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

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Date:   Fri, 25 Aug 2017 21:03:29 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Koichiro Den <den@...ipeden.com>, Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit
 path if no tx napi

On Fri, Aug 25, 2017 at 7:32 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Fri, Aug 25, 2017 at 06:44:36PM -0400, Willem de Bruijn wrote:
>> >> >> > We don't enable network watchdog on virtio but we could and maybe
>> >> >> > should.
>> >> >>
>> >> >> Can you elaborate?
>> >> >
>> >> > The issue is that holding onto buffers for very long times makes guests
>> >> > think they are stuck. This is funamentally because from guest point of
>> >> > view this is a NIC, so it is supposed to transmit things out in
>> >> > a timely manner. If host backs the virtual NIC by something that is not
>> >> > a NIC, with traffic shaping etc introducing unbounded latencies,
>> >> > guest will be confused.
>> >>
>> >> That assumes that guests are fragile in this regard. A linux guest
>> >> does not make such assumptions.
>> >
>> > Yes it does. Examples above:
>> >         > > - a single slow flow can occupy the whole ring, you will not
>> >         > >   be able to make any new buffers available for the fast flow
>>
>> Oh, right. Though those are due to vring_desc pool exhaustion
>> rather than an upper bound on latency of any single packet.
>>
>> Limiting the number of zerocopy packets in flight to some fraction
>> of the ring ensures that fast flows can always grab a slot.
>> Running
>> out of ubuf_info slots reverts to copy, so indirectly does this. But
>> I read it correclty the zerocopy pool may be equal to or larger than
>> the descriptor pool. Should we refine the zcopy_used test
>>
>>     (nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx
>>
>> to also return false if the number of outstanding ubuf_info is greater
>> than, say, vq->num >> 1?
>
>
> We'll need to think about where to put the threshold, but I think it's
> a good idea.
>
> Maybe even a fixed number, e.g. max(vq->num >> 1, X) to limit host
> resources.
>
> In a sense it still means once you run out of slots zcopt gets disabled possibly permanently.
>
> Need to experiment with some numbers.

I can take a stab with two flows, one delayed in a deep host qdisc
queue. See how this change affects the other flow and also how
sensitive that is to the chosen threshold value.