Estimate the number of file descriptors used by ouroboros-network

[coot]

This issue is related to:

• input-output-hk/cardano-node#305
• input-output-hk/ouroboros-consensus#20

TODO:

• [x] p2p
• [ ] non-p2p

[coot]

The network layer is using:

• 1 socket per upstream / downstream peer using node-to-node protocol. In P2P mode the socket is shared for upstream / downstream connections for the same peer, in non P2P mode 2 separate sockets are used;
• 1 socket per accepted node-to-client protocol connection;
• at most 2 listening sockets (one per IPv4 / IPv6);
• one listening Unix domain socket;
• the network layer occasionally reads /etc/resolv.conf file.
• As @karknu noted: we also need one file descriptor per dns name, they are resolved concurrently.

Note: The node will re-read the topology file on SIGHUP signal.

[coot]

ping @Jimbo4350 @disassembler

[coot]

Updated limits for P2P:

• 1 socket per upstream / downstream peer using node-to-node protocol. In P2P mode the socket is shared for upstream / downstream connections for the same peer, in non P2P mode 2 separate sockets are used;
• 1 socket per accepted node-to-client protocol connection;
• at most 2 listening sockets (one per IPv4 / IPv6);
• one listening Unix domain socket;
• the network layer occasionally reads /etc/resolv.conf file.
• at most 10 file descriptors for dns name resolutions (as of #4596).

The upper bound is n + m + l + 15 where

• n - number of upstream peers
• m - number of downstream peers
• l - number of local clients