Re: [dpdk-dev] [PATCH] parray: introduce internal API for dynamic arrays

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> >>>>>
> >>>>> 14/06/2021 15:15, Bruce Richardson:
> >>>>>> On Mon, Jun 14, 2021 at 02:22:42PM +0200, Morten Brørup wrote:
> >>>>>>>> From: dev [mailto:dev-bounces@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.

I probably I owe a better explanation to what I meant in first mail.
Otherwise it sounds confusing.
I'll try to write a more detailed one in next few days.

> > 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?

When we'll have just:
1. rte_eth_dev (hidden in .c)
2. rte_eth_dev_burst (visible)

And no specific public struct/array for callbacks - they will be hidden in rte_eth_dev.

> 
> 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.

I understand that, and as I said above - I think it is a good step forward.
Though even better would be to hide rte_eth_dev_cb too. 

> 
> > 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.

Cool, please keep me in a loop.
I'll try to free some cycles for 21.11 to get involved and help (if needed off-course).
Konstantin