Author: Uri Blass
Date: 09:52:30 10/28/05
Go up one level in this thread
On October 28, 2005 at 12:25:29, Vincent Diepeveen wrote: >On October 28, 2005 at 11:54:37, Uri Blass wrote: > >>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. > >I was waiting for you to bite in the bait. > >Because both explanations are true. > >What i find very simplistic minded from you is that the most important comments >i made, namely that history pruning in the long run isn't going to work, >provided you plan to work on your evaluation function, you completely ignore. > history pruning works for fabien and Fruit has a good evaluation function(otherwise it had no chance to get second place in WCCC inspite of using one processor). Uri
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