Author: Vasik Rajlich
Date: 15:41:09 06/16/05
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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. >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. >Regards, >Dieter 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. Even if you blunder a queen, the search will somehow manage to give it back and barely get the score it needs. You can also see this effect with an MTD (f) program if the root driver is really primitive. You'll start with 0.00 and fail high. Then 0.01. Then 0.02. Maybe all the way to 1.00, one centipawn by centipawn. That's not a very good fail soft! I think MTD (f) might be worth the trouble if this could somehow be solved. Vas
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