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Subject: Re: Zero-width Window Null Move Search

Author: Don Dailey

Date: 07:53:49 06/15/98

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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
>>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?
>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...


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.

- Don

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