Until you’re able to get to 100% CPU usage under the load you’re probably hitting some bottleneck somewhere. Here are some additional suggestions for discovering it:
- Whether you’re using exrm or not, make sure to load test the code compiled with
MIX_ENV=prod - If you’re load testing simple 204 response, make sure to keep
Plug.Parsersout. Using it might cause the body to be loaded (seePlug.Parsers.JSON), and the throughput of your simple CPU-bound code might become I/O bound. - Consider temporarily moving Plug/http out of the picture. Write a simple program that spawns a couple of processes (at least as many as you have cores), and make those processes CPU bound. They could run an infinite loop which invokes
:rand.uniform, or something similar. Just make sure they’re not receiving messages, sleeping, or doing I/O bound work. Check if you’re getting 100% CPU usage. If not, then take try playing with erl options and see if it helps. - Start the observer, then load test the system for a while. If you have a bottleneck in the system, it might show up in the processes tab during the load test. Sort the view by reds and/or message queue, and try to see if some process is constantly on/near the top of the list with a large message queue. If yes, that’s a possible bottleneck.
Only when you reach CPU usage of 100% can you consider comparing Scala vs Erlang/Elixir, and also think about possible optimizations in your code. Before that, some other bottleneck is likely making your system less efficient than it could be.
Good luck! 
