Author: martin fierz
Date: 12:36:04 11/25/01
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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. 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.. > > >> >>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, i don't do that - and i don't even know how to do it :-( can you elaborate on this a bit? mahalo martin
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