We will talk about implementing a compacting GC for MRI. This talk will cover compaction algorithms, along with implementation details, and challenges specific to MRI. Well cover low level and high level memory measurement tools, how to use them, and what they're telling us. We'll cover copy-on-write optimizations and how our GC impacts them. After this talk you should have a better understanding of how Ruby's memory management interacts with the system memory, how you can measure it, and how to adjust your application to work in concert with the system's memory.