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

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Date:	Mon, 26 Oct 2009 18:37:31 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Hagen Paul Pfeifer <hagen@...u.net>, netdev@...r.kernel.org,
	Octavian Purdila <opurdila@...acom.com>
Subject: Re: [PATCH next-next-2.6] netdev: better dev_name_hash

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.

Thanks,

- KK

#include <stdio.h>
#include <string.h>

#define NUM_ENTRIES 1024

static inline unsigned int
string_hash_n(const unsigned char *name, unsigned int len, int n)
{
        unsigned hash = 0;
        int i;

        for (i = 0; i < len; ++i)
                hash = n * hash + name[i];
        return hash;
}

int best_n = -1, best_n_val = -1;

void print_dist(unsigned int *dist, int n)
{
        unsigned int i;
        int perfect = 0;


printf("-----------------------------------------------------------\n");
        printf("n=%d[] = {\n", n);
        for (i = 0; i < 256; i++) {
                if (dist[i] == NUM_ENTRIES/256)
                        perfect++;
                printf("%d, ", dist[i]);
                if ((i & 15) == 15) putchar('\n');
        }
        if (perfect > best_n_val) {
                best_n_val = perfect;
                best_n = n;
        }
        printf("}; (PERFECT = %d (n=%d))\n", perfect, n);
}

int main(int argc, char *argv[])
{
        int n, i;
        char *str, name[16];
        unsigned int hash, hashdist[256];

        if (argc == 1)
                str="eth";
        else
                str=argv[1];

        for (n = 2; n < 63; n++) {
                memset(hashdist, 0, 256*sizeof(int));
                for (i = 0; i < NUM_ENTRIES; i++) {
                        unsigned int len = sprintf(name, "%s%d", str, i);

                        hash = string_hash_n(name, len, n);
                        hashdist[hash & 255]++;
                }
                print_dist(hashdist, n);
        }
        printf("Best N was %d, and perfect entries: %d\n",
                best_n, best_n_val);
        return 0;
}

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