Author: Robert Hyatt
Date: 18:46:45 11/24/01
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On November 24, 2001 at 21:04:34, Jesper Antonsson wrote: >On November 24, 2001 at 20:35:34, Robert Hyatt wrote: > >>On November 24, 2001 at 18:02:59, Jesper Antonsson wrote: >> >>>This is speculation, as I'm not a chess program author myself, but I wonder if >>>anyone has experimented recently and could explain things to me. >>> >>>Machines of today has a tremendous memory bottleneck and I asked myself the >>>other day how come the large hash tables used today are beneficial. I remember >>>figures from long ago when hash tables were said to give a speedup of perhaps >>>3%, but today when processors are 10-20 times faster than main memory, they >>>should give less, perhaps even be detrimental? If you turn of hash tables >>>entirely, how much of an increase does this give in NPS on a 1 Ghz+ processor? >>>Nothing? A lot? >> >>Hash tables have _nothing_ to do with NPS. > >You know better. Of course memory references costs. Not here. The typical chess engine does at _least_ 3000 instructions per node. One memory load in that mess won't change a thing... > >>They have a _lot_ to do with >>the size of the tree that is searched. IE try a fixed-depth search (say to >>12 plies) and vary the size of the hash table from small to large. The size >>of the tree will vary by 2-3X, which is a _significant_ advantage in terms of >>speed. Even though the raw NPS stays pretty much constant.. > >Ok, if the tree size decreases that much, I guess hash tables are ok for >now. You may continue. :-) > >>>Has anyone experimented with small hashtables, carefully tuned to fit in cache, >>>and used perhaps only in shallow parts of the tree, and compared the results to >>>the standard "use as much as you have"-approach? *Especially* in lightning >>>games, where a huge hash table won't be filled anyway, a cache-tuned table could >>>perhaps perform better? >> >> >>It would be way too small and get overwritten at a ridiculous rate. >> >>>And by the way, does anyone bother to try to make sure his/her engine itself >>>fits in instruction cache and that the search-function is so aligned that it >>>won't get pushed out of instruction cache by more seldomly used functions? >>> >>>br, >>>Jesper >> >>Most likely everybody does this... > >Oh, cool. I haven't heard anything about this, so I thought otherwise, but if >you say so... > >Jesper Chess programmers are probably as good at "optimizing" as any group you will ever find, as a whole... Speed is _everything_.
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