On 6/17/2021 3:58 PM, Ananyev, Konstantin wrote: > > >>>>> >>>>> 14/06/2021 15:15, Bruce Richardson: >>>>>> On Mon, Jun 14, 2021 at 02:22:42PM +0200, Morten Brørup wrote: >>>>>>>> From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Thomas Monjalon >>>>>>>> Sent: Monday, 14 June 2021 12.59 >>>>>>>> >>>>>>>> Performance of access in a fixed-size array is very good >>>>>>>> because of cache locality >>>>>>>> and because there is a single pointer to dereference. >>>>>>>> The only drawback is the lack of flexibility: >>>>>>>> the size of such an array cannot be increase at runtime. >>>>>>>> >>>>>>>> An approach to this problem is to allocate the array at runtime, >>>>>>>> being as efficient as static arrays, but still limited to a maximum. >>>>>>>> >>>>>>>> That's why the API rte_parray is introduced, >>>>>>>> allowing to declare an array of pointer which can be resized >>>>>>>> dynamically >>>>>>>> and automatically at runtime while keeping a good read performance. >>>>>>>> >>>>>>>> After resize, the previous array is kept until the next resize >>>>>>>> to avoid crashs during a read without any lock. >>>>>>>> >>>>>>>> Each element is a pointer to a memory chunk dynamically allocated. >>>>>>>> This is not good for cache locality but it allows to keep the same >>>>>>>> memory per element, no matter how the array is resized. >>>>>>>> Cache locality could be improved with mempools. >>>>>>>> The other drawback is having to dereference one more pointer >>>>>>>> to read an element. >>>>>>>> >>>>>>>> There is not much locks, so the API is for internal use only. >>>>>>>> This API may be used to completely remove some compilation-time >>>>>>>> maximums. >>>>>>> >>>>>>> I get the purpose and overall intention of this library. >>>>>>> >>>>>>> I probably already mentioned that I prefer "embedded style programming" >>>>>>> with fixed size arrays, rather than runtime configurability. >>>> It's >>>>> my personal opinion, and the DPDK Tech Board clearly prefers reducing the >>>>> amount of compile time configurability, so there is no way >> for >>>>> me to stop this progress, and I do not intend to oppose to this library. >>>>> :-) >>>>>>> >>>>>>> This library is likely to become a core library of DPDK, so I think it >>>>>>> is important getting it right. Could you please mention a few >>>> examples >>>>> where you think this internal library should be used, and where it should >>>>> not be used. Then it is easier to discuss if the border line >> between >>>>> control path and data plane is correct. E.g. this library is not intended >>>>> to be used for dynamically sized packet queues that grow and >> shrink >>>> in >>>>> the fast path. >>>>>>> >>>>>>> If the library becomes a core DPDK library, it should probably be >>>>>>> public instead of internal. E.g. if the library is used to make >>>>> RTE_MAX_ETHPORTS dynamic instead of compile time fixed, then some >>>>> applications might also need dynamically sized arrays for their >>>>> application specific per-port runtime data, and this library could serve >>>>> that purpose too. >>>>>>> >>>>>> >>>>>> Thanks Thomas for starting this discussion and Morten for follow-up. >>>>>> >>>>>> My thinking is as follows, and I'm particularly keeping in mind the cases >>>>>> of e.g. RTE_MAX_ETHPORTS, as a leading candidate here. >>>>>> >>>>>> While I dislike the hard-coded limits in DPDK, I'm also not convinced >>>>>> that >>>>>> we should switch away from the flat arrays or that we need fully dynamic >>>>>> arrays that grow/shrink at runtime for ethdevs. I would suggest a >>>>>> half-way >>>>>> house here, where we keep the ethdevs as an array, but one >>>>>> allocated/sized >>>>>> at runtime rather than statically. This would allow us to have a >>>>>> compile-time default value, but, for use cases that need it, allow use >>>>>> of a >>>>>> flag e.g. "max-ethdevs" to change the size of the parameter given to the >>>>>> malloc call for the array. This max limit could then be provided to apps >>>>>> too if they want to match any array sizes. [Alternatively those apps >>>>>> could >>>>>> check the provided size and error out if the size has been increased >>>>>> beyond >>>>>> what the app is designed to use?]. There would be no extra dereferences >>>>>> per >>>>>> rx/tx burst call in this scenario so performance should be the same as >>>>>> before (potentially better if array is in hugepage memory, I suppose). >>>>> >>>>> I think we need some benchmarks to decide what is the best tradeoff. >>>>> I spent time on this implementation, but sorry I won't have time for >>>>> benchmarks. >>>>> Volunteers? >>>> >>>> I had only a quick look at your approach so far. >>>> But from what I can read, in MT environment your suggestion will require >>>> extra synchronization for each read-write access to such parray element >>>> (lock, rcu, ...). >>>> I think what Bruce suggests will be much ligther, easier to implement and >>>> less error prone. >>>> At least for rte_ethdevs[] and friends. >>>> Konstantin >>> >>> One more thought here - if we are talking about rte_ethdev[] in particular, >>> I think we can: >>> 1. move public function pointers (rx_pkt_burst(), etc.) from rte_ethdev >>> into a separate flat array. >>> We can keep it public to still use inline functions for 'fast' calls >>> rte_eth_rx_burst(), etc. to avoid >>> any regressions. >>> That could still be flat array with max_size specified at application >>> startup. >>> 2. Hide rest of rte_ethdev struct in .c. >>> That will allow us to change the struct itself and the whole rte_ethdev[] >>> table in a way we like >>> (flat array, vector, hash, linked list) without ABI/API breakages. >>> >>> Yes, it would require all PMDs to change prototype for pkt_rx_burst() >>> function >>> (to accept port_id, queue_id instead of queue pointer), but the change is >>> mechanical one. >>> Probably some macro can be provided to simplify it. >>> >> >> We are already planning some tasks for ABI stability for v21.11, I think >> splitting 'struct rte_eth_dev' can be part of that task, it enables hiding >> more >> internal data. > > Ok, sounds good. > >> >>> The only significant complication I can foresee with implementing that >>> approach - >>> we'll need a an array of 'fast' function pointers per queue, not per device >>> as we have now >>> (to avoid extra indirection for callback implementation). >>> Though as a bonus we'll have ability to use different RX/TX funcions per >>> queue. >>> >> >> What do you think split Rx/Tx callback into its own struct too? >> >> Overall 'rte_eth_dev' can be split into three as: >> 1. rte_eth_dev >> 2. rte_eth_dev_burst >> 3. rte_eth_dev_cb >> >> And we can hide 1 from applications even with the inline functions. > > As discussed off-line, I think: > it is possible. > My absolute preference would be to have just 1/2 (with CB hidden).
How can we hide the callbacks since they are used by inline burst functions. > But even with 1/2/3 in place I think it would be a good step forward. > Probably worth to start with 1/2/3 first and then see how difficult it > would be to switch to 1/2. What do you mean by switch to 1/2? If we keep having inline functions, and split struct as above three structs, we can only hide 1, and 2/3 will be still visible to apps because of inline functions. This way we will be able to hide more still having same performance. > Do you plan to start working on it? > We are gathering the list of the tasks for the ABI stability, most probably they will be worked on during v21.11. I can take this one. > Konstantin > > > >