[ovs-discuss] [PATCH] hash: Improve 128-bit hash to 32-bit output.

Andrew Mann andrew at divvycloud.com
Tue Dec 30 18:28:20 UTC 2014


For what it's worth, Bob Jenkins has some good discussion around this, and
some code that can help measure such properties of hashes:

http://burtleburtle.net/bob/hash/index.html

Specifically the "tests" section may offer a good start for measuring the
characteristics of the hash output with quantitative results.

On Mon, Dec 29, 2014 at 3:08 PM, Ben Pfaff <blp at nicira.com> wrote:

> On Mon, Dec 22, 2014 at 03:35:22PM -0800, Joe Stringer wrote:
> > Previously, when using the 128-bit hash in conjunction with the 32-bit
> > hash tables, we would ignore the upper 96 bits rather than attempting to
> > redistribute the hash across the 32-bit output space. This patch adds a
> > new function to translate the hash down from 128 bits to 32 bits for
> > this use case.
>
> Suppose that we had 128-bit random numbers instead of 128-bit hashes.
> Then, if combining the 32-bit pieces of that random number gave us a
> higher-quality random 32-bit number than just taking any one 32-bit
> piece, it would mean that the random numbers weren't very random.
>
> By analogy, I think that this patch (without reading it) should only
> make a difference if the 128-bit hash isn't very high-quality.  If so,
> it might be better to consider improving our 128-bit hash function,
> instead of the approach taken here.
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-- 
Andrew Mann
DivvyCloud Inc.
www.divvycloud.com
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