What causes Ruby memory bloat?

Ruby apps can use a lot of memory. But why? Various people in the community attribute it to memory fragmentation, and provide two “hacky” solutions. Dissatisfied by the current explanations and provided solutions, I set out on a journey to discover the deeper truth and to find better.At Phusion we run a simple multithreaded HTTP proxy server written in Ruby (which serves our DEB and RPM packages). I've seen it consume 1.3 GB of memory. This is insane – the server is stateless and doesn't do all that much!.