On 6/14/2021 4:54 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. > 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.
