Author: Walter Faxon
Date: 19:05:14 12/20/02
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On December 20, 2002 at 05:40:35, Gerd Isenberg wrote: >On December 19, 2002 at 16:50:32, Dann Corbit wrote: > >>Suppose that we are searching at long time control, and the hash table rapidly >>fills up. This can throw move ordering in the toilet, if we can't find >>transpositions any more. >> >>What sort of replacement scheme is best for very long searches (several >>minutes)? > >Hi Dann, > >I use a two table approach. > >A primary big one, where i replace one of eight with the lowest >timestamp/draft/someFlags priority. Exact entries became a bit more resistance >against overwriting, the same for most left succesors of every root move (pv and >refutations lines). > >A smaller secondary one, with an always replacement scheme. > >In general i don't store or probe in qsearch. I use IID in __every__ interior >node without a move hint from hash. > >Regards, >Gerd Hi, Gerd. Good idea about IID! I have no chess program but it seems to me that one uses hashing for two reasons: (1) to save information that was expensive to compute but may not be immediately useful (closer to the root), and (2) to save information that wasn't very expensive to compute but may be useful very soon (close to the current node). That's why a two-table approach (or a one-table, two types of entry approach) is so often preferred. Reason (2) implies that some sort of hashing should be done even in quiescence search. The problem with qsearch hashing nowadays is the memory caching penalty. Nowadays, if you have to wait on a memory access that might only help a little, you might as well do the search directly. Maybe one might use a very small hash table (10K entries, say), reserved for qsearch nodes? (Or nodes beyond a certain depth or with reduced material.) Particularly if you could freeze it all into the cache... I think an older ICCAJ paper detailed experiments with hash node type allocation, but without reference to memory caching penalties. -- Walter
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