Compacting GC for MRI

on YouTube ↗51m

" summary

Compacting GC for MRI ships with Ruby 2.7 and takes three years of development; it implements a two-finger compaction over Ruby’s 40-byte slot, 16KB page heap, moving objects then updating references (reference-updating is ~80% of the patch). It adds pinning bits and compaction callbacks so C extensions don’t break, maintains object_id via a seen-ID map, cuts fragmentation to reclaim measurable Rails heap space, and currently runs as a 4-step GC that the team plans to shrink to 2–3 steps.

AI-written summary · about

" chapters (11)

1. 0:00Introduction and personal anecdotes
2. 13:12What is compaction
3. 22:27Ruby’s heap structure
4. 23:52Two-finger compaction algorithm
5. 27:53Updating references complexity
6. 29:00Supporting C extensions
7. 37:00Hidden references problem
8. 41:57Marking references rule
9. 42:17Object ID challenges
10. 46:33Impact on memory usage
11. 48:17Future improvements

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