>> >>>>Can anyone guide me to some good articles on the subject? I can't seem to find >>>>anything. Thanks in advance, >>>>Svein >>> >>>In a program with decent move ordering, you would expect a beta cutoff at >>>non-PV nodes to occur in one of the first moves played, or not at all. Beta >>>cutoffs late in the move list are very rare. This simple observation can be >>>used as a basis for reduction techniques. >>> >>>The basic idea is this: Search the first few moves at each node with full >>>depth. If no beta cutoff is found, search the remaining moves with reduced >>>depth. If one of the reduced moves returns a score >= beta, re-search this >>>move with full depth. >>> >>>You will probably find that this simple approach reduces your tree size >>>dramatically, but the risks are far too big. Blindly reducing all moves >>>late in the move list is too dangerous, and you need some extra conditions. >>>Most people never reduce captures, promotions, checks, or moves which are >>>extended for some reason. If you evaluate internal nodes, you can also >>>see how each move changes the components of the evaluation function, and >>>make exceptions for moves which dramatically increases your passed pawn >>>score, the pressure against the opponent king, and so on. There is lots >>>of scope for experiment here, and I suspect that the implementations in >>>current programs are very far from optimal. >>> >>>Another very popular condition is to collect statistics about how often >>>every move has failed high or low in the past, and to avoid reducing moves >>>which have a high (fail high)/(fail low) ratio. This condition is the >>>reason for the name "history pruning", which in my opinion is very >>>unfortunate. History is just one of several conditions which can be >>>used, and we are not talking about pruning, but reductions. I prefer >>>the term "late move reductions", but it seems I am quite alone. >>> >>>I have found the technique to work even better (especially in tactical >>>positions) with the following enhancement: If, at the node directly >>>following a reduction, the null move fails low, and the moving piece >>>in the move that refuted the null move is the same as the moving piece >>>in the reduced move, immediately cancel the reduced-depth search and >>>re-search the move with full depth. The point is that in cases like >>>this, the reduced depth move often contain some serious tactical >>>threat, and deserves a deeper search. >>> >>>Tord >> >>Do you any luck with those reductions? I mean provable benefit? >> >> >>Bas. > >At blitz there is no doubt, searching 2 ply deeper helps usual as it eliminates >a worst case. > >However at serious slow time controls, history pruning is positionally crippling >a program. > >There is 2 circumstances when history pruning might not hurt: > a) your evaluation function is extremely simple > b) you are doing so many dubious pruning things (multicut, last plies pruning >and so on) already that another dubious thing is not really a problem > >As in diep my evaluation function is not extremely well tuned, despite a >pathetic search depth of diep, history pruning is giving 2 ply search depth. > >However, just consider the problem for diep of history pruning. Even with +2 ply >i won't outsearch strong opponents. If i'm getting suddenly 15 ply instead of 13 >ply at dutch champs, then that's still less than the 17 ply or so from Zappa and >still less than the 16 ply from Fruit. > >In short you will realize that diep has to win games based upon positional >grounds anyway. It needs to get that fail high to a positional better move. The >bad thing from history pruning is that better positional moves, suddenly take +6 >or +7 ply more now to get found. > >Do you want to run a +6 or +7 ply extra depth risk just to search 2 ply deeper? > >When in a few years we search 20 ply search depth, the risk is not 6 nor 7 ply, >but the risk is 10-12 ply. > >In world champs 2005 i presented 1 improvement for history pruning. Which >limited the positional risks to less plies. Usually 5 ply positional loss it is >in that case. However, all those tiny improvements won't hide the fact that it >just positionally cripples a chessprogram and you will need to answer yourself >then whether getting down 300 rating points positional is worth 2 ply. > >In general this 300 points is true for forward pruning. The only forward pruning >i'm look at now is last plies pruning. > >Please note that the implementation as in Fruit 2.1 and Fruit WCCC 2005 is just >bringing 1.5 ply extra depth to Fruit initially and a 10 ply positional depth >risk. It doesn't bring as much as the implementation i did in Diep. Fruit is not >pruning any capture nor check. I did prune also captures. > >If i'm not pruning captures, then for diep i just win 1 ply search depth with >history pruning. Lucky my move ordering is pretty ok, meaning that i'll try good >captures as first anyway. > >In Diep what happens is that trying bad captures is a good idea to try at the >end of the move list. In a dumb beancounter experiment i saw that trying bad >captures *before* the remainder of the normal moves was a very clever idea and >simply gave extra search depth. > >In move ordering from Fruit 2.1 the score assigned to bad captures is far higher >than the score that can get assigned to history moves. Note that based on the author losing captures are searched last. see http://www.talkchess.com/forums/1/message.html?457839 There are 4 possibilities: 1)You understand fruit better than Fabien 2)Fabien made an error in the explanation 3)You did not understand fruit's code correctly. 4)I did not understand Fabien's post. I give the readers to decide which explanation they believe. Additional history moves >regurarly get put back to a default value. So Fruit is not very consequent in >trying moves. > >When i test games diep versus fruit, at slow time controls i saw Fruit not >deteriorate when turning off history pruning. Note that I suggested reducing default history threshold in fruit2.2 from 70 to 50(I found that fruit could solve faster some tactical positons with that change because it find them one or 2 ply earlier and the difference between history=50 and history=70 is less than 1 ply) and fruit2.2 Uri that has only that change did not perform better in CEGT rating list. http://www.husvankempen.de/nunn/ranglisteall.html Uri
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