Subject: Re: [PATCH next-next-2.6] netdev: better dev_name_hash

From: Eric Dumazet <[email-masked]>
Date: Mon, 26 Oct 2009 15:55:10 +0100

Octavian Purdila a écrit :
> On Monday 26 October 2009 15:07:31 you wrote:
>> Eric Dumazet wrote on 10/26/2009 10:58:34 AM:
>>> In fact, new10 should be the 'perfect' hash for the "eth%d"
>>> netdev use, not jhash (way more expensive in cpu cycles BTW)
>>>
>>> Most linux machines in the world have less than 10 interfaces, jhash
>>> would be really overkill.
>>>
>>>
>>> Thanks
>>>
>>>
>>> [PATCH net-next-2.6] netdev: better dev_name_hash
>> Changing Eric's test program to pass a multiplier to string_hash()
>> and calling string_hash with multipler=<2 - 63> confirms that 10
>> is almost always the best number for varying netdev names. I print
>> the number of times perfect 64 was scored, and for most passed
>> device names, the best is for n=10, followed by n=5 and others.
>> Almost the worst was n=31, slightly better was n=17.
>>
>> But other variables matter too, like fewer entries (4K or 1K) but
>> above values for are still better compared to n=31.
>>
> 
> Hmm, I've found out that it very much depends on the name as well:
> 
> 0 - orig, 1 - jhash, 2 - new10, 3 - new17, 4 - new31
> 
> $ ./dev_name_hash ixint 16000 0 16
> score 24741
> $ ./dev_name_hash ixint 16000 1 16
> score 17949
> $ ./dev_name_hash ixint 16000 2 16
> score 16000
> $ ./dev_name_hash ixint 16000 3 16
> score 16715
> $ ./dev_name_hash ixint 16000 4 16
> score 18125
> 
> 
> $ ./dev_name_hash ixunc 16000 0 16
> score 24741
> $ ./dev_name_hash ixunc 16000 1 16
> score 17904
> $ ./dev_name_hash ixunc 16000 2 16
> score 22180
> $ ./dev_name_hash ixunc 16000 3 16
> score 17065

> $ ./dev_name_hash ixunc 16000 4 16
> score 18038

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;
}

But should we really care ?