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.

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.