Re: Horribly slow hash join

Here's an interesting link that suggests that hyperthreading would be
much worse.

http://groups.google.com/groups?q=hyperthreading+dual+xeon+idle&start=10&hl=en&lr=&ie=UTF-8&c2coff=1&selm=aukkonen-FE5275.21093624062003%40shawnews.gv.shawcable.net&rnum=16

FWIW, I have anecdotal evidence that suggests that this is the case, on
of my clients was seeing very large context switches with HTT turned on,
and without it was much better.

Dave
On Mon, 2004-04-19 at 02:09, Tom Lane wrote:
> Greg Stark <gsstark(at)mit(dot)edu> writes:
> > If the hash tables were made a power of two then it would be possible to mix
> > the bits of the 32 bit value and just mask off the unneeded bits. I've found
> > one page via google that mentions mixing bits in a hash function, but I would
> > look for a more serious treatment somewhere.
> > http://burtleburtle.net/bob/hash/doobs.html
> > Incidentally, this text claims mod is extremely slow compared to bit
> > manipulations.
>
> Modding by a *non* power of 2 (esp. a prime) mixes the bits quite well,
> and is likely faster than any multiple-instruction way to do the same.
>
> The quoted article seems to be by someone who has spent a lot of time
> counting assembly cycles and none at all reading the last thirty years
> worth of CS literature. Knuth's treatment of hashing has some actual
> math to it...
>
> regards, tom lane
>
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Dave Cramer
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