In this keynote presentation titled "Exploring Memory in Ruby - Building a Compacting GC," Aaron Patterson, a key member of the Ruby and Rails core teams, discusses the intricacies of garbage collection (GC) and memory management in Ruby. The talk is structured around three main topics: copy-on-write (COW) optimizations, building a compacting garbage collector, and memory inspection tools in Ruby. Patterson underscores the importance of a compacting GC in reducing memory fragmentation and improving memory usage. He explains COW optimizations, which allow memory to be copied only when it's necessary, demonstrating this with examples in Ruby, particularly with strings and arrays.

Key Points Discussed:
- Introduction to Garbage Collection: The significance of GC in managing memory usage in Ruby applications.
- Copy-On-Write Optimizations: COW allows duplicate objects to share the same memory until one is modified, minimizing unnecessary memory usage. This is illustrated through Ruby examples and comparisons to how operating systems handle memory.
- Challenges with Forking: Patterson explains how using forking in web servers like Unicorn can lead to excessive memory overhead if COW optimizations are not utilized properly, emphasizing that it’s crucial to load shared libraries only once to conserve memory.
- Compaction Techniques: He introduces the concept of compaction, where live objects are rearranged to reduce fragmentation. Patterson describes a specific technique, two-finger compaction, outlining its advantages and complexities. He emphasizes that while compaction might seem slow, its effectiveness in merging free spaces can significantly alleviate memory problems during forking.
- Implementation Challenges: The talk covers technical challenges faced during implementation, including issues with hash keys, dual references, and string literals affecting memory allocation and management. Patterson shares how these obstacles can be mitigated through careful coding practices and heuristics.
- Memory Inspection Tools: Patterson also highlights the importance of memory inspection tools in evaluating GC efficiency and the effectiveness of COW optimizations, demonstrating a simple method using Ruby's object space capabilities.

In conclusion, Patterson motivates attendees to challenge the perception of impossible tasks in programming, proving through his work with GC compaction that innovative solutions can be formulated by questioning assumptions and exploring new ideas in the context of Ruby memory management. He advocates for community engagement and provides an open invitation for discussions, emphasizing collaboration in overcoming programming challenges.

Takeaways:
- Understanding and optimizing memory usage is critical in Ruby, especially in web applications.
- Compaction can be realized practically, despite perceived limitations, allowing for improved performance in memory management.
- Encouragement to experiment with perceived problems, fostering a mindset of innovation and creativity within coding practices.