Author: Gerd Isenberg
Date: 11:20:57 12/03/05
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On December 03, 2005 at 14:05:48, Gerd Isenberg wrote: >On December 03, 2005 at 11:56:42, Andreas Guettinger wrote: > >>On December 03, 2005 at 11:01:46, Paul Jacobean Sacral wrote: >> >>>I would appreciate a couple of clarifying remarks as well, because this is a >>>topic that's difficult to understand if you are not a progammer. Bacically, I >>>was studying explanations of this in the past but didnt't understand all of it, >>>and also do not remember all of it. >>> >>>My question is: >>> >>>How come that some solving times of test positions are worse (longer) with >>>bigger hash tables, than with smaller hash tables? >>> >>>Yours truly Paul J. Sacral >> >>Can you give an example? >>The size of the hashtable should not make a considerable difference in solving >>time, except if the engine clears the hashtable in analysis mode at the >>beginning of the search, which could take 1 or 2s on slow hardware. (Note during >>normal gameplay hastables usually don't get cleared.) >>Per position (in the search tree) it takes normally 1 (written ONE) probe per >>hashtable, doesn't matter if the hashtable is 1Mb or 1Gb. >>As a banal example, if you have a file register in your office, and you want to >>lookup file no. 56, it doesn't mater if you have 100 or 1000 files stored, you >>just walk to the shelf and take file number 56. >> >>regards >>Andy > >With changed table size you obviously map positions to other entries due to >hasIndex ::= someHashkey % tablesize. Two positions with disjoint slots with >some table size may share one slot with a bigger hash size. This is enough to >explain completely different search behaviour - and for some patological cases >even a longer solving time. > >Gerd Ok, the above statement is only true if table size if not power of two and you have hashIndex ::= someHashkey & (2**log2tblsize - 1). So different replacements of positions which map to same entries is the main cause to change search behaviour with the mentioned patological cases. Isn't "hash luck" is a common term? Gerd
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