Author: Uri Blass
Date: 12:49:15 09/24/03
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On September 24, 2003 at 15:04:36, Sune Fischer wrote: >On September 24, 2003 at 13:25:49, Leen Ammeraal wrote: > >>>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. > >My sentiments exactly. > >While Bob is right that speed is a point, there are certainly other points to >consider. > >In a typical modern machine we can expect something like 512 MB of system >memory. Assuming hash entries are of power two, that means an always power 2 >sized table could not be able to use more than 256 MB, or more generally half of >the system memory. I'd call that a waste of resources. > >I always go for the limit myself, which seems to lie somewhere around 300-350 >MB. So I basicly exchange my 0.05% modulo speedloss for a smaller tree due to >larger hash. 0.05% Did you compare speeds to find out that it is 0.05%? I suspect that it is clearly bigger than it. Uri
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