Ruby 2.7: Understand and debug problems with heap compaction

GitLab's recent update to Puma version 5, a multi-threaded Rack application server for the Ruby platform, introduced features like nakayoshi_fork and heap compaction to improve memory management by reducing fragmentation. The nakayoshi_fork feature leverages tenuring objects and compaction to optimize memory usage, with Ruby's GC.compact method compacting the heap to reclaim unused memory. However, when GitLab deployed these changes, they encountered production issues due to a segmentation fault caused by Hamlit's C-extension, which was not compaction-aware. This incident underscored the challenges of updating large applications with memory optimization techniques, highlighting the need for more reliable detection of compaction issues in CI environments and improved system configuration rollout capabilities. As a result, GitLab decided against using heap compaction in production until its benefits outweigh the associated risks.

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