Author: Vincent Diepeveen
Date: 10:06:18 06/15/98
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On June 15, 1998 at 04:52:16, Dezhi Zhao wrote: >In some earlier posts by Don Dailey and others, they mentioned >about zero-width window null move search and said it's an efficent >way to implement null move search. So I compared the zero-width null >move to the original full-width with my Xiangqi (Chinese chess) PVS >engine. >Here is the results of playing 20 moves (same path for both method, >without opening book) from the initial position. > >The m/c usually searches 2 to 4M nodes for each posoition. >If the transposition table is cleared between searches, the savings >of zero-width are generally around serveral hundreds of nodes, >and the max is 9K for one position. >If the transposition table is partially cleared (keep only last >iteration >entries) between searches, the savings become hard to interpret. You >save several hundreds of nodes in a postion, lose that in the >following position, and ocassionally lose much more than than previous >saving (save 8.4K and lose 140K in the next for example). > >I think that the savings are negligible, which are caused by the fact >that in PVS most of the nodes are of zero-width window already. > >These results also remind me of the word "vapor-ware" that Dr. Hyatt >called NegaScout over PVS. So I checked Crafty 14.13 again, and found >that Crafty uses full-width window null move search. Why? My best guess >is that Dr. Hyatt has done extensive tests over null move search >abnormaly >as he mentioned several times in CCC, and found null move search window >is >related to the abnormaly. Am I right? It all depends on how much do you extend when you have a nullmove search, and do you do checks in q-search? If you don't extend too much and don't do checks in q-search, then it will not make much different i guess. I use a higher reduction factor to limit the number of nodes needed for q-search. Diep extends so many stupid lines that R=2 versus R=3 saves me a ply sometimes, but average many tens of % Now that i use R=3 instead of R=2 i still see the difference between zero window of nullmove anymore, but less than with R=2. The difference however is still there in favor for the zero window nullmove, because you only need that >= beta bound. Greetings, Vincent
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