Computer Chess Club Archives


Search

Terms

Messages

Subject: Re: MTD(f)

Author: Dieter Buerssner

Date: 11:49:12 06/17/05

Go up one level in this thread


On June 16, 2005 at 18:41:09, Vasik Rajlich wrote:

>On June 16, 2005 at 16:20:38, Dieter Buerssner wrote:
>
>>On June 16, 2005 at 05:22:26, Vasik Rajlich wrote:
>>
>>>Fail soft helps when you need to re-search, so it helps more in MTD (f) than
>>>PVS, and doesn't matter at all in pure alpha-beta.
>>
>>Vas, fail soft vs. fail hard will change the search tree in "pure alpha-beta".
>>It will change move ordering. Fail soft has the potential, to give you better
>>move ordering. Assume some fail low position. Some refutations on ply deeper
>>mate, others just give the bound back as score, or scores in between. With fail
>>hard, you may order moves, that are refuted by mate early for the next search
>>(it needs some luck, to see mate scores, of course). In a later search with
>>different bounds and fail hard, you may search those moves early (depending on
>>all the move sorting heurists you use, of course. Yace for example stores a best
>>move in HT even when failing low). With fail soft, you might start with a more
>>reasonable move first.
>>
>
>Aha, yes - that's true.
>
>BTW I also store fail-low moves based on the fail-soft score. I doubt it helps
>much, but it at least shouldn't hurt.

Yes. However I remember what Ernst Heinz said to me once: "fail hard läuft
einfach besser". Seems not really logical - but I guess, he has a point. Perhaps
the point is, that it is much easier, to have some subtle bug in fail soft
search. alpha-beta is not really forgiving here - it will not crash with such
bugs, just not perform as good as it should (perhaps only in few positions).


>>This also reminds me to the typical move ordering statistics used: "How often do
>>I fail high in the first move relative to all node I failed high?". In many
>>positions, (almost) any move will fail high for the current beta. With a higher
>>beta, only one or a couple may do it. When you picked some stupid move in an
>>earlier search, instead of one really convincing one, it should be better for
>>later searches.
>>
>
>True - although fail soft won't help you with this.

It can help through the hash table (for example). It is very similar to the
situation of PVS - where fail soft can avoid some researches. The situation
arises, when you get a fail high/fail low at the root. Now you have to research
anyway - and fail soft scores from the HT may give a cutoff in this research,
while fail hard scores wouldn't.

>Some time ago, I was taking some statistics about my fail-soft. What I found is
>that it wasn't very soft. Once you do a search of above 6 or 7 plies, the search
>manages to get very close to the fail-hard value.

Same here.

Regards,
Dieter



This page took 0.03 seconds to execute

Last modified: Thu, 15 Apr 21 08:11:13 -0700

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