PING again. If the explanation and/or diff is too longwinded, just look at the resulting code instead - it is clean and easily readable.
This patch should not be controversial, so I would like to see it merged into the coming LTS release. (Unlike my other mempool patch [3], which changes the behavior of the mempool cache.) [3]: https://patchwork.dpdk.org/project/dpdk/patch/20220202103354.79832-1...@smartsharesystems.com/ Med venlig hilsen / Kind regards, -Morten Brørup > From: Morten Brørup [mailto:m...@smartsharesystems.com] > Sent: Wednesday, 15 June 2022 23.18 > > +CC: Beilei Xing <beilei.x...@intel.com>, i40e maintainer, may be > interested in the performance improvements achieved by this patch. > > > From: Morten Brørup [mailto:m...@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 <m...@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/
