User Code - No assumptions are made about how your program threads, if at all.
The event loop manages all socket IO for the cluster. If you start the cluster with dpp::st_return this will run under its own thread, otherwise if you use dpp::st_wait it will run in the same thread as the caller of the dpp::cluster::start method. The event loop will be either poll, epoll or kqueue based depending on your system capabilities. You should always start a cluster after forking, if your program forks, as various types of IO loop cannot be inherited by a forked process.
Set thread pool size via cluster constructor. Thread pool uses a priority queue and defaults in size to half the system concurrency value. Every callback or completed coroutine ends up executing here. The minimum concurrency of this pool is 4.
