Author: Robert Hyatt
Date: 11:13:03 01/21/04
Go up one level in this thread
On January 21, 2004 at 12:42:15, Sune Fischer wrote: >On January 21, 2004 at 06:54:49, Tord Romstad wrote: > >>On January 20, 2004 at 19:48:45, Sune Fischer wrote: >> >>>I'm surprised if a few PV moves can cut down the tree my any significant amount, >>>but then again I never understood why IID works.. :) >> >>IID is one of the few concepts even I, who doesn't even adhere to the >>"fundamental principle of chess programming" mentioned elsewhere in the >>thread, think I am capable of understanding. :-) >> >>When the remaining depth is big, there is no best move in the hash table, >>and it is likely that the search will fail high (typically because the >>static eval is above or at least not too far below beta), it makes sense >>to make some extra effort to obtain good move ordering for this node, >>because it will probably cut down the size of the big subtree considerably. >>A good way to do this is to first do a search with reduced depth. Most >>people seem to reduce the depth by 2, but I always had better results with >>a reduction of only 1 ply. > >I know how it is supposed to work, I just don't understand _why_ it works :) > >What if the window isn't fullwidth (alpha>-mate) and you fail low, you have just >done an expensive D-2 search to get a "random" move to search. nonono... We only do it on PV nodes.... I can't possibly do it if I can fail low, except for the rare cases where the root aspiration window alpha bound is too high and the best move is actually worse than the expected bound. But fail-low or fail-high at the root is not that common, and there I only do it when (a) I expect a hash move and don't have one, (b) I am at a non-null-window search (alpha < beta-1) and so forth. They are done _very_ infrequently. Not all over the tree. If you look at the definition laid down by Knuth/Moore, this only gets done at PV nodes, not at ALL or CUT nodes. > >Apparently that doesn't happen, but it should happen from time to time, >shouldn't it? No. If it did, it would be a _big_ waste... > >If it happens now and then, doesn't that eat away most of the savings? yes, but fortunately it doesn't happen. :) > >-S. >>Tord
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