RE: [RFC] lib/st_ring: add single thread ring

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> > From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> > Sent: Thursday, 24 August 2023 12.53
> >
> > On 2023-08-24 10:05, Morten Brørup wrote:
> > >> From: Honnappa Nagarahalli [mailto:Honnappa.Nagarahalli@arm.com]
> > >> Sent: Tuesday, 22 August 2023 07.47
> > >>
> > >>> From: Morten Brørup <mb@smartsharesystems.com>
> > >>> Sent: Monday, August 21, 2023 2:37 AM
> > >>>
> > >>>> From: Honnappa Nagarahalli
> [mailto:honnappa.nagarahalli@arm.com]
> > >>>> Sent: Monday, 21 August 2023 08.04
> > >>>>
> > >>>> Add a single thread safe and multi-thread unsafe ring data structure.
> > >>>> This library provides an simple and efficient alternative to
> > >>>> multi- thread safe ring when multi-thread safety is not required.
> > >>>>
> > >>>> Signed-off-by: Honnappa Nagarahalli
> > >>>> <honnappa.nagarahalli@arm.com>
> > >>>> ---
> > >>>
> > >>> Good idea.
> > >>>
> > >>> However, I prefer it to be implemented in the ring lib as one more
> > >>> ring
> > >> type.
> > >>> That would also give us a lot of the infrastructure (management
> > >>> functions, documentation and tests) for free.
> > >> IMO, the current code for rte_ring seems complex with C11 and
> > >> generic implementations, APIs for pointer objects vs APIs for
> > >> flexible element size etc. I did not want to introduce one more flavor and
> make it more complex.
> > >
> > >  From the user perspective, I think one more ring flavor is less
> > > complex
> > than an entirely separate (very similar) library with its own set of
> > (very
> > similar) APIs.
> > >
> > > I agree that the ring lib has grown somewhat over-engineered, but
> > > please
> > don't use that as an argument for making the same-thread ring a separate
> lib.
> > >
> >
> > What's being proposed is a double-ended queue, not a ring (in the DPDK
> > sense).
> >
> > If you want to Swiss army knifify the rte_ring further and make it a
> > deque, then rte_stack should scrapped as well, since it's will become
> > just a subset of the new rte_ring_now_really_a_deque.
> 
> OK. I accept that argument for not hacking it into the ring lib.
> 
> Then I will suggest that the new "deque" library should be designed with
> multi-threading in mind, like its two sibling libs (ring and stack). This makes it
> easier to use, and leaves it open for expansion to other flavors in the future.
> 
> It is perfectly acceptable that the first version only supports the same-thread
> deque flavor, and only the same-thread specialized APIs are exposed. I don't
> require any APIs or implementations supporting single-threaded (individual
> producer/consumer threads) or multi-threaded flavors, I only request that the
> design and API resemble those of its two sibling libraries. (And if there are no
> use cases for multi-threading flavors, they might never be added to this lib.)
+1, will aim for this

> 
> >
> > > On the other hand: If the addition of an optimized same-thread ring
> > > flavor
> > would require too many invasive modifications of the existing ring
> > lib, I would accept that as an argument for not adding it as another
> > ring flavor to the existing ring lib.
> > >
> > >> The requirements are different as well. For ex: single thread ring
> > >> needs
> > APIs
> > >> for dequeuing and enqueuing at both ends of the ring which is not
> > applicable
> > >> to existing RTE ring.
> > >
> > > Yes, I will address this topic at the end of this mail.
> > >
> > >>
> > >> But, I see how the existing infra can be reused easily.
> > >
> > > This also goes for future infrastructure. I doubt that new
> > > infrastructure
> > added to the ring lib will also be added to the same-thread ring
> > lib... for reference, consider the PMDs containing copy-pasted code
> > from the mempool lib... none of the later improvements of the mempool
> > lib were implemented in those PMDs.
> > >
> > > In essence, I think this lib overlaps the existing ring lib too much
> > > to
> > justify making it a separate lib.
> > >
> > >>
> > >>>
> > >>> The ring lib already has performance-optimized APIs for
> > >>> single-consumer
> > and
> > >>> single-producer use, rte_ring_sc_dequeue_bulk() and
> > >>> rte_ring_sp_enqueue_burst(). Similar performance-optimized APIs
> > >>> for
> > single-
> > >>> thread use could be added: rte_ring_st_dequeue_bulk() and
> > >>> rte_ring_st_enqueue_burst().
> > >> Yes, the names look fine.
> > >> Looking through the code. We have the sync type enum:
> > >>
> > >> /** prod/cons sync types */
> > >> enum rte_ring_sync_type {
> > >>          RTE_RING_SYNC_MT,     /**< multi-thread safe (default mode) */
> > >>          RTE_RING_SYNC_ST,     /**< single thread only */
> > >>          RTE_RING_SYNC_MT_RTS, /**< multi-thread relaxed tail sync */
> > >>          RTE_RING_SYNC_MT_HTS, /**< multi-thread head/tail sync */
> > >> };
> > >>
> > >> The type RTE_RING_SYNC_ST needs better explanation (not a problem).
> > >> But,
> > this
> > >> name would have been ideal to use for single thread ring.
> > >> This enum does not need to be exposed to the users. However, there
> > >> are rte_ring_get_prod/cons_sync_type etc which seem to be exposed to
> the user.
> > >> This all means, we need to have a sync type name
> > >> RTE_RING_SYNC_MT_UNSAFE
> > (any
> > >> other better name?) which then affects API naming.
> > >> rte_ring_mt_unsafe_dequeue_bulk?
> > >
> > > As always, naming is difficult.
> > > The enum rte_ring_sync_type describes the producer and consumer
> > independently, whereas this ring type uses the same thread for both
> > producer and consumer.
> > > I think we should avoid MT in the names for this variant. How about:
> > >
> > > RTE_RING_SYNC_STPC /**< same thread for both producer and consumer
> > > */
> > >
> > > And:
> > >
> > > rte_ring_spc_dequeue_bulk() and rte_ring_spc_enqueue_burst()
> > >
> > >>
> > >>>
> > >>> Regardless if added to the ring lib or as a separate lib,
> > >>> "reverse" APIs
> > >> (for single-
> > >>> thread use only) and zero-copy APIs can be added at any time later.
> > >
> > > As the only current use case for "reverse" (i.e. dequeue at tail,
> > > enqueue at
> > head) APIs is for the same-thread ring flavor, we could start by
> > adding only the specialized variants of the "reverse" APIs,
> > rte_ring_spc_reverse_xxx(), and initially omit the generic
> > rte_ring_reverse_xxx() APIs. (We need better names; I used "reverse"
> > for explanation only.)
> > >