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

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

 
> On 6/17/2021 4:17 PM, Morten Brørup wrote:
> >> From: Ananyev, Konstantin [mailto:konstantin.ananyev@intel.com]
> >> Sent: Thursday, 17 June 2021 16.59
> >>
> >>>>>>
> >>>>>> 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
> >
> > If you do proceed with this, be very careful. E.g. the inlined rx/tx burst functions should not touch more cache lines than they do today -
> especially if there are many active ports. The inlined rx/tx burst functions are very simple, so thorough code review (and possibly also of the
> resulting assembly) is appropriate. Simple performance testing might not detect if more cache lines are accessed than before the
> modifications.
> >
> > Don't get me wrong... I do consider this an improvement of the ethdev library; I'm only asking you to take extra care!
> >
> 
> ack
> 
> If we split as above, I think device specific data 'struct rte_eth_dev_data'
> should be part of 1 (rte_eth_dev). Which means Rx/Tx inline functions access
> additional cache line.
> 
> To prevent this, what about duplicating 'data' in 2 (rte_eth_dev_burst)? 

I think it would be better to change rx_pkt_burst() to accept port_id and queue_id,
instead of void *.
I.E:
typedef uint16_t (*eth_rx_burst_t)(uint16_t port_id, uint16_t queue_id, struct rte_mbuf **rx_pkts,  uint16_t nb_pkts);

And we can do actual de-referencing of private rxq data inside the actual rx function.

> We have
> enough space for it to fit into single cache line, currently it is:
> struct rte_eth_dev {
>         eth_rx_burst_t             rx_pkt_burst;         /*     0     8 */
>         eth_tx_burst_t             tx_pkt_burst;         /*     8     8 */
>         eth_tx_prep_t              tx_pkt_prepare;       /*    16     8 */
>         eth_rx_queue_count_t       rx_queue_count;       /*    24     8 */
>         eth_rx_descriptor_done_t   rx_descriptor_done;   /*    32     8 */
>         eth_rx_descriptor_status_t rx_descriptor_status; /*    40     8 */
>         eth_tx_descriptor_status_t tx_descriptor_status; /*    48     8 */
>         struct rte_eth_dev_data *  data;                 /*    56     8 */
>         /* --- cacheline 1 boundary (64 bytes) --- */
> 
> 'rx_descriptor_done' is deprecated and will be removed;