Puma
A Ruby/Rack web server built for concurrency
... bundle, start Puma. If you are not using Bundler, you can install Puma directly from the command line. On MRI, there is a Global VM Lock (GVL) that ensures only one thread can run Ruby code at a time. But if you're doing a lot of blocking IO (such as HTTP calls to external APIs like Twitter), Puma still improves MRI's throughput by allowing IO waiting to be done in parallel.