Subject: Re: [PATCH] net: fold network name hash (v2)

From: Eric Dumazet <[email-masked]>
Date: Wed, 28 Oct 2009 07:07:10 +0100

Stephen Hemminger a écrit :
> The full_name_hash does not produce a value that is evenly distributed
> over the lower 8 bits. This causes name hash to be unbalanced with large
> number of names. There is a standard function to fold in upper bits
> so use that.
> 
> This is independent of possible improvements to full_name_hash()
> in future.

>  static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)
>  {
>  	unsigned hash = full_name_hash(name, strnlen(name, IFNAMSIZ));
> -	return &net->dev_name_head[hash & ((1 << NETDEV_HASHBITS) - 1)];
> +	return &net->dev_name_head[hash_long(hash, NETDEV_HASHBITS)];
>  }
>  
>  static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)

full_name_hash() returns an "unsigned int", which is guaranteed to be 32 bits

You should therefore use hash_32(hash, NETDEV_HASHBITS),
not hash_long() that maps to hash_64() on 64 bit arches, which is
slower and certainly not any better with a 32bits input.

/* Compute the hash for a name string. */
static inline unsigned int
full_name_hash(const unsigned char *name, unsigned int len)
{
        unsigned long hash = init_name_hash();
        while (len--)
                hash = partial_name_hash(*name++, hash);
        return end_name_hash(hash);
}

static inline u32 hash_32(u32 val, unsigned int bits)
{
        /* On some cpus multiply is faster, on others gcc will do shifts */
        u32 hash = val * GOLDEN_RATIO_PRIME_32;

        /* High bits are more random, so use them. */
        return hash >> (32 - bits);
}

static inline u64 hash_64(u64 val, unsigned int bits)
{
        u64 hash = val;

        /*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
        u64 n = hash;
        n <<= 18;
        hash -= n;
        n <<= 33;
        hash -= n;
        n <<= 3;
        hash += n;
        n <<= 3;
        hash -= n;
        n <<= 4;
        hash += n;
        n <<= 2;
        hash += n;

        /* High bits are more random, so use them. */
        return hash >> (64 - bits);
}