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).
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