[winrid]

You can. You can with Node too.

With Node you can just use workers. I have tools I wrote in Node that can max out my 16 core MacBook.

[3np]

Some major differences here are how they interface with I/O and the mechanisms around memory sharing.

Nodejs workers are more like webworkers and mostly suitable for proper CPU-intensive parallelization whereas in Ruby it's not uncommon to run e.g. multithreaded web server in the same process and namespace.