Re: [PATCH RFC net-next v2 0/4] virtio/vsock: support datagrams

On Sat, Apr 15, 2023 at 03:55:05PM +0000, Bobby Eshleman wrote:

On Fri, Apr 28, 2023 at 12:43:09PM +0200, Stefano Garzarella wrote:

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On Sat, Apr 15, 2023 at 07:13:47AM +0000, Bobby Eshleman wrote:

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CC'ing virtio-dev@lists.oasis-open.org because this thread is starting
to touch the spec.

On Wed, Apr 19, 2023 at 12:00:17PM +0200, Stefano Garzarella wrote:

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Hi Bobby,

On Fri, Apr 14, 2023 at 11:18:40AM +0000, Bobby Eshleman wrote:

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CC'ing Cong.

On Fri, Apr 14, 2023 at 12:25:56AM +0000, Bobby Eshleman wrote:

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Hey all!

This series introduces support for datagrams to virtio/vsock.

Great! Thanks for restarting this work!

No problem!

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It is a spin-off (and smaller version) of this series from the summer:
  https://lore.kernel.org/all/cover.1660362668.git.bobby.eshleman@bytedance.com/ (local)

Please note that this is an RFC and should not be merged until
associated changes are made to the virtio specification, which will
follow after discussion from this series.

This series first supports datagrams in a basic form for virtio, and
then optimizes the sendpath for all transports.

The result is a very fast datagram communication protocol that
outperforms even UDP on multi-queue virtio-net w/ vhost on a variety
of multi-threaded workload samples.

For those that are curious, some summary data comparing UDP and VSOCK
DGRAM (N=5):

	vCPUS: 16
	virtio-net queues: 16
	payload size: 4KB
	Setup: bare metal + vm (non-nested)

	UDP: 287.59 MB/s
	VSOCK DGRAM: 509.2 MB/s

Some notes about the implementation...

This datagram implementation forces datagrams to self-throttle according
to the threshold set by sk_sndbuf. It behaves similar to the credits
used by streams in its effect on throughput and memory consumption, but
it is not influenced by the receiving socket as credits are.

So, sk_sndbuf influece the sender and sk_rcvbuf the receiver, right?

Correct.

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We should check if VMCI behaves the same.

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The device drops packets silently. There is room for improvement by
building into the device and driver some intelligence around how to
reduce frequency of kicking the virtqueue when packet loss is high. I
think there is a good discussion to be had on this.

Can you elaborate a bit here?

Do you mean some mechanism to report to the sender that a destination
(cid, port) is full so the packet will be dropped?

Correct. There is also the case of there being no receiver at all for
this address since this case isn't rejected upon connect(). Ideally,
such a socket (which will have 100% packet loss) will be throttled
aggressively.

Before we go down too far on this path, I also want to clarify that
using UDP over vhost/virtio-net also has this property... this can be
observed by using tcpdump to dump the UDP packets on the bridge network
your VM is using. UDP packets sent to a garbage address can be seen on
the host bridge (this is the nature of UDP, how is the host supposed to
know the address eventually goes nowhere). I mention the above because I
think it is possible for vsock to avoid this cost, given that it
benefits from being point-to-point and g2h/h2g.

If we're okay with vsock being on par, then the current series does
that. I propose something below that can be added later and maybe
negotiated as a feature bit too.

I see and I agree on that, let's do it step by step.
If we can do it in the first phase is great, but I think is fine to add
this feature later.

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Can we adapt the credit mechanism?

I've thought about this a lot because the attraction of the approach for
me would be that we could get the wait/buffer-limiting logic for free
and without big changes to the protocol, but the problem is that the
unreliable nature of datagrams means that the source's free-running
tx_cnt will become out-of-sync with the destination's fwd_cnt upon
packet loss.

We need to understand where the packet can be lost.
If the packet always reaches the destination (vsock driver or device),
we can discard it, but also update the counters.

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Imagine a source that initializes and starts sending packets before a
destination socket even is created, the source's self-throttling will be
dysfunctional because its tx_cnt will always far exceed the
destination's fwd_cnt.

Right, the other problem I see is that the socket aren't connected, so
we have 1-N relationship.

Oh yeah, good point.

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We could play tricks with the meaning of the CREDIT_UPDATE message and
fwd_cnt/buf_alloc fields, but I don't think we want to go down that
path.

I think that the best and simplest approach introduces a congestion
notification (VIRTIO_VSOCK_OP_CN?). When a packet is dropped, the
destination sends this notification. At a given repeated time period T,
the source can check if it has received any notifications in the last T.
If so, it halves its buffer allocation. If not, it doubles its buffer
allocation unless it is already at its max or original value.

An "invalid" socket which never has any receiver will converge towards a
rate limit of one packet per time T * log2(average pkt size). That is, a
socket with 100% packet loss will only be able to send 16 bytes every
4T. A default send buffer of MAX_UINT32 and T=5ms would hit zero within
160ms given at least one packet sent per 5ms. I have no idea if that is
a reasonable default T for vsock, I just pulled it out of a hat for the
sake of the example.

"Normal" sockets will be responsive to high loss and rebalance during
low loss. The source is trying to guess and converge on the actual
buffer state of the destination.

This would reuse the already-existing throttling mechanisms that
throttle based upon buffer allocation. The usage of sk_sndbuf would have
to be re-worked. The application using sendmsg() will see EAGAIN when
throttled, or just sleep if !MSG_DONTWAIT.

I see, it looks interesting, but I think we need to share that
information between multiple sockets, since the same destination
(cid, port), can be reached by multiple sockets.

Good point, that is true.

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Another approach could be to have both congestion notification and
decongestion, but maybe it produces double traffic.

I think this could simplify things and could reduce noise. It is also
probably sufficient for the source to simply halt upon congestion
notification and resume upon decongestion notification, instead of
scaling up and down like I suggested above. It also avoids the
burstiness that would occur with a "congestion notification"-only
approach where the source guesses when to resume and guesses wrong.

The congestion notification may want to have an expiration period after
which the sender can resume without receiving a decongestion
notification? If it receives congestion again, then it can halt again.

Yep, I agree.

Anyway the congestion/decongestion messages should be just a hint, 
because the other peer has to keep the state and a malicious host/guest 
could use it for DoS, so the peer could discard these packets if it has 
no more space to save the state.

Thanks,
Stefano

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