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

["Ananyev, Konstantin" <konstantin.ananyev@intel.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.

Yes, I am.
Thanks
Konstantin

> 
> 
> >
> > 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