On Sat, May 09, 2015 at 04:27:12PM +0000, Clark, Gilbert 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]. > > 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?
Indeed it doesn't free the mbufs, because this is not a PMD backed by real hardware so the packets are never actually transmitted anywhere, just passed to the other end of the ring. To behave strictly like a physical PMD, we would copy the sent packet to a new buffer on RX, and free the old one. However, in this case, we take a shortcut and just pass the same mbuf on RX as was passed on TX, which saves the cycles for buffer management. To see buffer freeing on TX occur, I suggest you look at some of the other PMDs, perhaps the e1000/igb PMD? /Bruce
