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Date:   Mon, 28 Dec 2020 14:51:09 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH rfc 0/3] virtio-net: add tx-hash, rx-tstamp and tx-tstamp

On Mon, Dec 28, 2020 at 12:29 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Dec 28, 2020 at 11:22:30AM -0500, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > RFC for three new features to the virtio network device:
> >
> > 1. pass tx flow hash and state to host, for routing + telemetry
> > 2. pass rx tstamp to guest, for better RTT estimation
> > 3. pass tx tstamp to host, for accurate pacing
> >
> > All three would introduce an extension to the virtio spec.
> > I assume this would require opening three ballots against v1.2 at
> > https://www.oasis-open.org/committees/ballots.php?wg_abbrev=virtio
> >
> > This RFC is to informally discuss the proposals first.
> >
> > The patchset is against v5.10. Evaluation additionally requires
> > changes to qemu and at least one back-end. I implemented preliminary
> > support in Linux vhost-net. Both patches available through github at
> >
> > https://github.com/wdebruij/linux/tree/virtio-net-txhash-1
> > https://github.com/wdebruij/qemu/tree/virtio-net-txhash-1
>
> Any data on what the benefits are?

For the general method, yes. For this specific implementation, not  yet.

Swift congestion control is delay based. It won the best paper award
at SIGCOMM this year. That paper has a lot of data:
https://dl.acm.org/doi/pdf/10.1145/3387514.3406591 . Section 3.1 talks
about the different components that contribute to delay and how to
isolate them.

BBR and BBRv2 also have an explicit ProbeRTT phase as part of the design.

The specific additional benefits for VM-based TCP depends on many
conditions, e.g., whether a vCPU is exclusively owned and pinned. But
the same reasoning should be even more applicable to this even longer
stack, especially in the worst case conditions.

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