Re: Re: An interesting performance thing ?

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Dec 14, 2005 at 10:51:40PM -0600, Iozone wrote:
>        One of the interesting things I noticed is that the general
>    purpose hash_long() function may not be as optimal
>    as a more focused hash_IP_addr() function might be,
>    even if GOLDEN were GOLDEN :-) And, trying
>    to smash 128 bit IPV6 addresses into a 8 bit hash
>    value, that is somehow uniformly distributed, well,
>    that's going to be quite a neat trick, and making it
>    work for 32 bit, 64, and 128 bit objects, with uniform
>    distribution over a variable number of output bits,
>    is approaching magical.

I'm not so pessimistic, but I'm also not expert enough about hash
functions to know how much magic is reasonable to expect out of a good
one.  Time to pull out Knuth, maybe.

There might also be an argument for experimenting with different data
structures.  I'd expect high temporal locality--in typical situations a
server with lots of clients may have only a few that are active at a
particular time--so one of those balanced trees that migrates recently
looked-up items to the top might be helpful.

Aside from seeing the race condition triggered, have you done any
profiling to make sure this is actually a big problem, even with the
current worst-case linear-search behaviour?

--b.


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