Option to configure event loop group thread count

While the same is configurable using a netty jvm option for client pools, for the server it's hardcoded at 2 * .availableProcessors

If there was an option to configure the event loop group thread counts (maybe even separately for the parent/child groups) users with enough understanding of their workloads would be able to configure more informed thread pools

Start by locating the server's hardcoded event-loop thread-count setup and the existing Netty JVM option used for client pools. Trace how server options are configured, then define and test configurable parent and child counts; done means server users can set the intended counts without changing defaults.

Written by the indexing model from the issue text.