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Subject: Re: Zero-width Window Null Move Search

Author: Dezhi Zhao

Date: 23:21:21 06/15/98

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On June 15, 1998 at 13:06:18, Vincent Diepeveen wrote:

>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
>>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
>>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
>>as he mentioned several times in CCC, and found null move search window
>>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 don't extend much. Currently only check and mate extensions are applied.
I will add capture/recapture extensions latter and do the experiment again.
>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 %

I had similar results with R = 3. However I found R = 3 produces weak
moves in defensive (behind) positions.
>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.
I use R = 2. Althougth R = 3
>The difference however is still there in favor for the zero window nullmove,
>because you only need that >= beta bound.
Yes. The impleamentation is easy, just 1 line to be modified. Hovever
the difference is hard to tell in my tests.  I got an impression that it helps
in highly tactical positions in the order of Kilo/Mega and if you keep
the old transposition entries between search, you lose the savings
in the next search. What concerns me more than the elusive speedup is
the null move search abnormaly. It *MIGHT* be related to null move search
window. I have no enough experience on that.  Hopefully after Dr. Hyatt
finishing the experiment, he could give a conclusion on this.

Best Regards

Dezhi Zhao


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