Re: [dpdk-dev] [RFC 1/3] eventdev: allow for event devices requiring maintenance

[Jerin Jacob <jerinjacobk@gmail.com>]

On Thu, Apr 9, 2020 at 5:51 PM Mattias Rönnblom
<mattias.ronnblom@ericsson.com> wrote:
>
> On 2020-04-08 21:36, Jerin Jacob wrote:
> > On Wed, Apr 8, 2020 at 11:27 PM Mattias Rönnblom
> > <mattias.ronnblom@ericsson.com> wrote:
> >> Extend Eventdev API to allow for event devices which require various
> >> forms of internal processing to happen, even when events are not
> >> enqueued to or dequeued from a port.
> >>
> >> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> >> ---
> >>   lib/librte_eventdev/rte_eventdev.h     | 65 ++++++++++++++++++++++++++
> >>   lib/librte_eventdev/rte_eventdev_pmd.h | 14 ++++++
> >>   2 files changed, 79 insertions(+)
> >>
> >> diff --git a/lib/librte_eventdev/rte_eventdev.h b/lib/librte_eventdev/rte_eventdev.h
> >> index 226f352ad..d69150792 100644
> >> --- a/lib/librte_eventdev/rte_eventdev.h
> >> +++ b/lib/librte_eventdev/rte_eventdev.h
> >> @@ -289,6 +289,15 @@ struct rte_event;
> >>    * single queue to each port or map a single queue to many port.
> >>    */
> >>
> >> +#define RTE_EVENT_DEV_CAP_REQUIRES_MAINT (1ULL << 9)
> >> +/**< Event device requires calls to rte_event_maintain() during
> > This scheme would call for DSW specific API handling in fastpath.
>
>
> Initially this would be so, but buffering events might yield performance
> benefits for more event devices than DSW.
>
>
> In an application, it's often convenient, but sub-optimal from a
> performance point of view, to do single-event enqueue operations. The
> alternative is to use an application-level buffer, and the flush this
> buffer with rte_event_enqueue_burst(). If you allow the event device to
> buffer, you get the simplicity of single-event enqueue operations, but
> without taking any noticeable performance hit.

IMO, It is better to aggregate the burst by the application,  as sending
event by event to the driver to aggregate has performance due to cost
function pointer overhead.

Another concern is the frequency of calling rte_event_maintain() function by
the application, as the timing requirements will vary differently by
the driver to driver and application to application.
IMO, It is not portable and I believe the application should not be
aware of those details. If the driver needs specific maintenance
function for any other reason then better to use DPDK SERVICE core infra.

>
>
> >> + * periods when neither rte_event_dequeue_burst() nor
> > The typical worker thread will be
> > while (1) {
> >                  rte_event_dequeue_burst();
> >                   ..proess..
> >                  rte_event_enqueue_burst();
> > }
> > If so, Why DSW driver can't do the maintenance in driver context in
> > dequeue() call.
> >
>
> DSW already does maintenance on dequeue, and works well in the above
> scenario. The typical worker does not need to care about the
> rte_event_maintain() functions, since it dequeues events on a regular basis.
>
>
> What this RFC addresses is the more atypical (but still fairly common)
> case of a port being neither dequeued to or enqueued from on a regular
> basis. The timer and ethernet rx adapters are examples of such.

If it is an Adapter specific use case problem then maybe, we have
an option to fix the problem in adapter specific API usage or in that area.


>
>
> >> + * rte_event_enqueue_burst() are called on a port. This will allow the
> >> + * event device to perform internal processing, such as flushing
> >> + * buffered events, return credits to a global pool, or process
> >> + * signaling related to load balancing.
> >> + */
>
>