NonBlockingServletIo

Use non-blocking reads and writes. Available only on containers that support Servlet 3.1.

This can support more concurrent connections on a smaller request thread pool than BlockingServletIo, but consumes more CPU per request. It is also known to cause IllegalStateExceptions in the logs under high load up through at least Tomcat 8.0.15. These appear to be harmless, but are operationally annoying.