file: /proc/sys/net/ipv4/ip_early_demux
variable: net.ipv4.ip_early_demux
Official reference

Optimize input packet processing down to one demux for certain kinds of local sockets. Currently we only do this for established TCP and connected UDP sockets.

It may add an additional cost for pure routing workloads that reduces overall throughput, in such case you should disable it. Default: 1