Author: Robert Hyatt
Date: 10:58:28 11/22/00
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On November 22, 2000 at 12:28:05, martin fierz wrote: >On November 22, 2000 at 10:06:24, Robert Hyatt wrote: > >>On November 22, 2000 at 05:38:32, martin fierz wrote: >> >>>hi, >>> >>>i was wondering about this PV-hashtable thing: in my checkers program, i do not >>>generate a separate PV. i have two hashtables, a 'shallow' and a 'deep' one, >>>positions close to the root of the tree are stored in shallow, the rest in deep. >>>in shallow, i take care not to fill the table and not to overwrite entries. i >>>took this idea from the crafty source code. in deep i overwrite everything >>>without looking twice. question: i have no PV, but i have a guarantee that for >>>the first 10 ply i have a hash move - because thats about what fits in shallow. >>>for the next 10 ply i won't have a hash move. do you think that this is a >>>serious problem? should i be generating a PV and stuffing hash moves back in the >>>table (that surely costs some speed...)? typical search depths are 15-19 ply in >>>a couple of seconds. do these last 5-9 ply really matter? >>> >>>cheers >>> martin >> >> >>They are often important for debugging, more than anything else. IE the goal >>is to play the right move. Nothing says you have to have the right PV, when >>playing a game. But if you can't see the PV when testing, you have a harder >>problem to understand what is going on and why. >> >>DB didn't have a separate PV because the hardware didn't support the idea. They >>seemed to play just fine. So yes, it can work... > >thanks for the answer. of course i can get a PV from the hashtable this way, as >long as the position is still in the hashtable, which it is for the first 10 >ply. but i see what you mean, i could get the whole PV to see which position >determines the final evaluation and also to see if the quiescense search is >performing properly. i suppose i should implement it :-) > >cheers > martin Remember that hashing is all about probability and distribution. There is absolutely no guarantee that you will get all 10 PV moves for ply 1 through 10, even if you have 1 million entries in that hash table. You can still get overwrites that clobber the positions. The larger the tree, the greater the probability that your PV will come up 'short' on occasion.
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