[PATCH v2 3/3] mempool: use cache for frequently updated statistics

[Mattias Rönnblom]

On 2022-10-31 12:26, Morten Brørup wrote:
> When built with statistics enabled (RTE_LIBRTE_MEMPOOL_STATS defined), the
> performance of mempools with caches is improved as follows.
> 
> When accessing objects in the mempool, either the put_bulk and put_objs or
> the get_success_bulk and get_success_objs statistics counters are likely
> to be incremented.
> 
> By adding an alternative set of these counters to the mempool cache
> structure, accesing the dedicated statistics structure is avoided in the
> likely cases where these counters are incremented.
> 
> The trick here is that the cache line holding the mempool cache structure
> is accessed anyway, in order to access the 'len' or 'flushthresh' fields.
> Updating some statistics counters in the same cache line has lower
> performance cost than accessing the statistics counters in the dedicated
> statistics structure, which resides in another cache line.
> 
> mempool_perf_autotest with this patch shows the follwing change in
> rate_persec.
> 
> Compared to only spliting statistics from debug:
> +1.5 % and +14.4 %, respectively without and with cache.
> 
> Compared to not enabling mempool stats:
> -4.4 % and -9.9 %, respectively without and with cache.
> 
> v2:
> * Move the statistics counters into a stats structure.
> 
> Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
> ---
>   lib/mempool/rte_mempool.c |  9 +++++
>   lib/mempool/rte_mempool.h | 73 ++++++++++++++++++++++++++++++++-------
>   2 files changed, 69 insertions(+), 13 deletions(-)
> 
> diff --git a/lib/mempool/rte_mempool.c b/lib/mempool/rte_mempool.c
> index e6208125e0..a18e39af04 100644
> --- a/lib/mempool/rte_mempool.c
> +++ b/lib/mempool/rte_mempool.c
> @@ -1286,6 +1286,15 @@ rte_mempool_dump(FILE *f, struct rte_mempool *mp)
>   		sum.get_success_blks += mp->stats[lcore_id].get_success_blks;
>   		sum.get_fail_blks += mp->stats[lcore_id].get_fail_blks;
>   	}
> +	if (mp->cache_size != 0) {
> +		/* Add the statistics stored in the mempool caches. */
> +		for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++) {
> +			sum.put_bulk += mp->local_cache[lcore_id].stats.put_bulk;
> +			sum.put_objs += mp->local_cache[lcore_id].stats.put_objs;
> +			sum.get_success_bulk += mp->local_cache[lcore_id].stats.get_success_bulk;
> +			sum.get_success_objs += mp->local_cache[lcore_id].stats.get_success_objs;
> +		}
> +	}
>   	fprintf(f, "  stats:\n");
>   	fprintf(f, "    put_bulk=%"PRIu64"\n", sum.put_bulk);
>   	fprintf(f, "    put_objs=%"PRIu64"\n", sum.put_objs);
> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> index 16e7e62e3c..5806e75609 100644
> --- a/lib/mempool/rte_mempool.h
> +++ b/lib/mempool/rte_mempool.h
> @@ -86,6 +86,21 @@ struct rte_mempool_cache {
>   	uint32_t size;	      /**< Size of the cache */
>   	uint32_t flushthresh; /**< Threshold before we flush excess elements */
>   	uint32_t len;	      /**< Current cache count */
> +	uint32_t unused0;
> +#ifdef RTE_LIBRTE_MEMPOOL_STATS
> +	/*
> +	 * Alternative location for the most frequently updated mempool statistics (per-lcore),
> +	 * providing faster update access when using a mempool cache.
> +	 */
> +	struct {
> +		uint64_t put_bulk;          /**< Number of puts. */
> +		uint64_t put_objs;          /**< Number of objects successfully put. */
> +		uint64_t get_success_bulk;  /**< Successful allocation number. */
> +		uint64_t get_success_objs;  /**< Objects successfully allocated. */
> +	} stats;                        /**< Statistics */
> +#else
> +	uint64_t unused1[4];

Are a particular DPDK version supposed to be ABI compatible with itself, 
with different configuration options? E.g., with and without 
RTE_LIBRTE_MEMPOOL_STATS. Is that why you have those 4 unused uint64_ts?

> +#endif
>   	/**
>   	 * Cache objects
>   	 *
> @@ -296,14 +311,14 @@ struct rte_mempool {
>   	| RTE_MEMPOOL_F_NO_IOVA_CONTIG \
>   	)
>   /**
> - * @internal When debug is enabled, store some statistics.
> + * @internal When stats is enabled, store some statistics.
>    *
>    * @param mp
>    *   Pointer to the memory pool.
>    * @param name
>    *   Name of the statistics field to increment in the memory pool.
>    * @param n
> - *   Number to add to the object-oriented statistics.
> + *   Number to add to the statistics.
>    */
>   #ifdef RTE_LIBRTE_MEMPOOL_STATS
>   #define RTE_MEMPOOL_STAT_ADD(mp, name, n) do {                  \
> @@ -312,6 +327,23 @@ struct rte_mempool {
>   #else
>   #define RTE_MEMPOOL_STAT_ADD(mp, name, n) do {} while (0)
>   #endif
> +/**
> + * @internal When stats is enabled, store some statistics.
> + *
> + * @param cache
> + *   Pointer to the memory pool cache.
> + * @param name
> + *   Name of the statistics field to increment in the memory pool cache.
> + * @param n
> + *   Number to add to the statistics.
> + */
> +#ifdef RTE_LIBRTE_MEMPOOL_STATS
> +#define RTE_MEMPOOL_CACHE_STAT_ADD(cache, name, n) do { \
> +		(cache)->stats.name += n;               \
> +	} while (0)
> +#else
> +#define RTE_MEMPOOL_CACHE_STAT_ADD(cache, name, n) do {} while (0)

Somewhat unrelated comment: maybe <rte_common.h> should have a RTE_NOP 
macro.

> +#endif
>   
>   /**
>    * @internal Calculate the size of the mempool header.
> @@ -1327,13 +1359,17 @@ rte_mempool_do_generic_put(struct rte_mempool *mp, void * const *obj_table,
>   {
>   	void **cache_objs;
>   
> +	/* No cache provided */
> +	if (unlikely(cache == NULL))
> +		goto driver_enqueue;
> +
>   	/* increment stat now, adding in mempool always success */
> -	RTE_MEMPOOL_STAT_ADD(mp, put_bulk, 1);
> -	RTE_MEMPOOL_STAT_ADD(mp, put_objs, n);
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_bulk, 1);
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, n);
>   
> -	/* No cache provided or the request itself is too big for the cache */
> -	if (unlikely(cache == NULL || n > cache->flushthresh))
> -		goto driver_enqueue;
> +	/* The request itself is too big for the cache */
> +	if (unlikely(n > cache->flushthresh))
> +		goto driver_enqueue_stats_incremented;
>   
>   	/*
>   	 * The cache follows the following algorithm:
> @@ -1358,6 +1394,12 @@ rte_mempool_do_generic_put(struct rte_mempool *mp, void * const *obj_table,
>   
>   driver_enqueue:
>   
> +	/* increment stat now, adding in mempool always success */
> +	RTE_MEMPOOL_STAT_ADD(mp, put_bulk, 1);
> +	RTE_MEMPOOL_STAT_ADD(mp, put_objs, n);
> +
> +driver_enqueue_stats_incremented:
> +
>   	/* push objects to the backend */
>   	rte_mempool_ops_enqueue_bulk(mp, obj_table, n);
>   }
> @@ -1464,8 +1506,8 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
>   	if (remaining == 0) {
>   		/* The entire request is satisfied from the cache. */
>   
> -		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> -		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> +		RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> +		RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
>   
>   		return 0;
>   	}
> @@ -1494,8 +1536,8 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
>   
>   	cache->len = cache->size;
>   
> -	RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> -	RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
>   
>   	return 0;
>   
> @@ -1517,8 +1559,13 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
>   		RTE_MEMPOOL_STAT_ADD(mp, get_fail_bulk, 1);
>   		RTE_MEMPOOL_STAT_ADD(mp, get_fail_objs, n);
>   	} else {
> -		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> -		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> +		if (likely(cache != NULL)) {
> +			RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> +			RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
> +		} else {
> +			RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> +			RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> +		}
>   	}
>   
>   	return ret;