[PATCH v2] mempool: fix get objects from mempool with cache

[Morten Brørup]

+CC: Beilei Xing <beilei.xing at intel.com>, i40e maintainer, may be interested in the performance improvements achieved by this patch.

> From: Morten Brørup [mailto:mb at smartsharesystems.com]
> Sent: Wednesday, 2 February 2022 09.14
> 
> A flush threshold for the mempool cache was introduced in DPDK version
> 1.3, but rte_mempool_do_generic_get() was not completely updated back
> then, and some inefficiencies were introduced.
> 
> This patch fixes the following in rte_mempool_do_generic_get():
> 
> 1. The code that initially screens the cache request was not updated
> with the change in DPDK version 1.3.
> The initial screening compared the request length to the cache size,
> which was correct before, but became irrelevant with the introduction
> of
> the flush threshold. E.g. the cache can hold up to flushthresh objects,
> which is more than its size, so some requests were not served from the
> cache, even though they could be.
> The initial screening has now been corrected to match the initial
> screening in rte_mempool_do_generic_put(), which verifies that a cache
> is present, and that the length of the request does not overflow the
> memory allocated for the cache.
> 
> This bug caused a major performance degradation in scenarios where the
> application burst length is the same as the cache size. In such cases,
> the objects were not ever fetched from the mempool cache, regardless if
> they could have been.
> This scenario occurs e.g. if an application has configured a mempool
> with a size matching the application's burst size.
> 
> 2. The function is a helper for rte_mempool_generic_get(), so it must
> behave according to the description of that function.
> Specifically, objects must first be returned from the cache,
> subsequently from the ring.
> After the change in DPDK version 1.3, this was not the behavior when
> the request was partially satisfied from the cache; instead, the
> objects
> from the ring were returned ahead of the objects from the cache.
> This bug degraded application performance on CPUs with a small L1
> cache,
> which benefit from having the hot objects first in the returned array.
> (This is probably also the reason why the function returns the objects
> in reverse order, which it still does.)
> Now, all code paths first return objects from the cache, subsequently
> from the ring.
> 
> The function was not behaving as described (by the function using it)
> and expected by applications using it. This in itself is also a bug.
> 
> 3. If the cache could not be backfilled, the function would attempt
> to get all the requested objects from the ring (instead of only the
> number of requested objects minus the objects available in the ring),
> and the function would fail if that failed.
> Now, the first part of the request is always satisfied from the cache,
> and if the subsequent backfilling of the cache from the ring fails,
> only
> the remaining requested objects are retrieved from the ring.
> 
> The function would fail despite there are enough objects in the cache
> plus the common pool.
> 
> 4. The code flow for satisfying the request from the cache was slightly
> inefficient:
> The likely code path where the objects are simply served from the cache
> was treated as unlikely. Now it is treated as likely.
> And in the code path where the cache was backfilled first, numbers were
> added and subtracted from the cache length; now this code path simply
> sets the cache length to its final value.
> 
> v2 changes
> - Do not modify description of return value. This belongs in a separate
> doc fix.
> - Elaborate even more on which bugs the modifications fix.
> 
> Signed-off-by: Morten Brørup <mb at smartsharesystems.com>
> ---
>  lib/mempool/rte_mempool.h | 75 ++++++++++++++++++++++++++++-----------
>  1 file changed, 54 insertions(+), 21 deletions(-)
> 
> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> index 1e7a3c1527..2898c690b0 100644
> --- a/lib/mempool/rte_mempool.h
> +++ b/lib/mempool/rte_mempool.h
> @@ -1463,38 +1463,71 @@ rte_mempool_do_generic_get(struct rte_mempool
> *mp, void **obj_table,
>  	uint32_t index, len;
>  	void **cache_objs;
> 
> -	/* No cache provided or cannot be satisfied from cache */
> -	if (unlikely(cache == NULL || n >= cache->size))
> +	/* No cache provided or if get would overflow mem allocated for
> cache */
> +	if (unlikely(cache == NULL || n > RTE_MEMPOOL_CACHE_MAX_SIZE))
>  		goto ring_dequeue;
> 
> -	cache_objs = cache->objs;
> +	cache_objs = &cache->objs[cache->len];
> +
> +	if (n <= cache->len) {
> +		/* The entire request can be satisfied from the cache. */
> +		cache->len -= n;
> +		for (index = 0; index < n; index++)
> +			*obj_table++ = *--cache_objs;
> +
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> 
> -	/* Can this be satisfied from the cache? */
> -	if (cache->len < n) {
> -		/* No. Backfill the cache first, and then fill from it */
> -		uint32_t req = n + (cache->size - cache->len);
> +		return 0;
> +	}
> 
> -		/* How many do we require i.e. number to fill the cache +
> the request */
> -		ret = rte_mempool_ops_dequeue_bulk(mp,
> -			&cache->objs[cache->len], req);
> +	/* Satisfy the first part of the request by depleting the cache.
> */
> +	len = cache->len;
> +	for (index = 0; index < len; index++)
> +		*obj_table++ = *--cache_objs;
> +
> +	/* Number of objects remaining to satisfy the request. */
> +	len = n - len;
> +
> +	/* Fill the cache from the ring; fetch size + remaining objects.
> */
> +	ret = rte_mempool_ops_dequeue_bulk(mp, cache->objs,
> +			cache->size + len);
> +	if (unlikely(ret < 0)) {
> +		/*
> +		 * We are buffer constrained, and not able to allocate
> +		 * cache + remaining.
> +		 * Do not fill the cache, just satisfy the remaining part
> of
> +		 * the request directly from the ring.
> +		 */
> +		ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, len);
>  		if (unlikely(ret < 0)) {
>  			/*
> -			 * In the off chance that we are buffer constrained,
> -			 * where we are not able to allocate cache + n, go to
> -			 * the ring directly. If that fails, we are truly out
> of
> -			 * buffers.
> +			 * That also failed.
> +			 * No further action is required to roll the first
> +			 * part of the request back into the cache, as both
> +			 * cache->len and the objects in the cache are
> intact.
>  			 */
> -			goto ring_dequeue;
> +			RTE_MEMPOOL_STAT_ADD(mp, get_fail_bulk, 1);
> +			RTE_MEMPOOL_STAT_ADD(mp, get_fail_objs, n);
> +
> +			return ret;
>  		}
> 
> -		cache->len += req;
> +		/* Commit that the cache was emptied. */
> +		cache->len = 0;
> +
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
> +		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> +
> +		return 0;
>  	}
> 
> -	/* Now fill in the response ... */
> -	for (index = 0, len = cache->len - 1; index < n; ++index, len--,
> obj_table++)
> -		*obj_table = cache_objs[len];
> +	cache_objs = &cache->objs[cache->size + len];
> 
> -	cache->len -= n;
> +	/* Satisfy the remaining part of the request from the filled
> cache. */
> +	cache->len = cache->size;
> +	for (index = 0; index < len; index++)
> +		*obj_table++ = *--cache_objs;
> 
>  	RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
>  	RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
> @@ -1503,7 +1536,7 @@ rte_mempool_do_generic_get(struct rte_mempool
> *mp, void **obj_table,
> 
>  ring_dequeue:
> 
> -	/* get remaining objects from ring */
> +	/* Get the objects from the ring. */
>  	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, n);
> 
>  	if (ret < 0) {
> --
> 2.17.1

PING.

According to Patchwork [1], this patch provides up to 10.9 % single thread throughput improvement on XL710 with x86, and 0.7 % improvement with ARM.

Still no interest?

PS: Bruce reviewed V1 of this patch [2], but I don't think it is appropriate copying a Reviewed-by tag from one version of a patch to another, regardless how small the changes are.

[1] http://mails.dpdk.org/archives/test-report/2022-February/256462.html
[2] http://inbox.dpdk.org/dev/YeaDSxj%2FuZ0vPMl+@bricha3-MOBL.ger.corp.intel.com/