Subject: Re: epoll_wait() performance

From: Jakub Sitnicki <[email-masked]>
Date: Sat, 30 Nov 2019 14:29:41 +0100

On Sat, Nov 30, 2019 at 02:07 AM CET, Eric Dumazet wrote:
> On 11/28/19 2:17 AM, David Laight wrote:
>> From: Eric Dumazet
>>> Sent: 27 November 2019 17:47
>> ...
>>> A QUIC server handles hundred of thousands of ' UDP flows' all using only one UDP socket
>>> per cpu.
>>>
>>> This is really the only way to scale, and does not need kernel changes to efficiently
>>> organize millions of UDP sockets (huge memory footprint even if we get right how
>>> we manage them)
>>>
>>> Given that UDP has no state, there is really no point trying to have one UDP
>>> socket per flow, and having to deal with epoll()/poll() overhead.
>>
>> How can you do that when all the UDP flows have different destination port numbers?
>> These are message flows not idempotent requests.
>> I don't really want to collect the packets before they've been processed by IP.
>>
>> I could write a driver that uses kernel udp sockets to generate a single message queue
>> than can be efficiently processed from userspace - but it is a faff compiling it for
>> the systems kernel version.
>
> Well if destinations ports are not under your control,
> you also could use AF_PACKET sockets, no need for 'UDP sockets' to receive UDP traffic,
> especially it the rate is small.

Alternatively, you could steer UDP flows coming to a certain port range
to one UDP socket using TPROXY [0, 1].

TPROXY has the same downside as AF_PACKET, meaning that it requires at
least CAP_NET_RAW to create/set up the socket.

OTOH, with TPROXY you can gracefully co-reside with other services,
filering on just the destination addresses you want in iptables/nftables.

Fan-out / load-balancing with reuseport to have one socket per CPU is
not possible, though. You would need to do that with Netfilter.

-Jakub

[0] https://www.kernel.org/doc/Documentation/networking/tproxy.txt
[1] https://blog.cloudflare.com/how-we-built-spectrum/