diff mbox series

[v3] mempool: fix get objects from mempool with cache

Message ID 20221004125354.3438386-1-andrew.rybchenko@oktetlabs.ru (mailing list archive)
State Superseded, archived
Delegated to: Thomas Monjalon
Headers show
Series [v3] mempool: fix get objects from mempool with cache | expand

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Commit Message

Andrew Rybchenko Oct. 4, 2022, 12:53 p.m. UTC
From: Morten Brørup <mb@smartsharesystems.com>

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.

Fix 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.

Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
---
v3 changes (Andrew Rybchenko)
 - Always get first objects from the cache even if request is bigger
   than cache size. Remove one corresponding condition from the path
   when request is fully served from cache.
 - Simplify code to avoid duplication:
    - Get objects directly from backend in single place only.
    - Share code which gets from the cache first regardless if
      everythihg is obtained from the cache or just the first part.
 - Rollback cache length in unlikely failure branch to avoid cache
   vs NULL check in success branch.

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.

 lib/mempool/rte_mempool.h | 74 +++++++++++++++++++++++++--------------
 1 file changed, 48 insertions(+), 26 deletions(-)

Comments

Morten Brørup Oct. 4, 2022, 2:42 p.m. UTC | #1
> From: Andrew Rybchenko [mailto:andrew.rybchenko@oktetlabs.ru]
> Sent: Tuesday, 4 October 2022 14.54
> To: Olivier Matz
> Cc: dev@dpdk.org; Morten Brørup; Beilei Xing; Bruce Richardson; Jerin
> Jacob Kollanukkaran
> Subject: [PATCH v3] mempool: fix get objects from mempool with cache
> 
> From: Morten Brørup <mb@smartsharesystems.com>
> 
> 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.
> 
> Fix 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.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> ---
> v3 changes (Andrew Rybchenko)
>  - Always get first objects from the cache even if request is bigger
>    than cache size. Remove one corresponding condition from the path
>    when request is fully served from cache.
>  - Simplify code to avoid duplication:
>     - Get objects directly from backend in single place only.
>     - Share code which gets from the cache first regardless if
>       everythihg is obtained from the cache or just the first part.
>  - Rollback cache length in unlikely failure branch to avoid cache
>    vs NULL check in success branch.
> 
> 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.
> 
>  lib/mempool/rte_mempool.h | 74 +++++++++++++++++++++++++--------------
>  1 file changed, 48 insertions(+), 26 deletions(-)

Thank you, Andrew.

I haven't compared the resulting assembler output (regarding performance), but I have carefully reviewed the resulting v3 source code for potential bugs in all code paths and for performance, and think it looks good.

The RTE_MIN() macro looks like it prefers the first parameter, so static branch prediction for len=RTE_MIN(remaining, cache->len) should be correct.

You could consider adding likely() around (cache != NULL) near the bottom of the function, so it matches the unlikely(cache == NULL) at the top of the function; mainly for symmetry in the source code, as I expect it to be the compiler default anyway.

Also, you could add "remaining" to the comment:
/* Get the objects directly from the ring. */

As is, or with suggested modifications...

Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
diff mbox series

Patch

diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
index a3c4ee351d..58e41ed401 100644
--- a/lib/mempool/rte_mempool.h
+++ b/lib/mempool/rte_mempool.h
@@ -1443,41 +1443,54 @@  rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
 			   unsigned int n, struct rte_mempool_cache *cache)
 {
 	int ret;
+	unsigned int remaining = n;
 	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 */
+	if (unlikely(cache == NULL))
 		goto ring_dequeue;
 
-	cache_objs = cache->objs;
+	/* Use the cache as much as we have to return hot objects first */
+	len = RTE_MIN(remaining, cache->len);
+	cache_objs = &cache->objs[cache->len];
+	cache->len -= len;
+	remaining -= len;
+	for (index = 0; index < len; index++)
+		*obj_table++ = *--cache_objs;
 
-	/* 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);
+	if (remaining == 0) {
+		/* The entire request is satisfied from the cache. */
 
-		/* 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);
-		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.
-			 */
-			goto ring_dequeue;
-		}
+		RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
+		RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
 
-		cache->len += req;
+		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];
+	/* if dequeue below would overflow mem allocated for cache */
+	if (unlikely(remaining > RTE_MEMPOOL_CACHE_MAX_SIZE))
+		goto ring_dequeue;
 
-	cache->len -= n;
+	/* Fill the cache from the ring; fetch size + remaining objects. */
+	ret = rte_mempool_ops_dequeue_bulk(mp, cache->objs,
+			cache->size + remaining);
+	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.
+		 */
+		goto ring_dequeue;
+	}
+
+	/* Satisfy the remaining part of the request from the filled cache. */
+	cache_objs = &cache->objs[cache->size + remaining];
+	for (index = 0; index < remaining; index++)
+		*obj_table++ = *--cache_objs;
+
+	cache->len = cache->size;
 
 	RTE_MEMPOOL_STAT_ADD(mp, get_success_bulk, 1);
 	RTE_MEMPOOL_STAT_ADD(mp, get_success_objs, n);
@@ -1486,10 +1499,19 @@  rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
 
 ring_dequeue:
 
-	/* get remaining objects from ring */
-	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, n);
+	/* Get the objects directly from the ring. */
+	ret = rte_mempool_ops_dequeue_bulk(mp, obj_table, remaining);
 
 	if (ret < 0) {
+		if (cache != NULL) {
+			cache->len = n - remaining;
+			/*
+			 * No further action is required to roll the first part
+			 * of the request back into the cache, as objects in
+			 * the cache are intact.
+			 */
+		}
+
 		RTE_MEMPOOL_STAT_ADD(mp, get_fail_bulk, 1);
 		RTE_MEMPOOL_STAT_ADD(mp, get_fail_objs, n);
 	} else {