[shrughes]

You get less overhead by using less often the low-level concurrency primitives that involve cross-core synchronization. Cross-core synchronization happens in rethinkdb mainly when you see an on_thread_t object constructed or destroyed (and also in a few other places) and those get batched when you have more than one per event loop (which is not necessarily good, inflated queue sizes is also something to be wary of). So if you want to attach a bunch of network cards and high-speed storage devices on opposite ends of a handful of CPUs, your throughput won't be hindered by the fact that millions of threads are trying to talk to one another.