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Subject: Re: Beowulf hot spots shown pictorially

Author: Koundinya Veluri

Date: 02:47:47 06/21/02

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On June 21, 2002 at 05:12:28, Sune Fischer wrote:

>On June 21, 2002 at 04:01:41, Sven Reichard wrote:
>
>>Maybe I'm missing something here, but if you furthermore agree to use a hash
>>table of size 2^i, i <= 32, you can replace all mods, divs and muls by a 32 bit
>>'and'. The performance hit of the slightly smaller table (less than factor 2)
>>should be outweighed by the faster access.
>>
>>just my 2 bits
>
>How do you know that?
>If you use power 2 size then you get pretty big jumps; 64,128,256,512.
>So you really have only 4 sizes of the hash. I think it is better to use 200 MB
>with modulo than 128 MB with AND (there are not divs or muls), the save is not
>_that_ much compared to what you save when getting those extra hashhits.
>Of course I don't _know_, I'm just guessing :)
>
>Some claim the distribution of the keys are better if the number of entries is
>prime, though I'm not convinced of that I can't rule it out.
>
>just my two øre.
>-S.
>
>>Sven.

If you use two hash tables that use power 2 size, then there will be many more
total hash sizes to choose from. If you have 200 MB available, you can divide
that into 128 MB for one and 64 MB for the other, that's 192 MB total.

Koundinya



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