Author: Ross Boyd
Date: 06:49:09 09/11/04
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On September 11, 2004 at 00:08:20, Stuart Cracraft wrote: >Hi, > >I added keeping a triangular pv in main search and quiescence >to compare it with the output of my walk-the-hashtable-pv. > >The two differ frequently but quite often are also mostly >identical all the way through. > >Which should I trust? Seems like the hash table is getting >overwritten with other variations (not sure why). What >kind of scenario would cause that? My algorithm is >length >= depth to replace. > >Seems like triangular would be better and then use that to >prefill the hash table from the last iteration before the >next iteration would be best, as Bob Hyatt mentioned. > >In looking at both pv's, they look fine and lead to positions >that are often equal in material without any gross material errors that >could be attributed to a misimplementation but there are definite >differences between the two methods for me. > >Before I throw out the walk-the-hashtablepv, I want to be sure >that it is really no good. So I want to know exactly how a hash >table can be damaged such that its PV is corrupted. Under precisely >what circumstances can this happen? > >I would have thought length >= depth would have prevented it as long >as there are no collisions. My hashkey is 64-bits and I am searching ><1M nodes per move for my 1 second searches so I'd expect no collisons. >Should I have some hook in there to check the collisions aren't happening >and are causing damage to the pv or is there another hash table side >effect that results in that damage? Note that two or more different positions can 'map' to the same hash entry. (I'm not talking about 2 positions sharing the same 64bit key, I'm talking about 2 positions with different keys which MAP (after masking) to the same hash entry.) Imagine you had a hash table with only 16 entries... (crazy, I know)... you would see your PV entries getting overwritten constantly... even though there are no hashkey collisions. So, maybe this is the cause of the PV damage you are seeing. Regards, Ross
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