Author: Amir Ban
Date: 11:08:42 10/08/97
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On October 08, 1997 at 12:51:44, Robert Hyatt wrote: >On October 08, 1997 at 07:08:01, Amir Ban wrote: > > > >>Don't believe it. The fact that hash collisions occur does not mean that >>it affects the PV, and if it does, it is a really freak accident. I do >>48-bit hashing with almost no validation. If you wait for my program to >>fail because of that you will get old in waiting. >> > >this is not necessarily true. Several of us, in a long thread in >r.g.c.c a couple of years ago, very carefully measured the number of >hash collisions produced using a 32 bit, 48 bit, and 64 bit hash key. >32 bits is totally hopeless. 48 bits was better, but still produced a >large number of collisions at high node rates. 64 bits produced a >*significant* number of hash collisions as well. These were all run on >machines that were then searching 20-30K nodes per second, except for me >(and the 64 bit numbers) where I ran the test on a C90 at 500K nodes per >second or so.) > >We are getting far more collisions than you imagine I suspect, based on >the numbers from Crafty, ZarkovX, I believe Ed contributed some results, >and I don't know who else was involved. To think that multiplying by >2000 is really like removing 11 bits from the hash signature is a >sobering thought. It is likely that they are on the fringe of seeing >bad things happen, particularly when they search for 20 minutes at a >pop. That's interesting, but how often do you get your PV and your actual best move changed ? Anyway, adding bits in hash is cheap, so if the damage is considerable you would need to be downright foolish to accept it. Amir
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