netdev - Re: [NETFILTER] xt_hashlimit : speedups hash

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Message-Id: <20071214.103031.39124025.davem@davemloft.net>
Date:	Fri, 14 Dec 2007 10:30:31 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	kaber@...sh.net, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [NETFILTER] xt_hashlimit : speedups hash_dst()

From: Eric Dumazet <dada1@...mosbay.com>
Date: Fri, 14 Dec 2007 12:09:31 +0100

> 1) Using jhash2() instead of jhash() is a litle bit faster if applicable.
> 
> 2) Thanks to jhash, hash value uses full 32 bits.
>     Instead of returning hash % size (implying a divide)
>     we return the high 32 bits of the (hash * size) that will
>     give results between [0 and size-1] and same hash distribution.
> 
>    On most cpus, a multiply is less expensive than a divide, by an order
>    of magnitude.
>   
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

As a side note, Jenkins performs nearly optimally (unlike
most traditional hash functions) with power of two hash
table sizes.

Using a pow2 hash table size would completely obviate the
issues solved by #2.

I don't know if that is feasible here in xt_hashlimit, but
if it is that is how we should solve this expensive
modulo.
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