Re: [dpdk-dev] [PATCH v6 10/10] doc: update ring guide

[David Marchand <david.marchand@redhat.com>]

On Mon, Apr 20, 2020 at 2:12 PM Konstantin Ananyev
<konstantin.ananyev@intel.com> wrote:
>
> Changed the rte_ring chapter in programmer's guide to reflect
> the addition of new sync modes and peek style API.

I'd like to split this as follows, see below.
I have a couple of typos too.


If you are fine with it, I'll proceed and squash when merging.


>
> Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> ---
>  doc/guides/prog_guide/ring_lib.rst | 95 ++++++++++++++++++++++++++++++
>  1 file changed, 95 insertions(+)
>
> diff --git a/doc/guides/prog_guide/ring_lib.rst b/doc/guides/prog_guide/ring_lib.rst
> index 8cb2b2dd4..668e67ecb 100644
> --- a/doc/guides/prog_guide/ring_lib.rst
> +++ b/doc/guides/prog_guide/ring_lib.rst
> @@ -349,6 +349,101 @@ even if only the first term of subtraction has overflowed:
>      uint32_t entries = (prod_tail - cons_head);
>      uint32_t free_entries = (mask + cons_tail -prod_head);
>

From here, this first part would go to patch2 "ring: prepare ring to
allow new sync schemes".

> +Producer/consumer synchronization modes
> +---------------------------------------
> +
> +rte_ring supports different synchronization modes for porducer and consumers.

producers*

> +These modes can be specified at ring creation/init time via ``flags`` parameter.
> +That should help  user to configure ring in way most suitable for his

double space to remove.
users?


> +specific usage scenarios.
> +Currently supported modes:
> +
> +MP/MC (default one)
> +~~~~~~~~~~~~~~~~~~~
> +
> +Multi-producer (/multi-consumer) mode. This is a default enqueue (/dequeue)
> +mode for the ring. In this mode multiple threads can enqueue (/dequeue)
> +objects to (/from) the ring. For 'classic' DPDK deployments (with one thread
> +per core) this is usually most suitable and fastest synchronization mode.

the most*

> +As a well known limitaion - it can perform quite pure on some overcommitted

limitation*

> +scenarios.
> +
> +SP/SC
> +~~~~~
> +Single-producer (/single-consumer) mode. In this mode only one thread at a time
> +is allowed to enqueue (/dequeue) objects to (/from) the ring.

End of first part.

Then the second part that would go to patch3 "ring: introduce RTS ring mode".

> +
> +MP_RTS/MC_RTS
> +~~~~~~~~~~~~~
> +
> +Multi-producer (/multi-consumer) with Relaxed Tail Sync (RTS) mode.
> +The main difference from original MP/MC algorithm is that

from the original*

> +tail value is increased not by every thread that finished enqueue/dequeue,
> +but only by the last one.
> +That allows threads to avoid spinning on ring tail value,
> +leaving actual tail value change to the last thread at a given instance.
> +That technique helps to avoid Lock-Waiter-Preemtion (LWP) problem on tail

the Lock-Waiter-Preemption*

> +update and improves average enqueue/dequeue times on overcommitted systems.
> +To achieve that RTS requires 2 64-bit CAS for each enqueue(/dequeue) operation:
> +one for head update, second for tail update.
> +In comparison original MP/MC algorithm requires one 32-bit CAS

the original*

> +for head update and waiting/spinning on tail value.
> +

End of second part.

Third part that would go to patch 5 "ring: introduce HTS ring mode".


> +MP_HTS/MC_HTS
> +~~~~~~~~~~~~~
> +
> +Multi-producer (/multi-consumer) with Head/Tail Sync (HTS) mode.
> +In that mode enqueue/dequeue operation is fully serialized:
> +at any given moment only one enqueue/dequeue operation can proceed.
> +This is achieved by allowing a thread to proceed with changing ``head.value``
> +only when ``head.value == tail.value``.
> +Both head and tail values are updated atomically (as one 64-bit value).
> +To achieve that 64-bit CAS is used by head update routine.
> +That technique also avoids Lock-Waiter-Preemtion (LWP) problem on tail

the Lock-Waiter-Preemption*


> +update and helps to improve ring enqueue/dequeue behavior in overcommitted
> +scenarios. Another advantage of fully serialized producer/consumer -
> +it provides ability to implement MT safe peek API for rte_ring.

it provides the ability*

End of 3rd part.

Last part would go to patch 7 "ring: introduce peek style API".

> +
> +
> +Ring Peek API
> +-------------
> +
> +For ring with serialized producer/consumer (HTS sync mode) it is  possible

double space.

> +to split public enqueue/dequeue API into two phases:
> +
> +*   enqueue/dequeue start
> +
> +*   enqueue/dequeue finish
> +
> +That allows user to inspect objects in the ring without removing them
> +from it (aka MT safe peek) and reserve space for the objects in the ring
> +before actual enqueue.
> +Note that this API is available only for two sync modes:
> +
> +*   Single Producer/Single Consumer (SP/SC)
> +
> +*   Multi-producer/Multi-consumer with Head/Tail Sync (HTS)
> +
> +It is a user responsibility to create/init ring with appropriate sync modes
> +selected. As an example of usage:
> +
> +.. code-block:: c
> +
> +    /* read 1 elem from the ring: */
> +    uint32_t n = rte_ring_dequeue_bulk_start(ring, &obj, 1, NULL);
> +    if (n != 0) {
> +        /* examine object */
> +        if (object_examine(obj) == KEEP)
> +            /* decided to keep it in the ring. */
> +            rte_ring_dequeue_finish(ring, 0);
> +        else
> +            /* decided to remove it from the ring. */
> +            rte_ring_dequeue_finish(ring, n);
> +    }
> +
> +Note that between ``_start_`` and ``_finish_`` none other thread can proceed
> +with enqueue(/dequeue) operation till ``_finish_`` completes.
> +



-- 
David Marchand