<snip> > > +To: Bruce also showed interest in this topic, and might have more insights. > > > From: Honnappa Nagarahalli [mailto:honnappa.nagaraha...@arm.com] > > Sent: Wednesday, 9 November 2022 18.58 > > > > <snip> > > > > > > > > > From: Honnappa Nagarahalli [mailto:honnappa.nagaraha...@arm.com] > > > > Sent: Sunday, 6 November 2022 00.11 > > > > > > > > + Akshitha, she is working on similar patch > > > > > > > > Few comments inline > > > > > > > > > From: Morten Brørup <m...@smartsharesystems.com> > > > > > Sent: Saturday, November 5, 2022 8:40 AM > > > > > > > > > > Zero-copy access to the mempool cache is beneficial for PMD > > > > performance, > > > > > and must be provided by the mempool library to fix [Bug 1052] > > > > > without > > > > a > > > > > performance regression. > > > > > > > > > > [Bug 1052]: https://bugs.dpdk.org/show_bug.cgi?id=1052 > > > > > > > > > > > > > > > This RFC offers a conceptual zero-copy put function, where the > > > > application > > > > > promises to store some objects, and in return gets an address > > where > > > > to store > > > > > them. > > > > > > > > > > I would like some early feedback. > > > > > > > > > > Notes: > > > > > * Allowing the 'cache' parameter to be NULL, and getting it from > > the > > > > > mempool instead, was inspired by rte_mempool_cache_flush(). > > > > I am not sure why the 'cache' parameter is required for this API. > > This > > > > API should take the mem pool as the parameter. > > > > > > > > We have based our API on 'rte_mempool_do_generic_put' and removed > > > the > > > > 'cache' parameter. > > > > > > I thoroughly considered omitting the 'cache' parameter, but included > > it for > > > two reasons: > > > > > > 1. The function is a "mempool cache" function (i.e. primarily > > > working > > on the > > > mempool cache), not a "mempool" function. > > > > > > So it is appropriate to have a pointer directly to the structure it > > is working on. > > > Following this through, I also made 'cache' the first parameter and > > 'mp' the > > > second, like in rte_mempool_cache_flush(). > > I am wondering if the PMD should be aware of the cache or not. For ex: > > in the case of pipeline mode, the RX and TX side of the PMD are > > running on different cores. > > In that example, the PMD can store two cache pointers, one for each of the > RX and TX side. I did not understand this. If RX core and TX core have their own per-core caches the logic would not work. For ex: the RX core cache would not get filled.
In the case of pipeline mode, there will not be a per-core cache. The buffers would be allocated and freed from a global ring or a global lockless stack. > > And if the PMD is unaware of the cache pointer, it can look it up at runtime > using rte_lcore_id(), like it does in the current Intel PMDs. > > > However, since the rte_mempool_cache_flush API is provided, may be > > that decision is already done? Interestingly, rte_mempool_cache_flush > > is called by just a single PMD. > > I intentionally aligned this RFC with rte_mempool_cache_flush() to maintain > consistency. > > However, the API is not set in stone. It should always be acceptable to > consider improved alternatives. > > > > > So, the question is, should we allow zero-copy only for per-core cache > > or for other cases as well. > > I suppose that the mempool library was designed to have a mempool > associated with exactly one mempool cache per core. (Alternatively, the > mempool can be configured with no mempool caches at all.) > > We should probably stay loyal to that design concept, and only allow zero- > copy for per-core cache. > > If you can come up with an example of the opposite, I would like to explore > that option too... I can't think of a good example myself, and perhaps I'm > overlooking a relevant use case. The use case I am talking about is the pipeline mode as I mentioned above. Let me know if you agree. > > > > > > > > > 2. In most cases, the function only accesses the mempool structure > > > in > > order to > > > get the cache pointer. Skipping this step improves performance. > > > > > > And since the cache is created along with the mempool itself (and > > thus never > > > changes for a mempool), it would be safe for the PMD to store the > > 'cache' > > > pointer along with the 'mp' pointer in the PMD's queue structure. > > Agreed > > > > > > > > E.g. in the i40e PMD the i40e_rx_queue structure could include a > > "struct > > > rte_mempool_cache *cache" field, which could be used > > > i40e_rxq_rearm() > > [1] > > > instead of "cache = rte_mempool_default_cache(rxq->mp, > > rte_lcore_id())". > > > > > > [1] https://elixir.bootlin.com/dpdk/v22.11- > > > rc2/source/drivers/net/i40e/i40e_rxtx_vec_avx512.c#L31 > > > > > > > This new API, on success, returns the pointer to memory where the > > > > objects are copied. On failure it returns NULL and the caller has > > to > > > > call 'rte_mempool_ops_enqueue_bulk'. Alternatively, the new API > > could > > > > do this as well and PMD does not need to do anything if it gets a > > NULL > > > > pointer. > > > > > > Yes, we agree about these two details: > > > > > > 1. The function should return a pointer, not an integer. > > > It would be a waste to use a another CPU register to convey a > > success/error > > > integer value, when the success/failure information is just as > > > easily > > conveyed > > > by the pointer return value (non-NULL/NULL), and rte_errno for > > various error > > > values in the unlikely cases. > > > > > > 2. The function should leave it up to the PMD what to do if direct > > access to > > > the cache is unavailable. > > Just wondering about the advantage of this. I do not think PMD's have > > much of a choice other than calling 'rte_mempool_ops_enqueue_bulk' > > I agree, but that was not my point. Let me try to rephrase: > > The PMD is more likely to know how to efficiently build the array of mbufs to > pass to rte_mempool_ops_enqueue_bulk() than the mempool library - many > PMDs already implement a variety of vector instruction variants to do exactly > that. So we should not try to be clever and add a fallback path - this job > belongs in the PMD. > > The PMD might not even have the array of mbufs lined up when calling > rte_mempool_cache_put_bulk_promise(). The PMD could have an array of > internal structures, where the mbuf pointer is an element in that structure. Agree, you are correct. We should leave it to PMD to handle the failure case. > > > > > > > > > > > > > > We should think about providing similar API on the RX side to > > > > keep > > it > > > > symmetric. > > > > > > I sent an RFC for that too: > > > > http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35D87488@ > > > smartserver.smartshare.dk/T/#u > > > > > > > > > > > > > > > * Asserting that the 'mp' parameter is not NULL is not done by > > other > > > > > functions, so I omitted it here too. > > > > > > > > > > NB: Please ignore formatting. Also, this code has not even been > > > > compile > > > > > tested. > > > > We are little bit ahead, tested the changes with i40e PF PMD, > > > > wrote unit test cases, going through internal review, will send > > > > out RFC > > on > > > > Monday > > > > > > Sounds good. Looking forward to review. > > > > > > > > > > > > > > > > > /** > > > > > * Promise to put objects in a mempool via zero-copy access to a > > > > user-owned > > > > > mempool cache. > > > > > * > > > > > * @param cache > > > > > * A pointer to the mempool cache. > > > > > * @param mp > > > > > * A pointer to the mempool. > > > > > * @param n > > > > > * The number of objects to be put in the mempool cache. > > > > > * @return > > > > > * The pointer to where to put the objects in the mempool > > cache. > > > > > * NULL on error > > > > > * with rte_errno set appropriately. > > > > > */ > > > > > static __rte_always_inline void * > > > > > rte_mempool_cache_put_bulk_promise(struct rte_mempool_cache > > > *cache, > > > > > struct rte_mempool *mp, > > > > > unsigned int n) > > > > > { > > > > > void **cache_objs; > > > > > > > > > > if (cache == NULL) > > > > > cache = rte_mempool_default_cache(mp, rte_lcore_id()); > > Any reason we need this? If we are expecting the PMD to store the > > pointer to cache and a NULL is passed, it would mean it is a mempool > > with no per-core cache? > > We could also leave the NULL check to the PMD. > > Personally, I would strongly prefer requiring the cache pointer to be valid, > and use RTE_ASSERT() here, instead of allowing a NULL pointer as a special > case to look it up inside the function. I consider this special NULL case > useless > bloat, which does not belong in a fast path library function. > > But I copied this approach from rte_mempool_cache_flush(). The API definition does not bind it to do this check. We might be able to delete the check in rte_mempool_cache_flush. > > We could expose an "unsafe" function where is not allowed to pass NULL > pointers, and a "safe" function (fixing the cache pointer if NULL) for > consistency. > > If the rte_mempool_cache_flush() function is popular, we could also expose > an "unsafe" variant where passing NULL pointers are disallowed. > > I wonder if there are any examples of such safe/unsafe variants in DPDK? It > would be nice with a common naming convention for such function variants. > > > > > > > > if (cache == NULL) { > > > > > rte_errno = EINVAL; > > > > > return NULL; > > > > > } > > > > > > > > > > rte_mempool_trace_cache_put_bulk_promise(cache, mp, n); > > > > > > > > > > /* The request itself is too big for the cache */ > > > > > if (unlikely(n > cache->flushthresh)) { > > > > > rte_errno = EINVAL; > > > > > return NULL; > > > > > } > > > > > > > > > > /* > > > > > * The cache follows the following algorithm: > > > > > * 1. If the objects cannot be added to the cache without > > > > crossing > > > > > * the flush threshold, flush the cache to the backend. > > > > > * 2. Add the objects to the cache. > > > > > */ > > > > > > > > > > if (cache->len + n <= cache->flushthresh) { > > > > > cache_objs = &cache->objs[cache->len]; > > > > > cache->len += n; > > > > > } else { > > > > > cache_objs = &cache->objs[0]; > > > > > rte_mempool_ops_enqueue_bulk(mp, cache_objs, cache->len); > > > > > cache->len = n; > > > > > } > > > > > > > > > > RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_bulk, 1); > > > > > RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, n); > > These are new stats. Do these break ABI compatibility (though these > > are under DEBUG flag)? > > They are not mempool cache stats, they are only kept in the cache structure > to provide alternative (i.e. faster) update access to some (i.e. the most > often > updated) of the existing mempool stats. The patch is [1], and part of a series > currently being discussed if should go into 22.11-rc3 or not [2]. > > [1]: > https://patchwork.dpdk.org/project/dpdk/patch/20221109181852.109856-3- > m...@smartsharesystems.com/ > [2]: > http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35D874A6 > @smartserver.smartshare.dk/T/#m41bf4e8bd886db49f11c8dbd63741b35327 > 7082f > > > > > > > > > > > > > return cache_objs; > > > > > } > > > > > > > > > > > > > > > Med venlig hilsen / Kind regards, -Morten Brørup > > > > > > > > > > >
