Re: [dpdk-dev] vhost-net stops sending to virito pmd -- already fixed?

[Kyle Larose <eomereadig@gmail.com>]

Hi Huawei,

> Kyle:
> Could you tell us how did you produce this issue, very small pool size
> or you are using pipeline model?

If I understand correctly, by pipeline model you mean a model whereby
multiple threads handle a given packet, with some sort IPC (e.g. dpdk
rings) between them? If so, yes: we are using such a model. And I
suspect that this model is where we run into issues: the length of the
pipeline, combined with the queuing between stages, can lead to us
exhausting the mbufs, particularly when a stage's load causes queuing.

When I initially ran into this issue, I had a fairly large mbuf pool
(32K entries), with 3 stages in the pipeline: rx, worker, tx. There
were two worker threads, with a total of 6 rings. I was sending some
fairly bursty traffic, at a high packet rate (it was bursting up to
around 1Mpkt/s). There was a low chance that this actually caused the
problem. However, when I decreased the mbuf pool to 1000 entries, it
*always* happened.

In summary: the pipeline model is important here, and a small pool
size definitely exacerbates the problem.

I was able to reproduce the problem using the load_balancer sample
application, though it required some modification to get it to run
with virtio. I'm not sure if this is because I'm using DPDK 1.8,  or
something else. Either way, I made the number of mbufs configurable
via an environment variable, and was able to show that decreasing it
from the default of 32K to 1K would cause the problem to always happen
when using the same traffic as with my application. Applying the below
patch fixed the problem.

The following patch seems to fix the problem for me, though I'm not
sure it's the optimal solution. It does so by removing the early exit
which prevents us from allocating mbufs. After we skip over the packet
processing loop since there are no packets, the mbuf allocation loop
runs.  Note that the patch is on dpdk 1.8.

diff --git a/lib/librte_pmd_virtio/virtio_rxtx.c
b/lib/librte_pmd_virtio/virtio_rxtx.c
index c013f97..7cadf52 100644
--- a/lib/librte_pmd_virtio/virtio_rxtx.c
+++ b/lib/librte_pmd_virtio/virtio_rxtx.c
@@ -463,9 +463,6 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf
**rx_pkts, uint16_t nb_pkts)
        if (likely(num > DESC_PER_CACHELINE))
                num = num - ((rxvq->vq_used_cons_idx + num) %
DESC_PER_CACHELINE);

-       if (num == 0)
-               return 0;
-
        num = virtqueue_dequeue_burst_rx(rxvq, rcv_pkts, len, num);
        PMD_RX_LOG(DEBUG, "used:%d dequeue:%d", nb_used, num);
        for (i = 0; i < num ; i++) {
@@ -549,9 +546,6 @@ virtio_recv_mergeable_pkts(void *rx_queue,

        rmb();

-       if (nb_used == 0)
-               return 0;
-
        PMD_RX_LOG(DEBUG, "used:%d\n", nb_used);

        while (i < nb_used) {

Thanks,

Kyle