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From: David Miller <davem@davemloft.net>
Subject: Re: ipv4: Simplify ARP hash function.
Date: Sun, 10 Jul 2011 12:07:32 -0700 (PDT)
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From: Roland Dreier <roland@purestorage.com>
Date: Fri, 8 Jul 2011 16:11:00 -0700

> Maybe * of hash key with a random odd value is good enough?

Yes, from what I've read over the past few days it should
be.  More precisely:

	(key * hash_rnd) >> (32 - hash_table_size_log2)

where "hash_rnd" is odd.

The reason we want the top bits is because multiplies intrinsically
work such that bits in the inputs can only effect the same or higher
bits in the result.