netdev - Re: [PATCH next-next-2.6] netdev: better dev_name

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Date:	Mon, 26 Oct 2009 17:52:51 +0200
From:	Octavian Purdila <opurdila@...acom.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Krishna Kumar2 <krkumar2@...ibm.com>,
	Hagen Paul Pfeifer <hagen@...u.net>, netdev@...r.kernel.org
Subject: Re: [PATCH next-next-2.6] netdev: better dev_name_hash

On Monday 26 October 2009 16:55:10 you wrote:
> 
> This is because you chose a 65536 slots hash table, to store 16000 elements
> 
> The ideal function should be :
> 
> $ ./dev_name_hash ixunc 16000 5 16
> score 16000
> 
> unsigned int dev_name_hash_new10bis(const char *name)
> {
> 	unsigned hash = 0;
> 	int len = strnlen(name, IFNAMSIZ);
> 	int i;
> 
> 	for (i = 0; i < len; ++i)
> 		hash = 10 * hash + (name[i] - '0');
> 	return hash;
> }
> 

Eric, thanks a lot for your help. This is turning into a very instructive 
thread for me :)

10bis performs better for ixunc but interestingly performs worse for ixint 
now. I also get mixed results for the two when using other names like ppp or 
gtp.

2 - new10, 3 - new10bis

score 49852
$ ./dev_name_hash ixint 32000 3 14
score 53194
$ ./dev_name_hash ixunc 32000 2 14
score 55232
$ ./dev_name_hash ixunc 32000 3 14
score 48168

> But should we really care ?

I'm just testing various common cases we use here ({ixint,ixunc,gtp,ppp,gre} 
{1000,16000,32000,128000} {14,16}). 

Ideally we want a hash function that performs better in all cases  - but I 
understand that it may not be possible.

I will play more with it and see if I can come up with something better, but 
in any case the new{10,10bis,17,31} performs much better than full_name_hash 
and most of the time better that jhash .
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