RE: [PATCH v2] mempool: simplify get objects

Morten Brørup
2026-02-03T10:03:53Z

PING for review.

Here's some elaboration for reviewers...

Clearly, when the request can be served from the cache (n <= cache->len), the patch is correct, regardless if n is constant or variable:

__rte_assume(cache->len <= RTE_MEMPOOL_CACHE_MAX_SIZE * 2);
if (likely(n <= cache->len)) {
/*
* The entire request can be satisfied from the cache.
* If the request size is known at build time,
* the compiler unrolls the fixed length copy loop.
*/
cache->len -= n;
for (index = 0; index < n; index++)
*obj_table++ = *--cache_objs;

RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);

return 0;
}

Now, let's see what happens when the request cannot be served from the cache,
i.e. when n > cache-len:

__rte_assume(cache->len <= RTE_MEMPOOL_CACHE_MAX_SIZE * 2);
if (__rte_constant(n) && n <= cache->len) {
// FALSE, because n > cache->len
// Regardless if n is constant or variable
// /*
// * The request size is known at build time, and
// * the entire request can be satisfied from the cache,
// * so let the compiler unroll the fixed length copy loop.
// */
// cache->len -= n;
// for (index = 0; index < n; index++)
// *obj_table++ = *--cache_objs;
//
// RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
// RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
//
// return 0;
// }

/*
* Use the cache as much as we have to return hot objects first.
* If the request size 'n' is known at build time, the above comparison
* ensures that n > cache->len here, so omit RTE_MIN().
*/
len = __rte_constant(n) ? cache->len : RTE_MIN(n, cache->len);
// ALWAYS: len = cache->len
// When n is constant:
// len = cache->len
// When n is variable:
// len = RTE_MIN(n, cache->len)
// = cache->len, because n > cache->len
cache->len -= len;
// ALWAYS: cache->len = 0, because len == cache->len
remaining = n - len;
for (index = 0; index < len; index++)
*obj_table++ = *--cache_objs;

/*
* If the request size 'n' is known at build time, the case
* where the entire request can be satisfied from the cache
* has already been handled above, so omit handling it here.
*/
if (!__rte_constant(n) && likely(remaining == 0)) {
// FALSE, because remaining > 0
// /* The entire request is satisfied from the cache. */
//
// RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
// RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
//
// return 0;
// }

/* Dequeue below would overflow mem allocated for cache? */
if (unlikely(remaining > RTE_MEMPOOL_CACHE_MAX_SIZE))
goto driver_dequeue;

Venlig hilsen / Kind regards,
-Morten Brørup

> -----Original Message-----
> From: Morten Brørup [mailto:mb@smartsharesystems.com]
> Sent: Tuesday, 20 January 2026 11.17
> To: Andrew Rybchenko; dev@dpdk.org
> Cc: Morten Brørup
> Subject: [PATCH v2] mempool: simplify get objects
>
> Removed explicit test for build time constant request size,
> and added comment that the compiler loop unrolls when request size is
> build time constant, to improve source code readability.
>
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---
> v2:
> * Removed unrelated microoptimization from
> rte_mempool_do_generic_put(),
> which was also described incorrectly.
> ---
> lib/mempool/rte_mempool.h | 35 ++++++++---------------------------
> 1 file changed, 8 insertions(+), 27 deletions(-)
>
> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
> index aedc100964..4213784e14 100644
> --- a/lib/mempool/rte_mempool.h
> +++ b/lib/mempool/rte_mempool.h
> @@ -1531,11 +1531,11 @@ rte_mempool_do_generic_get(struct rte_mempool
> *mp, void **obj_table,
> cache_objs = &cache->objs[cache->len];
>
> __rte_assume(cache->len <= RTE_MEMPOOL_CACHE_MAX_SIZE * 2);
> - if (__rte_constant(n) && n <= cache->len) {
> + if (likely(n <= cache->len)) {
> /*
> - * The request size is known at build time, and
> - * the entire request can be satisfied from the cache,
> - * so let the compiler unroll the fixed length copy loop.
> + * The entire request can be satisfied from the cache.
> + * If the request size is known at build time,
> + * the compiler unrolls the fixed length copy loop.
> */
> cache->len -= n;
> for (index = 0; index < n; index++)
> @@ -1547,31 +1547,13 @@ rte_mempool_do_generic_get(struct rte_mempool
> *mp, void **obj_table,
> return 0;
> }
>
> - /*
> - * Use the cache as much as we have to return hot objects first.
> - * If the request size 'n' is known at build time, the above
> comparison
> - * ensures that n > cache->len here, so omit RTE_MIN().
> - */
> - len = __rte_constant(n) ? cache->len : RTE_MIN(n, cache->len);
> - cache->len -= len;
> + /* Use the cache as much as we have to return hot objects first.
> */
> + len = cache->len;
> remaining = n - len;
> + cache->len = 0;
> for (index = 0; index < len; index++)
> *obj_table++ = *--cache_objs;
>
> - /*
> - * If the request size 'n' is known at build time, the case
> - * where the entire request can be satisfied from the cache
> - * has already been handled above, so omit handling it here.
> - */
> - if (!__rte_constant(n) && likely(remaining == 0)) {
> - /* The entire request is satisfied from the cache. */
> -
> - RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> - RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
> -
> - return 0;
> - }
> -
> /* Dequeue below would overflow mem allocated for cache? */
> if (unlikely(remaining > RTE_MEMPOOL_CACHE_MAX_SIZE))
> goto driver_dequeue;
> @@ -1592,11 +1574,10 @@ rte_mempool_do_generic_get(struct rte_mempool
> *mp, void **obj_table,
> __rte_assume(cache->size <= RTE_MEMPOOL_CACHE_MAX_SIZE);
> __rte_assume(remaining <= RTE_MEMPOOL_CACHE_MAX_SIZE);
> cache_objs = &cache->objs[cache->size + remaining];
> + cache->len = cache->size;
> for (index = 0; index < remaining; index++)
> *obj_table++ = *--cache_objs;
>
> - cache->len = cache->size;
> -
> RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
> RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
>
> --
> 2.43.0