Computer Chess Club Archives


Search

Terms

Messages

Subject: Re: When to do a null move search - an experiment

Author: Robert Hyatt

Date: 12:04:32 04/26/04

Go up one level in this thread


On April 26, 2004 at 14:05:14, Vincent Diepeveen wrote:

>On April 26, 2004 at 13:51:03, Robert Hyatt wrote:
>
>>On April 26, 2004 at 11:50:18, Vincent Diepeveen wrote:
>>
>>>On April 26, 2004 at 06:31:58, Tord Romstad wrote:
>>>
>>>I wonder how you came to your conclusion.
>>>
>>>I did more experiments and concluded that always nullmoving saves most time.
>>>Double nullmove is just performing slightly worse (less than a percent in search
>>>time) and not missing zugzwangs, so i use it for that reason.
>>
>>
>>This is one of those rare occasions where I agree with Vincent.
>>
>>Several years ago Bruce Moreland and I were talking, about this very subject.  I
>>was explaining how I did it in Cray Blitz (and also in Crafty).  I classified a
>>node as "ALL", "CUT" or "PV" according to Marsland/Schaeffer terminology, and if
>>the node was classified as PV I avoided doing the null-search at all.
>>
>>Bruce did some testing and doing this restriction actually slowed him down a
>>bit.  So we both set about testing this stuff extensively and we came to the
>>conclusion that null-move is best used _everywhere_ because we don't have
>>perfect ordering and node classification is not that accurate as a result.  The
>>only reason for avoiding a null is:
>>
>>(1) you are in some sort of zugzwang position where a null-move will fail high
>>for the wrong reason and wreck the search.  Classic examples here are positions
>>with very few pieces.  IE pawns vs a knight where the knight can be zugged.
>>Most require some minimal amount of material on the board to avoid this problem.
>>
>>(2) there is a tactical issue that is hidden with the R reduction.  IE the
>>classic position with white pawns at f2, g3 and h2, black queen at h3 and black
>>pawn or bishop at f3, threatening mate on the move.  If the R reduction prevents
>>you from seeing the mate, you can have problems.
>>
>>(3) The hash table proves that the null-move search will not fail high, meaning
>>that the search will be wasted effort.
>>
>>(4) Obvious positions such as when the side on move is in check.  Not moving
>>can't fail high here as the king is lost.
>>
>>(5) I don't allow two consecutive nulls.  It is a potentially cute way of
>>eliminating zugzwang problems, but it is only good for that, and it is not free
>>in positions where no zugzwang is possible.  I choose to not deal with it
>>although I have this on my "to do" list to test with (say) pawn-only endings.
>
>Not allowing 2 consecutive nulls will for sure not find zugzwang problems.
>
>Not allowing 3 consecutive nulls will however find many zugzwang problems. As
>pointed out hashtable could do difficult, but that very seldom happens.
>
>Double nullmove does allow 2 consecutive nulls but not a 3d. So for a correct
>implementation you just need 1 'if then else' to also not allow null in a
>transposition table cutoff. This is very trivial to do.

That is exactly what I was talking about.  More than 2 can't ever be good.  In
middlegame positions, I believe that 2 in a row introduces unnecessary overhead.
 In endgame it might be worthwhile.


>
>>In short, try null everywhere.  But, as always, test, test, test.  Theory says
>>that doing null on a PV node is a waste.  Practice says that not all PV nodes
>>are really PV nodes after the search has started...  Go with practice. :)
>
>I'm using PVS in its pure implementation, so i start with window [-inf;inf]
>i also disallow nullmove when beta == inf.
>
>That's trivial stuff however.
>
>All other 'pv nodes' doing a nullmove is very good for different reasons. Even
>when it gets a nullmove. double nullmove has a bit of a IID effect.
>
>>>Your results give IMHO only the indication that your qsearch needs to be fixed
>>>and/or that you did your experiments at a too fast time control where tactics
>>>dominate.
>>>
>>>>Last week, there was an interesting discussion on this forum about which
>>>>criterions should be used to decide whether or not to do a null move search.
>>>>In an attempt to find out what works best for my engine, I played a self-play
>>>>tournament between four slightly different versions of my engine.  The
>>>>following pseudo-code explains the null-move criterions used by each of
>>>>the four versions:
>>>>
>>>>/* Gothmog A: */
>>>>int ok_to_do_nullmove() {
>>>>  if(in_check(SideToMove)) return 0;
>>>>  if(static_eval - value_of_biggest_hanging_piece(SideToMove) >= beta)
>>>>    return 1;
>>>>  return 0;
>>>>}
>>>>
>>>>/* Gothmog B: */
>>>>int ok_to_do_nullmove() {
>>>>  if(in_check(SideToMove)) return 0;
>>>>  if(static_eval - value_of_biggest_hanging_piece(SideToMove)
>>>>                 + value_of_biggest_hanging_piece(Opponent) >= beta)
>>>>    return 1;
>>>>  return 0;
>>>>}
>>>>
>>>>/* Gothmog C: */
>>>>int ok_to_do_nullmove() {
>>>>  if(in_check(SideToMove)) return 0;
>>>>  else return 1;
>>>>}
>>>>
>>>>Gothmog D is similar to Gothmog C, except that when the remaining depth
>>>>is 4 plies or more, a null move search with the depth reduced to zero
>>>>is done before the real null move search, and the real null move search
>>>>is avoided if the zero-depth search fails low.
>>>>
>>>>The criterion in Gothmog A is the one I have always used in the public
>>>>versions of Gothmog.  Our discussion last week seemed to indicate that
>>>>most others use the simpler criterion in Gothmog C.
>>>>
>>>>In my tournament, I all four versions face each other 50 times.  The
>>>>results:
>>>>
>>>>           Gothmog A   Gothmog B   Gothmog C   Gothmog D       Sum
>>>>Gothmog A    XXXX        26.5        31.0        29.5         87.0/150
>>>>Gothmog B    23.5        XXXX        30.5        26.0         80.0/150
>>>>Gothmog C    19.0        19.5        XXXX        31.5         70.0/150
>>>>Gothmog D    20.5        24.0        18.5        XXXX         63.0/150
>>>>
>>>>It is interesting that the version with the most restrictive criterion
>>>>achieves the highest score.  This confirms my growing suspicion that
>>>>recursive null move pruning doesn't work nearly as well as most people
>>>>believe, and that it might be a good idea to make an effort to reduce
>>>>its use as much as possible.  I will try to experiment with even more
>>>>restrictive criterions (perhaps static_eval-biggest_hanging >= beta+margin)
>>>>and see whether that improves the strength further.
>>>>
>>>>The number of games is of course still not sufficiently big to make any
>>>>definitive conclusions.  If I find the time, I will add more games and
>>>>more versions to the tournament.
>>>>
>>>>Tord



This page took 0.04 seconds to execute

Last modified: Thu, 07 Jul 11 08:48:38 -0700

Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.