> >>> > >>> 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). 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. Do you plan to start working on it? Konstantin
