Author: Leen Ammeraal
Date: 10:25:49 09/24/03
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On September 24, 2003 at 11:49:58, Robert Hyatt wrote: >On September 24, 2003 at 11:20:13, William Penn wrote: > >>For example, is 512MB hash table size really better than 513MB (512+1), or >>better than 528MB (512+16)? >> >>Another example, is 768MB (3x256) better than 784MB (3x256+16)? >> >>I'm also wondering if these old rules (truisms) regarding optimum numbers for >>hash table size only apply to the Windows 9x/Me op systems which had problems >>with allocation of resources? I'm doubtful that they still apply to 2000/XP op >>systems. >> >>Does anyone really know? > >The point is a bit of speed. You have to convert a hash signature into a >hash table index. For a tablesize that is a power of 2, you can simply >AND (mask) off the upper bits leaving a power-of-2 table index. For other >sizes, you will end up doing a divide (mod) to get the remainder. The divide >is not fast. > >How significant this is is debatable, but for some of us, "every cycle counts." > > I agree that masking is to be preferred to the modulo operation. However, what about tournaments (organized by others) that allow a hash table size which corresponds to, for example, 12 MB entries in your table? It seems not wise to me to use only 8 MB entries in this case just to make the table length a power of 2. Leen Ammeraal
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