netdev - Re: [NETFILTER] xt_hashlimit : speedups hash

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Date:	Fri, 14 Dec 2007 22:37:15 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	Patrick McHardy <kaber@...sh.net>, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [NETFILTER] xt_hashlimit : speedups hash_dst()

Jarek Poplawski a écrit :
> Eric Dumazet wrote, On 12/14/2007 12:09 PM:
> ...
> 
>> +	/*
>> +	 * Instead of returning hash % ht->cfg.size (implying a divide)
>> +	 * we return the high 32 bits of the (hash * ht->cfg.size) that will
>> +	 * give results between [0 and cfg.size-1] and same hash distribution,
>> +	 * but using a multiply, less expensive than a divide
>> +	 */
>> +	return ((u64)hash * ht->cfg.size) >> 32;
> 
> Are we sure of the same hash distribution? Probably I miss something,
> but: if this 'hash' is well distributed on 32 bits, and ht->cfg.size
> is smaller than 32 bits, e.g. 256 (8 bits), then this multiplication
> moves to the higher 32 of u64 only max. 8 bits of the most significant
> byte, and the other three bytes are never used, while division is
> always affected by all four bytes...

Not sure what you are saying... but if size=256, then, yes, we want a final 
result between 0 and 255, so three bytes are nul.


'size' is the size of hashtable, its not a random 32bits value :)

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