Doing some rough math and my limited understanding of linux network internals, it's about 40KB per connection in this benchmark. I know that cowboy is going to require ~4KB or so per connection. Consulting a local ubuntu install, the default minimum buffer sizes in TCP will be at least 4KB each (2* for read and write), but by default 16KB each, and by default the max goes to 1.5MB or so each. This is required for TCP retransmits and such. If you have clients on shoddy connections or see packet loss, your memory could skyrocket on you. I remember reading of a case where someone had a service die despite memory overhead of 33% when the TCP packet loss rate went up (still under 1%), but it caused their buffer sizes to grow large enough to run out of memory.

So that's 8KB (will be higher with more usage) for TCP buffers in the kernel, 4KB or so for cowboy, and 28KB or so left for various other bits of the system when amortized per connection.