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Subject: Re: Zero-width Window Null Move Search

Author: Vincent Diepeveen

Date: 10:09:31 06/15/98

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On June 15, 1998 at 10:53:49, Don Dailey wrote:

>On June 15, 1998 at 09:41:02, Robert Hyatt 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
>>>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?
>>
>>
>>no...  I do it that way simply because 99.999% of the nodes searched are
>>already using alpha,alpha+1...  There is nothing wrong with doing every
>>null-move search using beta-1,beta, for example, and it probably makes
>>sense to do it that way, I just did not because of PVS already taking
>>care of that for the most part.
>>
>>I will run some tests to see if it makes any difference at all, but
>>suspect that it will have virtually no effect, because it will only
>>affect a very few searches, total...
>
>Bob,
>
>I'm not convinced that this is the case.  Also it may not matter if
>99 selective nodes are zero width and 1 is not if that one takes
>significantly more  time to search.
>
>I do not know that you are wrong either, I'm just not sure.  When
>I made this change I definitely got a noticable speedup, not huge
>but enough to imply something was going on.
>
>If you make the change can you instrument this for us?  You could
>count the nodes that would be zero width and also count the non-zero
>width nodes and report them to us.

Theoretically it can never be worse to use  beta-1 and beta.

>- Don



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