Author: Dieter Buerssner
Date: 11:49:12 06/17/05
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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
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