Author: Tord Romstad
Date: 02:40:20 01/22/04
Go up one level in this thread
On January 21, 2004 at 20:53:00, Robert Hyatt wrote: >On January 21, 2004 at 18:21:28, Tord Romstad wrote: >>I use MTD(f), hence I *never* get any "true scores" anywhere in the tree. > >Why? I played with mtd(f) for several months right after Don Daily started >his "you got to try this" many years ago... I got exact scores, because the >last two searches have a common "edge" and that "edge" is the true score. OK, in that sense I get true scores, of course. But I don't see any good way to guess whether this is going to happen at a specific node. >> My >>intuition >>regarding PVS is not very good (I have used MTD(f) almost since the beginning), >>but >>I don't see any reason not to use IID at all expected fail-high nodes in a PVS >>search, >>too. At a fail-high node, you can return a value before all moves are searched. >> If >>your move ordering is perfect, you will only need to search one move. It >>therefore >>makes sense to make some extra effort to make sure that the first move searched >>will really return a score >= beta, and the most obvious way to do this is to >>first >>do a search with reduced depth. What am I missing? >> > >for the majority of moves, it is easier to find a capture refutation than to >do an N-iteration search... For this reason, I don't to IID when there is a hanging piece which are sufficiently valuable that capturing it will almost certainly cause a fail high. Basically, I do IID when I expect a fail high and have no obvious candidate (hash table moves, winning captures, safe promotions, mates in 1, etc.) for a move to search first. >I tried it both ways (plus other ideas) when I >first did this. What I am doing right now has really proven to be effective >with no particular down-side or risk... > >For normal fail-high positions we have already been doing iterated searches >for previous depths, so hopefully we get decent ordering here most of the >time using normal ideas. The critical positions are the PV nodes, because >there searching a good move first is not good enough. We need the _best_ move >first. At normal fail-high positions, we only need a move good enough to cause >a cutoff, not the best move. IID will provide the best move generally, but at >significant cost. No, IID will not generally provide the best move, at least not the way I have implemented it. It will only give you a move good enough to fail high at a reduced search depth. Usually, this move will fail high even at full search depth. >>Another thing that has always puzzled me is that reducing the depth by only 1 >>ply >>gives better results for me than reducing by 2 plies, like you and almost >>everybody >>else do. I have tested this very thorougly, and depth-1 always seems to be a >>bit >>better. Really weird. > >I tried it both ways years ago. I found the depth-2 worked better for me. I >ran the test set (a bunch of positions) with IID off, using D-1 and D-2, and I >chose the case that produced the shortest time overall... for me that was D-2. >I should probably test it again just to see if things have changed, as it is >certainly possible after so many versions under the bridge. :) Perhaps it's time for me to try D-2 again, too. Tord
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