Author: Robert Hyatt
Date: 17:35:34 11/24/01
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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...
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