Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

[Jason Wang <jasowang@redhat.com>]

On 2017年08月30日 11:11, Willem de Bruijn wrote:
> On Tue, Aug 29, 2017 at 9:45 PM, Jason Wang <jasowang@redhat.com> wrote:
>>
>> On 2017年08月30日 03:35, Willem de Bruijn wrote:
>>> On Fri, Aug 25, 2017 at 9:03 PM, Willem de Bruijn
>>> <willemdebruijn.kernel@gmail.com> wrote:
>>>> On Fri, Aug 25, 2017 at 7:32 PM, Michael S. Tsirkin <mst@redhat.com>
>>>> wrote:
>>>>> On Fri, Aug 25, 2017 at 06:44:36PM -0400, Willem de Bruijn wrote:

[...]

>>> Incomplete results at this stage, but I do see this correlation between
>>> flows. It occurs even while not running out of zerocopy descriptors,
>>> which I cannot yet explain.
>>>
>>> Running two threads in a guest, each with a udp socket, each
>>> sending up to 100 datagrams, or until EAGAIN, every msec.
>>>
>>> Sender A sends 1B datagrams.
>>> Sender B sends VHOST_GOODCOPY_LEN, which is enough
>>> to trigger zcopy_used in vhost net.
>>>
>>> A local receive process on the host receives both flows. To avoid
>>> a deep copy when looping the packet onto the receive path,
>>> changed skb_orphan_frags_rx to always return false (gross hack).
>>>
>>> The flow with the larger packets is redirected through netem on ifb0:
>>>
>>>     modprobe ifb
>>>     ip link set dev ifb0 up
>>>     tc qdisc add dev ifb0 root netem limit $LIMIT rate 1MBit
>>>
>>>     tc qdisc add dev tap0 ingress
>>>     tc filter add dev tap0 parent ffff: protocol ip \
>>>         u32 match ip dport 8000 0xffff \
>>>         action mirred egress redirect dev ifb0
>>>
>>> For 10 second run, packet count with various ifb0 queue lengths $LIMIT:
>>>
>>> no filter
>>>     rx.A: ~840,000
>>>     rx.B: ~840,000
>>
>> Just to make sure I understand the case here. What did rx.B mean here? I
>> thought all traffic sent by Sender B has been redirected to ifb0?
> It has been, but the packet still arrives at the destination socket.
> IFB is a special virtual device that applies traffic shaping and
> then reinjects it back at the point it was intercept by mirred.
>
> rx.B is indeed arrival rate at the receiver, similar to rx.A.
>

I see, then ifb looks pretty fit to the test.