Author: Andreas Guettinger
Date: 11:41:48 12/03/05
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On December 03, 2005 at 14:20:57, Gerd Isenberg wrote: >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 To Uri and you: I wanted to explain very simply to others that probing a hashtable does not contain any searching in the hashtable, because that seemed to be unclear. Furthermore, also the solving time of a solution can be much faster with "hash luck" and can be different with different sizes of hashtables, the probe itself takes exactly the same time, independant of hashtable size, doesn't it. I think this point was not clear. regards Andy
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