Re: [RFC PATCH net-next] net/udp: Add 4-tuple hash for connected socket

[Eric Dumazet]

On Fri, Sep 13, 2024 at 12:09 PM Philo Lu <lulie@xxxxxxxxxxxxxxxxx> wrote:
>
> This RFC patch introduces 4-tuple hash for connected udp sockets, to
> make udp lookup faster. It is a tentative proposal and any comment is
> welcome.
>
> Currently, the udp_table has two hash table, the port hash and portaddr
> hash. But for UDP server, all sockets have the same local port and addr,
> so they are all on the same hash slot within a reuseport group. And the
> target sock is selected by scoring.
>
> In some applications, the UDP server uses connect() for each incoming
> client, and then the socket (fd) is used exclusively by the client. In
> such scenarios, current scoring method can be ineffcient with a large
> number of connections, resulting in high softirq overhead.
>
> To solve the problem, a 4-tuple hash list is added to udp_table, and is
> updated when calling connect(). Then __udp4_lib_lookup() firstly
> searches the 4-tuple hash list, and return directly if success. A new
> sockopt UDP_HASH4 is added to enable it. So the usage is:
> 1. socket()
> 2. bind()
> 3. setsockopt(UDP_HASH4)
> 4. connect()
>
> AFAICT the patch (if useful) can be further improved by:
> (a) Support disable with sockopt UDP_HASH4. Now it cannot be disabled
> once turned on until the socket closed.
> (b) Better interact with hash2/reuseport. Now hash4 hardly affects other
> mechanisms, but maintaining sockets in both hash4 and hash2 lists seems
> unnecessary.
> (c) Support early demux and ipv6.
>
> Signed-off-by: Philo Lu <lulie@xxxxxxxxxxxxxxxxx>

Adding a 4-tuple hash for UDP has been discussed in the past.

Main issue is that this is adding one cache line miss per incoming packet.

Most heavy duty UDP servers (DNS, QUIC), use non connected sockets,
because having one million UDP sockets has huge kernel memory cost,
not counting poor cache locality.