Author: Tony Werten
Date: 13:06:50 10/29/05
Go up one level in this thread
On October 28, 2005 at 12:52:30, Uri Blass wrote: >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). Flawed reasoning. Maybe without the history pruning he would have become first ? Or maybe 10th. Point is that not every single thing in Fruit is perfect because he became second. All things together gave it that 2nd place. Tony > >Uri
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