diff mbox series

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

Message ID 98CBD80474FA8B44BF855DF32C47DC35D8738E@smartserver.smartshare.dk (mailing list archive)
State Superseded, archived
Delegated to: Thomas Monjalon
Headers show
Series [v2] mempool: fix get objects from mempool with cache | expand


Context Check Description
ci/iol-x86_64-compile-testing success Testing PASS
ci/iol-x86_64-unit-testing success Testing PASS
ci/github-robot: build success github build: passed
ci/iol-aarch64-unit-testing success Testing PASS
ci/iol-aarch64-compile-testing success Testing PASS
ci/iol-intel-Functional success Functional Testing PASS
ci/iol-intel-Performance success Performance Testing PASS
ci/iol-mellanox-Performance success Performance Testing PASS
ci/checkpatch warning coding style issues

Commit Message

Morten Brørup Oct. 4, 2022, 4:39 p.m. UTC
RESENT for test purposes.

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
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@smartsharesystems.com>
 lib/mempool/rte_mempool.h | 75 ++++++++++++++++++++++++++++-----------
 1 file changed, 54 insertions(+), 21 deletions(-)
diff mbox series


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,
-	/* get remaining objects from ring */
+	/* Get the objects from the ring. */
 	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, n);
 	if (ret < 0) {