Re: [dpdk-dev] [PATCH v3 1/4] hash: reorder hash structure

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Sep 06, 2016 at 08:34:01PM +0100, Pablo de Lara wrote:
> In order to optimize lookup performance, hash structure
> is reordered, so all fields used for lookup will be
> in the first cache line.
> 
> Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
> ---
>  lib/librte_hash/rte_cuckoo_hash.h | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/librte_hash/rte_cuckoo_hash.h b/lib/librte_hash/rte_cuckoo_hash.h
> index e290dab..701531a 100644
> --- a/lib/librte_hash/rte_cuckoo_hash.h
> +++ b/lib/librte_hash/rte_cuckoo_hash.h
> @@ -182,9 +182,7 @@ struct rte_hash_bucket {
>  
>  /** A hash table structure. */
>  struct rte_hash {
> -	char name[RTE_HASH_NAMESIZE];   /**< Name of the hash. */
> -	uint32_t entries;               /**< Total table entries. */
> -	uint32_t num_buckets;           /**< Number of buckets in table. */
> +	/* first cache line - fields used in lookup */
>  	uint32_t key_len;               /**< Length of hash key. */
>  	rte_hash_function hash_func;    /**< Function used to calculate hash. */
>  	uint32_t hash_func_init_val;    /**< Init value used by hash_func. */
> @@ -196,12 +194,13 @@ struct rte_hash {
>  						from hash signature. */
>  	uint32_t key_entry_size;         /**< Size of each key entry. */
>  
> -	struct rte_ring *free_slots;    /**< Ring that stores all indexes
> -						of the free slots in the key table */
>  	void *key_store;                /**< Table storing all keys and data */
>  	struct rte_hash_bucket *buckets;	/**< Table with buckets storing all the
>  							hash values and key indexes
>  							to the key table*/
> +
> +	struct rte_ring *free_slots;    /**< Ring that stores all indexes
> +						of the free slots in the key table */
>  	uint8_t hw_trans_mem_support;	/**< Hardware transactional
>  							memory support */
>  	struct lcore_cache *local_free_slots;
> @@ -209,6 +208,9 @@ struct rte_hash {
>  	enum add_key_case add_key; /**< Multi-writer hash add behavior */
>  
>  	rte_spinlock_t *multiwriter_lock; /**< Multi-writer spinlock for w/o TM */
> +	char name[RTE_HASH_NAMESIZE];   /**< Name of the hash. */
> +	uint32_t entries;               /**< Total table entries. */
> +	uint32_t num_buckets;           /**< Number of buckets in table. */

Hi Pablo,

While I've no strong objection to the change, having the name at the start
is a common paradigm in DPDK. Rather than place these fields at the end,
can you get the same effect by just marking the key_len function
__rte_cache_aligned? It may use a little more memory per table, but given that
the size of the hash table is going to be largely governed by the table data,
I don't see an extra 64 bytes in the structure as being an issue.

Regards,
/Bruce