For this test were used two instances in Google-Cloud n1-highmem-16 (16 vCPUs, 104 GB memory) running Debian GNU/Linux 9. One for server, one for client.
Each of 5-million WebSocket clients continuously sends messages to server. Once message is sent client sends another one.
Both server and client applications are running asynchronous oatpp server/client based on oatpp coroutines.