netdev - Re: [RFC 0/2] Delayed binding of UDP sockets for Quic per-connection sockets

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Date:   Wed, 31 Oct 2018 22:08:53 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Christoph Paasch <cpaasch@...le.com>
Cc:     netdev@...r.kernel.org, Ian Swett <ianswett@...gle.com>,
        Leif Hedstrom <lhedstrom@...le.com>,
        Jana Iyengar <jri.ietf@...il.com>
Subject: Re: [RFC 0/2] Delayed binding of UDP sockets for Quic per-connection
 sockets



On 10/31/2018 08:50 PM, Christoph Paasch wrote:
> On 31/10/18 - 17:53:22, Eric Dumazet wrote:
>> On 10/31/2018 04:26 PM, Christoph Paasch wrote:
>>> Implementations of Quic might want to create a separate socket for each
>>> Quic-connection by creating a connected UDP-socket.
>>>
>>
>> Nice proposal, but I doubt a QUIC server can afford having one UDP socket per connection ?
>>
>> It would add a huge overhead in term of memory usage in the kernel,
>> and lots of epoll events to manage (say a QUIC server with one million flows, receiving
>> very few packets per second per flow)
>>
>> Maybe you could elaborate on the need of having one UDP socket per connection.
> 
> I let Leif chime in on that as the ask came from him. Leif & his team are
> implementing Quic in the Apache Traffic Server.
> 
> 
> One advantage I can see is that it would allow to benefit from fq_pacing as
> one could set sk_pacing_rate simply on the socket. That way there is no need
> to implement the pacing in the user-space anymore.

Our plan is to use EDT model for UDP packets, so that we can
still use one (not connected) UDP socket per cpu/thread.

We added in linux-4.20 the EDT model for TCP, and I intend to add the remaining part for sch_fq for 4.21.

UDP can use an ancillary message (SCM_TXTIME) to attach to the skb (which can be a GSO btw) a tstamp,
and pacing will happen just fine.