netdev - Re: rhashtable: Add cap on number of elements in hash table

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Message-Id: <20150518.161201.1959117179761854761.davem@davemloft.net>
Date:	Mon, 18 May 2015 16:12:01 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	johannes@...solutions.net, netdev@...r.kernel.org, kaber@...sh.net,
	tgraf@...g.ch, johannes.berg@...el.com
Subject: Re: rhashtable: Add cap on number of elements in hash table

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Sun, 17 May 2015 09:38:29 +0800

> On Sat, May 16, 2015 at 06:09:46PM -0400, David Miller wrote:
>>
>> Obviously something like 50 or 100 is too much.
>> 
>> Perhaps something between 5 and 10.
> 
> You are even more parsimonious than I :) Because the maximum chain
> length grows at a rate of logN * loglogN, I had chosen the number
> 16 which should be sufficient even if you had a 2^32 table (which
> you currently can't as we max out somewhere below that).
> 
> FWIW Thomas did some numbers on this and apparent 10 gets breached
> in a 2^24 table at 100% utilisation.

Ok, this of course depends upon the distribution of the input data
and the strength/suitability of the hash function.

I'm a little bit disappointed in what Thomas found.  I would expect
the distribution to be at least a little bit better.
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