Re: Question about tcp hash function tcp_hashfn()

[Evgeniy Polyakov <johnpol@2ka.mipt.ru>]

On Wed, May 31, 2006 at 11:41:27AM -0700, David Miller (davem@davemloft.net) wrote:
> > >   Worse: he folded the jenkins algorith result with
> > > 
> > >    h ^= h >> 16;
> > >    h ^= h >> 8;
> > > 
> > >   Destroying the coverage of the function.
> > 
> > It was done to simulate socket code which uses the same folding.
> > Leaving 32bit space is just wrong, consider hash table size with that
> > index.
> 
> You absolutely show not do this shifting on the jenkins hash
> result, you destroy the distribution entirely.  Just mask
> it with the hash mask and that's all you need to do.
> 
> Brian is right, this is absolutely critical to using the Jenkins hash
> correctly.  You're "unmixing" the bits it worked so hard to mix.

That is wrong. And I have a code and picture to show that, 
and you dont - prove me wrong :)

Attached code and picture which shows _exactly_ the same distribution for
folded and not folded Jenkins hash distribution, and it's artifact
compared to XOR hash.

Fairly distributed values being XORed produce still fairly distributed
value.

-- 
	Evgeniy Polyakov

Attachment: hash_comparison.png
Description: PNG image

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

typedef unsigned int u32;
typedef unsigned short u16;
typedef unsigned char u8;

typedef unsigned int __u32;
typedef unsigned short __u16;
typedef unsigned char __u8;

#include "jhash.h"

struct hash_entry
{
	unsigned long		counter;
};

static inline num2ip(__u8 a1, __u8 a2, __u8 a3, __u8 a4)
{
	__u32 a = 0;

	a |= a1;
	a << 8;
	a |= a2;
	a << 8;
	a |= a3;
	a << 8;
	a |= a4;

	return a;
}

static inline __u8 get_random_byte(void)
{
	return 1 + (int) (255.0 * (rand() / (RAND_MAX + 1.0)));
}

static inline __u16 get_random_word(void)
{
	return 1 + (int) (65536.0 * (rand() / (RAND_MAX + 1.0)));
}

unsigned int hash_addr(__u32 faddr, __u16 fport, __u32 laddr, __u16 lport)
{
	unsigned int h = (laddr ^ lport) ^ (faddr ^ fport);
	h ^= h >> 16;
	h ^= h >> 8;
	return h;
}

unsigned int hash_addr1(__u32 faddr, __u16 fport, __u32 laddr, __u16 lport)
{
	u32 ports;
	unsigned int h;

	ports = lport;
	ports <<= 16;
	ports |= fport;

	h = jhash_2words(faddr, laddr, ports);
	//h ^= h >> 16;
	//h ^= h >> 8;

	return h;
}

int main()
{
	struct hash_entry *table;
	__u32 saddr, daddr;
	__u16 sport, dport;
	unsigned int hash, i, *results;
	unsigned int hash_size = 65536, iter_num = 100;

	table = malloc(hash_size * sizeof(struct hash_entry));
	if (!table)
		return -ENOMEM;
	
	results = malloc(hash_size * sizeof(unsigned int));
	if (!results)
		return -ENOMEM;

	for (i=0; i<hash_size; ++i) {
		results[i] = 0;
		table[i].counter = 0;
	}

	srand(time(NULL));

	daddr = num2ip(192, 168, 0, 1);
	dport = htons(80);
	
	for (i=0; i<hash_size*iter_num; ++i) {
		saddr = num2ip(get_random_byte(), get_random_byte(), get_random_byte(), get_random_byte());
		sport = get_random_word();

		hash = hash_addr(saddr, sport, daddr, dport);
		hash &= (hash_size - 1);

		table[hash].counter++;
	}

	for (i=0; i<hash_size; ++i)
		results[table[i].counter]++;
	
	for (i=0; i<hash_size; ++i)
		printf("%u %u\n", i, results[i]);
		//printf("%u %u\n", i, table[i].counter);
}