Author: Tord Romstad
Date: 10:34:27 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. Yes. This is why it is a good idea not to do IID at all nodes, but only at nodes where you are reasonably sure that there will be a fail high. You should always check the value of the static eval before doing the internal search. There are some possible improvements to IID which I haven't found the time to experiment with yet. You could for instance abort the internal search if none of the n first moves fail high, or if one of the "sub-IID" searches with a even more reduced search depth does not fail high. >Apparently that doesn't happen, but it should happen from time to time, >shouldn't it? > >If it happens now and then, doesn't that eat away most of the savings? I am sure it does, if you are not careful about where you use IID. Tord
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