From: "Iozone" <capps@iozone.org>
Subject: Re: An interesting performance thing ?
Date: Wed, 14 Dec 2005 17:47:47 -0600
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 <17312.39940.985507.704832@cse.unsw.edu.au> <43A0A0D5.4040804@citi.umich.edu>
Reply-To: "Iozone" <capps@iozone.org>
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Cc: <wli@holomorphy.com>, <nfs@lists.sourceforge.net>
To: <cel@citi.umich.edu>, "Neil Brown" <neilb@suse.de>
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----- Original Message ----- 
From: "Chuck Lever" <cel@citi.umich.edu>
To: "Neil Brown" <neilb@suse.de>
Cc: "Iozone" <capps@iozone.org>; <wli@holomorphy.com>; 
<nfs@lists.sourceforge.net>
Sent: Wednesday, December 14, 2005 4:46 PM
Subject: Re: An interesting performance thing ?


> Neil Brown wrote:
>> William, Chunk:  Any suggestions on whether a straight multiply would
>> be too expensive, or what else we could do to make hash_long both fast
>> and effective?
>
> my original proposal was a hash function which computed the index by a
> single multiplication with a large prime number.  i was told that
> multiplication was too expensive, especially on older platforms, so the
> existing computation took its place.  but i don't think anyone ever did
> any real studies.  i can't imagine that multiplication would be worse
> than extra elements in a hash chain, especially on modern CPU 
> architectures.
>

Neil,

    In a perfect world, a perfect hash would be ideal...but,

    1. It is fairly normal practice for code to call inet_lnaof( in_addr )
        so that the host native format is used by the host. Just
        grabbing s_addr is a bit odd.
    2. Simply adding the inet_lnaof( in_addr ) would do nice
        things with the existing hash algorithm, for CDIR allocated
        monotonically incremented IP address spaces.
    3. Gosh, it seems like a pretty easy and safe change.

        I think I understand Neil's position. If one hands a 64 bit
    opaque object to a hash function, it should do a better
    job of distribution, so who cares if it was in the native
    format or not.  Ok..  But, given 1->3 above, it seems
    like there might be a lower risk, less controversial, solution
    for the immediate future, and permit folks to do a study
    while the world goes on its happy way :-)

    Thanks again for being kind to new kid tossing
    spitballs :-)

Enjoy,
Don Capps


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