Re: [dpdk-dev] Ring PMD: why are stats counters atomic?

["Mauricio Vásquez" <mauricio.vasquezbernal@studenti.polito.it>]

Hello Bruce,

Although having this support does not harm anyone, I am not convinced that
it is useful, mainly because there exists the single-thread limitation in
other PMDs. Then, if an application has to use different kind of NICs (i.e,
different PMDs) it has to implement the locking strategies. On the other
hand, if an application  only uses rte_rings, it could just use the
rte_ring library.

Thanks, Mauricio V

On Tue, May 10, 2016 at 11:36 AM, Bruce Richardson <
bruce.richardson@intel.com> wrote:

> On Tue, May 10, 2016 at 11:13:08AM +0200, Mauricio Vásquez wrote:
> > Hello,
> >
> > Per-queue stats counters are defined as rte_atomic64_t, in the tx/rx
> > functions, they are atomically increased if the rings have the multiple
> > consumers/producer flag enabled.
> >
> > According to the design principles, the application should not invoke
> those
> > functions on the same queue on different cores, then I think that atomic
> > increasing is not necessary.
> >
> > Is there something wrong with my reasoning?, If not, I am willing to
> send a
> > patch.
> >
> > Thank you very much,
> >
> Since the rte_rings, on which the ring pmd is obviously based, have
> multi-producer
> and multi-consumer support built-in, I thought it might be useful in the
> ring
> PMD itself to allow multiple threads to access the ring queues at the same
> time,
> if the underlying rings are marked as MP/MC safe. When doing enqueues and
> dequeue
> from the ring, the stats are either incremented atomically, or
> non-atomically,
> depending on the underlying queue type.
>
>         const uint16_t nb_rx = (uint16_t)rte_ring_dequeue_burst(r->rng,
>                         ptrs, nb_bufs);
>         if (r->rng->flags & RING_F_SC_DEQ)
>                 r->rx_pkts.cnt += nb_rx;
>         else
>                 rte_atomic64_add(&(r->rx_pkts), nb_rx);
>
> If people don't think this behaviour is worthwhile keeping, I'm ok with
> removing
> it, since all other PMDs have the restriction that the queues are
> single-thread
> only.
>
> Regards,
> /Bruce
>