Author: Tom Kerrigan
Date: 01:05:47 05/11/03
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On May 10, 2003 at 15:07:55, Peter Stayne wrote: >The more RAM you give it for hash over what it needs, the slower it will >perform. > >Take a look at the manual page 18. 380 is way overboard. it says 4-12MB is good >for blitz. 100MB for 3 minute/move tournament games. if you take an average of >60 moves in a game, that's a 3 hour timelimit game. > >Set it for 64 if you plan on playing blitz a lot, or 128 for tournament games. >anything more will indeed slow it down. > >I'm not sure actually why it slows it down, anybody know exactly? > >pete There are a couple reasons why a chess program would slow down with bigger hash tables: 1. It clears the hash tables for each move. This will obviously take longer with bigger hash tables (up to a couple seconds). The performance hit is only at the beginning of the search, so it will matter a lot for blitz and almost not at all for tournament. I think programs that do this are kind of stupid--it's easy enough to store a number in each hash entry indicating which search it belongs to, and if it's from an old search, ignore it. Effectively the same as clearing the hash table, but basically takes zero time. 2. You made the hash table so big that it takes a long time to allocate, i.e., caches must be resized and data for other programs must be written to the hard drive in order to fit the entire hash table into physical memory. But once the table IS in memory, program speed will not be affected. 3. You made the hash table WAY too big and it can't all fit in physical memory. In this case, performance will be abysmal and your hard drive will always be busy. 4. Data from the hash table causes the program to spend more time on "slower" nodes. NPS might get marginally slower but playing strength is not affected. In other words, use a hash table that's as big as possible without causing hard drive crunching, unless you're affected by (1), in which case, you might want to consider using a different program. -Tom
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