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

Author: Vincent Diepeveen

Date: 07:41:08 04/28/04

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On April 28, 2004 at 06:06:37, Vasik Rajlich wrote:

>On April 27, 2004 at 21:05:11, Vincent Diepeveen wrote:
>
>>On April 27, 2004 at 17:10:16, Dann Corbit wrote:
>>
>>>On April 27, 2004 at 06:10:04, Vasik Rajlich wrote:
>>>
>>>>On April 27, 2004 at 01:38:49, Uri Blass wrote:
>>>>
>>>>>On April 27, 2004 at 00:44:34, rasjid chan wrote:
>>>>>
>>>>>>On April 26, 2004 at 20:07:00, Uri Blass wrote:
>>>>>>
>>>>>>>On April 26, 2004 at 13:41:53, Vincent Diepeveen wrote:
>>>>>>>
>>>>>>>>On April 26, 2004 at 12:14:33, José Carlos wrote:
>>>>>>>>
>>>>>>>>>On April 26, 2004 at 11:57:43, Vincent Diepeveen wrote:
>>>>>>>>>
>>>>>>>>>>On April 26, 2004 at 11:48:35, José Carlos wrote:
>>>>>>>>>>
>>>>>>>>>>>On April 26, 2004 at 11:32:26, Tord Romstad wrote:
>>>>>>>>>>>
>>>>>>>>>>>>On April 26, 2004 at 10:39:42, José Carlos wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>>  An interesting experiment, of course. But I think your conditions are rather
>>>>>>>>>>>>>different from 'most' programs. I mean:
>>>>>>>>>>>>>  - You allow any number of null moves in a row (most programs don't do even
>>>>>>>>>>>>>two)
>>>>>>>>>>>>
>>>>>>>>>>>>This has no importance, I think.  My experience is that I almost always get the
>>>>>>>>>>>>same score and PV when I enable/disable several null moves in a row, and that
>>>>>>>>>>>>the difference in number of moves searched is *very* tiny.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>  You're probably right, as you've tested and I speak from intuition, but at
>>>>>>>>>>>first sight, it seems that the fact that you allow several null moves in a row
>>>>>>>>>>>will increase your percentage of null-moves-tries/total-nodes-searched, and thus
>>>>>>>>>>>that avoiding unnecessary null moves will be a good idea.
>>>>>>>>>>
>>>>>>>>>>In *all* experiments i did with nullmove and a program not using *any* forward
>>>>>>>>>>pruning other than nullmove, the best thing was to *always* nullmove.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>  Yes, that's what other programmers also said (including me) in the thread we
>>>>>>>>>had last week. That's pretty intuitive. With not any other forward pruning (or
>>>>>>>>>very little) but null move, the cost of not trying a null move that would have
>>>>>>>>>produced a cutoff it terrible compared to the benefit of saving an useless null
>>>>>>>>>move try. So avoid null move, in this case, must be only in a very few cases
>>>>>>>>>where you're 99.99% certain you'll fail low... if any.
>>>>>>>>
>>>>>>>>99.99% means 1 in 10k nodes.
>>>>>>>
>>>>>>>No
>>>>>>>
>>>>>>>You can be 99.99% sure about fail low more often than 1 in 10k nodes.
>>>>>>>
>>>>>>>>
>>>>>>>>So doing nullmove always is cheaper, because in a lot of cases
>>>>>>>>transpositiontable is doing its good job and in other cases you search more than
>>>>>>>>10k nodes which you avoid searching now.
>>>>>>>>
>>>>>>>>>  Gothmog is very different from that 'paradigm' (he does a lot of forward
>>>>>>>>>prunning and applies many ideas he has commented here), hence it works pretty
>>>>>>>>>well for him.
>>>>>>>>
>>>>>>>>I get impression evaluation function plays a major role in when something is
>>>>>>>>useful or when it isn't.
>>>>>>>>
>>>>>>>>Checks in qsearch is also a typical example of this.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>Double nullmove i invented to proof nullmove gives the same results like a
>>>>>>>>>>normal fullwidth search for depth n which i may pick, and i use it as it finds
>>>>>>>>>>zugzwangs and i am sure that is very helpful, because the weakest chain counts.
>>>>>>>>>>
>>>>>>>>>>So double nullmove always completely outgunned doing a single nullmove then
>>>>>>>>>>disallowing a nullmove and then allowing the next one.
>>>>>>>>>
>>>>>>>>>  I tried double null move some time ago, and it didn't work for me. Probably I
>>>>>>>>>did something wrong, but I recall an old post (see the archives) from C. Theron
>>>>>>>>>where he gave some points why double null move should not work. I, myself,
>>>>>>>>>didn't invest too much time though as I had much weaker points to fix in my
>>>>>>>>>program before.
>>>>>>>>
>>>>>>>>Christophe didn't post it doesn't work AFAIK.
>>>>>>>>
>>>>>>>>Further i must remind you that majority of commercial programmers posting here
>>>>>>>>is not busy letting you know what works for them or doesn't work for them.
>>>>>>>>
>>>>>>>>To quote Johan: "don't inform the amateurs".
>>>>>>>
>>>>>>>What reason do you have to tell other what works for you and what does not work
>>>>>>>for you?
>>>>>>>
>>>>>>>You do not plan to inform the amateurs about better code for tablebases than the
>>>>>>>nalimov tablebases so I do not see you as a person who try to help the amateurs.
>>>>>>>
>>>>>>>>
>>>>>>>>I remember that Christophe also posted that evaluation function is not so
>>>>>>>>important.
>>>>>>>>
>>>>>>>>His latest postings here made more sense however than the crap posted before
>>>>>>>>that.
>>>>>>>
>>>>>>
>>>>>>>I understand that you claim that basically Christophe's claim that most of the
>>>>>>>improvement in tiger came from better search and not from better evaluation was
>>>>>>>disinformation.
>>>>>>
>>>>>>Firstly there is not that BIG a stake for disinformation and posting
>>>>>>here is also just normal human behaviour that does not require
>>>>>>asking "....why do I post ? ". Then also ask why do I talk.
>>>>>>
>>>>>>I think Christophe was quite clear about the reasons why chess programming
>>>>>>is NOT about evaluation(not dumb evaluation). After pawn structures, passed
>>>>>>pawns etc, it is very difficult to try to improve on it. The curve for
>>>>>>evaluation is logarithmic for elo-increase/code-increase + huge overhead,
>>>>>>the very reverse of exponential.Search almost have no trend patterns and
>>>>>>search improvements usually have no overhead, you just need to be smarter
>>>>>>then the rest. Assume your opponent searches on average 3 plys ahead.
>>>>>>How do you do a good evaluation that can see 3 plys ahead? Evaluation is horizon
>>>>>>dumb.
>>>>>>
>>>>>>Rasjid
>>>>>
>>>>>I did not claim that christophe claimed wrong things.
>>>>>It is Vincent who claimed it.
>>>>>
>>>>>I prefer not to talk about the top programs.
>>>>>I can only say that it is clear for me that I can get much by search
>>>>>improvements.
>>>>>
>>>>>Certainly searching 3 plies forward or doing something equivalent can help
>>>>>significantly but the problem is how to do it.
>>>>>If you are optimistic about doing it with no price by intelligent extensions and
>>>>>reductions
>>>>>and better order of moves then it is clear that going for search is the right
>>>>>direction.
>>>>>
>>>>>If you are not optimistic even about getting something equivalent to 1 ply
>>>>>forward then evaluation is the right direction.
>>>>>
>>>>>Uri
>>>>
>>>>I don't think you need objective answers to these questions.
>>>>
>>>>You just need a game plan.
>>>>
>>>>A plain, reasonably tuned eval combined with a state of the art selective search
>>>>seems like a perfectly reasonable game plan to me.
>>>>
>>>>Ditto for plain search combined with a state-of-the art evaluation.
>>>
>>>Bruce Moreland (who's program Ferret was at one time among the top two or three
>>>in the world) found a great annoyance when as he improved his evaluation: he
>>>discovered that he was then being outsearched.
>>>
>>>I think the lesson is simple:
>>>If your new smarter eval makes the program stronger, then keep the new
>>>evaluation terms.  If not, rip them out.
>>
>>I feel search is overrated too much. It's so so hard to make a good eval, only
>>when you have a real well tested and tuned eval making a good search is
>>important.
>>
>>Even searching fullwidth i can beat Crafty.
>>
>>In fact i saw diep at a P4 2.8Ghz in blitz beat crafty at its dual Xeon lately
>>at icc. I was pretty amazed because i always say that you need at least 12-14
>>ply to search outside tactical barrier.
>>
>>Perhaps i'll change that back to 10 ply one day :)
>
>Eval is hard, true. So is selective search.

The difference is that Eval brings more points. Selective search doesn't
(assuming you use nullmove and have a pretty efficient search, some programs we
cannot say that from though).

>Moreland's approach is definitely not the way to end up with a great eval. When
>your eval is cheap, you can't just add one expensive thing to it and expect it
>to pay off. One expensive idea might cost you 20% of your nodes. With this
>approach, if you're testing honestly, you'll always end up throwing out the
>expensive stuff.

I'm not commenting here on Bruce in a negative sense. In the days that ferret
was strong, chessprograms simply didn't search deep enough yet. Around 1997 the
search dominated because it was hard to get > 10 ply.

So his approach in those days made sense.

>If you want a program with a great eval, you have to commit to it, let
>everything slow down, and go from there.

Big eval also means that a lot of dubious search stuff no longer works.

Correctness of search and worst case behaviour of forward pruning plays a big
role.

>Another thing to keep in mind: a few years ago, it might well be the case that
>allowing onesself to be outsearched was a death wish. This may be changing now.

When posting in 1997 at RGCC that in the future just searching deeper wasn't
holy, i remember that all the internet guys were laughing loud for me (also
there programmers hardly posted).

>Chess Genius dominated computer chess with a very unusual selective algorithm
>that worked great at low depths. Eventually, it became irrelevant. As hardware
>changes, all of these tradeoffs change.

In the days that scientific programs searched at 100 nodes a second at 512
processors (Zugzwang), there genius was searching at a single cpu 10Mhz machine
already like what was it 2000 nodes a second or so?

Of course using very dubious selective search and dubious qsearch methods.

Even till today i do not know 100% sure what he was doing, but i know that at a
10Mhz machine Genius will beat diep, because diep will search 3 ply at it in C
and genius will get a tactical 10 ply and completely outgun it.

Same holds true for Rebel. His dubious searching methods of course were genius
finds around 1991.

This has changed however in 2004.

>Vas



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