From: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
To: "Morten Brørup" <mb@smartsharesystems.com>, olivier.matz@6wind.com
Cc: bruce.richardson@intel.com, jerinjacobk@gmail.com, dev@dpdk.org
Subject: Re: [PATCH v2] mempool: fix get objects from mempool with cache
Date: Tue, 4 Oct 2022 15:57:47 +0300 [thread overview]
Message-ID: <43c333f7-83fa-3072-8db1-8b3a6ece5999@oktetlabs.ru> (raw)
In-Reply-To: <20220202081426.77975-1-mb@smartsharesystems.com>
Hi Morten,
In general I agree that the fix is required.
In sent v3 I'm trying to make it a bit better from my point of
view. See few notes below.
On 2/2/22 11:14, Morten Brørup wrote:
> 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.
I've just sent v3 with suggested changes to the patch.
>
> 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 --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))
The second condition is unnecessary until we try to fill in
cache from backend.
> 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;
I dislike duplication of these lines here and above. See v3.
> +
> + /* 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);
I dislike the duplication as well. We can goto ring_dequeue
instead. See v3.
> 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) {
next prev parent reply other threads:[~2022-10-04 12:57 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-26 15:34 [RFC] mempool: rte_mempool_do_generic_get optimizations Morten Brørup
2022-01-06 12:23 ` [PATCH] mempool: optimize incomplete cache handling Morten Brørup
2022-01-06 16:55 ` Jerin Jacob
2022-01-07 8:46 ` Morten Brørup
2022-01-10 7:26 ` Jerin Jacob
2022-01-10 10:55 ` Morten Brørup
2022-01-14 16:36 ` [PATCH] mempool: fix get objects from mempool with cache Morten Brørup
2022-01-17 17:35 ` Bruce Richardson
2022-01-18 8:25 ` Morten Brørup
2022-01-18 9:07 ` Bruce Richardson
2022-01-24 15:38 ` Olivier Matz
2022-01-24 16:11 ` Olivier Matz
2022-01-28 10:22 ` Morten Brørup
2022-01-17 11:52 ` [PATCH] mempool: optimize put objects to " Morten Brørup
2022-01-19 14:52 ` [PATCH v2] mempool: fix " Morten Brørup
2022-01-19 15:03 ` [PATCH v3] " Morten Brørup
2022-01-24 15:39 ` Olivier Matz
2022-01-28 9:37 ` Morten Brørup
2022-02-02 8:14 ` [PATCH v2] mempool: fix get objects from " Morten Brørup
2022-06-15 21:18 ` Morten Brørup
2022-09-29 10:52 ` Morten Brørup
2022-10-04 12:57 ` Andrew Rybchenko [this message]
2022-10-04 15:13 ` Morten Brørup
2022-10-04 15:58 ` Andrew Rybchenko
2022-10-04 18:09 ` Morten Brørup
2022-10-06 13:43 ` Aaron Conole
2022-10-04 16:03 ` Morten Brørup
2022-10-04 16:36 ` Morten Brørup
2022-10-04 16:39 ` Morten Brørup
2022-02-02 10:33 ` [PATCH v4] mempool: fix mempool cache flushing algorithm Morten Brørup
2022-04-07 9:04 ` Morten Brørup
2022-04-07 9:14 ` Bruce Richardson
2022-04-07 9:26 ` Morten Brørup
2022-04-07 10:32 ` Bruce Richardson
2022-04-07 10:43 ` Bruce Richardson
2022-04-07 11:36 ` Morten Brørup
2022-10-04 20:01 ` Morten Brørup
2022-10-09 11:11 ` [PATCH 1/2] mempool: check driver enqueue result in one place Andrew Rybchenko
2022-10-09 11:11 ` [PATCH 2/2] mempool: avoid usage of term ring on put Andrew Rybchenko
2022-10-09 13:08 ` Morten Brørup
2022-10-09 13:14 ` Andrew Rybchenko
2022-10-09 13:01 ` [PATCH 1/2] mempool: check driver enqueue result in one place Morten Brørup
2022-10-09 13:19 ` [PATCH v4] mempool: fix mempool cache flushing algorithm Andrew Rybchenko
2022-10-04 12:53 ` [PATCH v3] mempool: fix get objects from mempool with cache Andrew Rybchenko
2022-10-04 14:42 ` Morten Brørup
2022-10-07 10:44 ` [PATCH v4] " Andrew Rybchenko
2022-10-08 20:56 ` Thomas Monjalon
2022-10-11 20:30 ` Copy-pasted code should be updated Morten Brørup
2022-10-11 21:47 ` Honnappa Nagarahalli
2022-10-30 8:44 ` Morten Brørup
2022-10-30 22:50 ` Honnappa Nagarahalli
2022-10-14 14:01 ` [PATCH v4] mempool: fix get objects from mempool with cache Olivier Matz
2022-10-09 13:37 ` [PATCH v6 0/4] mempool: fix mempool cache flushing algorithm Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 1/4] mempool: check driver enqueue result in one place Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 2/4] mempool: avoid usage of term ring on put Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 3/4] mempool: fix cache flushing algorithm Andrew Rybchenko
2022-10-09 14:31 ` Morten Brørup
2022-10-09 14:51 ` Andrew Rybchenko
2022-10-09 15:08 ` Morten Brørup
2022-10-14 14:01 ` Olivier Matz
2022-10-14 15:57 ` Morten Brørup
2022-10-14 19:50 ` Olivier Matz
2022-10-15 6:57 ` Morten Brørup
2022-10-18 16:32 ` Jerin Jacob
2022-10-09 13:37 ` [PATCH v6 4/4] mempool: flush cache completely on overflow Andrew Rybchenko
2022-10-09 14:44 ` Morten Brørup
2022-10-14 14:01 ` Olivier Matz
2022-10-10 15:21 ` [PATCH v6 0/4] mempool: fix mempool cache flushing algorithm Thomas Monjalon
2022-10-11 19:26 ` Morten Brørup
2022-10-26 14:09 ` Thomas Monjalon
2022-10-26 14:26 ` Morten Brørup
2022-10-26 14:44 ` [PATCH] mempool: cache align mempool cache objects Morten Brørup
2022-10-26 19:44 ` Andrew Rybchenko
2022-10-27 8:34 ` Olivier Matz
2022-10-27 9:22 ` Morten Brørup
2022-10-27 11:42 ` Olivier Matz
2022-10-27 12:11 ` Morten Brørup
2022-10-27 15:20 ` Olivier Matz
2022-10-28 6:35 ` [PATCH v3 1/2] " Morten Brørup
2022-10-28 6:35 ` [PATCH v3 2/2] mempool: optimized debug statistics Morten Brørup
2022-10-28 6:41 ` [PATCH v4 1/2] mempool: cache align mempool cache objects Morten Brørup
2022-10-28 6:41 ` [PATCH v4 2/2] mempool: optimized debug statistics Morten Brørup
2022-10-30 9:09 ` Morten Brørup
2022-10-30 9:16 ` Thomas Monjalon
2022-10-30 9:17 ` [PATCH v4 1/2] mempool: cache align mempool cache objects Thomas Monjalon
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