Author: Uri Blass
Date: 13:53:48 10/29/05
Go up one level in this thread
On October 29, 2005 at 16:06:50, Tony Werten wrote: >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 I will make my post clearer I meant that the following claims are correct 1)history pruning works for fabien 2)Fruit has a good evaluation function(otherwise it had no chance to get second place in WCCC inspite of using one processor). Note that 1 is correct based on testing and it is clear that fruit without history pruning is stronger. I did not mean that the second place of fruit is a proof that history pruning works for it but only that it proves that it has good evaluation function. Uri
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