Re: [dpdk-dev] Getting started - sanity check

["Wiles, Keith" <keith.wiles@intel.com>]

Sent from my iPhone

> On May 9, 2015, at 9:27 AM, Clark, Gilbert <gc355804@ohio.edu> wrote:
> 
> 
> Hi folks:
> 
> I'm brand new to DPDK.  Read about it off and on occasionally, but never had the chance to sit down and play with things until now.  It's been fun so far: just been working on a few toy applications to get myself started.
> 
> I have run into a question, though: when calling rte_eth_tx_burst with a ring-backed PMD I've set up, the mbufs I've sent never seem to be freed.  This seems to make some degree of sense, but ... since I'm new, and because the documentation says rte_eth_tx_burst should eventually free mbufs that are sent [1], I wanted to make sure I'm on track and not just misunderstanding the way something works [2].

The mbufs are free as needed or when a watermark is hit in the driver. One other thing I found is you need to send enough packets to hit the water mark for the tx mbufs start to get freed. Also you have to have enough mbufs allocated to hit these watermarks. I just pick two times the ring size just to be safe. 

Hope that helps. 

> 
> Thanks,
> Gilbert Clark
> 
> [1] From http://dpdk.org/doc/api/rte__ethdev_8h.html :
> 
> It is the responsibility of the rte_eth_tx_burst() function to transparently free the memory buffers of packets previously sent
> 
> [2] From lib/librte_pmd_ring.c:
> 
> static uint16_t
> eth_ring_tx(void *q, struct rte_mbuf **bufs, uint16_t nb_bufs)
> {
>    void **ptrs = (void *)&bufs[0];
>    struct ring_queue *r = q;
>    const uint16_t nb_tx = (uint16_t)rte_ring_enqueue_burst(r->rng,
>            ptrs, nb_bufs);
>    if (r->rng->flags & RING_F_SP_ENQ) {
>        r->tx_pkts.cnt += nb_tx;
>        r->err_pkts.cnt += nb_bufs - nb_tx;
>    } else {
>        rte_atomic64_add(&(r->tx_pkts), nb_tx);
>        rte_atomic64_add(&(r->err_pkts), nb_bufs - nb_tx);
>    }
>    return nb_tx;
> }
> 
> This doesn't ever appear to free a transmitted mbuf ... unless there's code to do that somewhere else that I'm missing?