netdev - Re: [PATCH 1/2] neigh: Store hash shift instead of mask.

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Date:	Mon, 11 Jul 2011 14:08:28 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mirqus@...il.com
Cc:	roland@...estorage.com, johnwheffner@...il.com, mj@....cz,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] neigh: Store hash shift instead of mask.

From: Micha┅ Miros┅aw <mirqus@...il.com>
Date: Mon, 11 Jul 2011 13:58:41 +0200

> 2011/7/11 David Miller <davem@...emloft.net>:
>> And mask the hash function result by simply shifting
>> down the "->hash_shift" most significant bits.
>>
>> Currently which bits we use is arbitrary since jhash
>> produces entropy evenly across the whole hash function
>> result.
>>
>> But soon we'll be using universal hashing functions,
>> and in those cases more entropy exists in the higher
>> bits than the lower bits, because they use multiplies.
> 
> You could use some evil shift tricks to cut some instructions if you like. ;-)

If these were fast paths I would seriously consider it :-)

But they really are not.
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