The Whippet Embeddable Garbage Collection Library
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Whippet is a minimal, embed-only, highly parallel, pure-C garbage collection library, designed to replace Guile's use of the Boehm-Demers-Weiser collector, but designed also to be usable by other languages that might appreciate a zero-dependency, state-of-the-art upgrade to their memory manager. In this talk we present Whippet, compare Guile-on-Whippet to Guile-on-BDW, and outline a roadmap to getting Whippet merged into Guile. (c) FOSDEM 2025 1 & 2 February 2025 https://fosdem.org/2025/schedule/event/fosdem-2025-6066-the-whippet-embeddable-garbage-collection-library/
The Whippet Embeddable Garbage Collection Library
- 1. Whippet: A practical memory management upgrade for Guile & beyond 2 February 2025 – FOSDEM ’25 Andy Wingo Igalia, S.L.
- 2. Agenda The big idea ❧ The results ❧ The future ❧
- 3. The big idea Whippet is a practical memory management upgrade for Guile & beyond
- 4. A practical memory management upgrade for Guile & beyond
- 5. struct gc_options *options = NULL; struct gc_stack_addr *stack_base = NULL; struct gc_heap *heap; struct gc_mutator *mut; void *event_listener_data = NULL; gc_init(options, stack_addr, &heap, &mut, GC_NULL_EVENT_LISTENER, event_listener_data); void *obj = gc_allocate(mut, 42);
- 6. options = gc_allocate_options(); gc_options_parse_and_set_many(options, getenv("GC_OPTIONS")); struct gc_mutator_roots roots; // Embedder-defined gc_mutator_set_roots(mut, &roots); // For generational configurations gc_write_barrier(mut, obj, obj_size, edge, new_val); // For cooperative safepoints gc_safepoint(mut); // For collectors that don't require copying gc_pin_object(mut, ref);
- 7. static inline void gc_trace_object(struct gc_ref ref, void (*visit)(struct gc_edge edge, struct gc_heap *heap, void *visit_data), struct gc_heap *heap, void *trace_data, size_t *size) { /* ... */ } static inline void gc_trace_mutator_roots(struct gc_mutator_roots *roots, void (*trace_edge)(struct gc_edge edge, struct gc_heap *heap, void *trace_data), struct gc_heap *heap, void *trace_data) { /* ... */ }
- 8. A practical memory management upgrade for Guile & beyond
- 9. Whippet: An upgrade relative to BDW-GC Performance: Bump-pointer allocation, better parallelism Features: Ephemerons and finalizers that work Behavior: Choice of workload- appropriate collectors Memory use: Compaction, adaptive heap sizing (membalancer)
- 10. Whippet: An upgrade with a migration path Bridge, n.: Construction with two ends and a path in between Whippet: a GC library with compile- time abstraction over embedder needs and collector construction Collector variants: MMC, PCC, BDW MMC collector has optional conservative tracing stack roots ❧ global roots ❧ (optionally) intra-heap edges ❧
- 11. MMC Mostly-marking collector MMC = nofl space + lospace
- 12. Nofl space “No free-list” For objects less than 8192 bytes Bump-pointer allocation Excellent parallelism Mostly-marking (Immix-derived), occasionally compacting 12% overhead Pinning (transitively due to conservative roots, or permanently) Optionally generational (sticky-mark)
- 13. Lospace “Large object space” mmap allocation, freelist, deferred release Optionally generational (sticky-mark)
- 14. Whippet: An upgrade relative to bespoke GCs Language run-times often get stuck with their GC Whippet’s compile-time API abstraction enables evolution
- 15. PCC Parallel copying collector PCC = copy space + lospace
- 16. Copy space For objects less than 8192 bytes Bump-pointer allocation Excellent parallelism Always compacting 100% overhead
- 17. Generational PCC Generational PCC = copy space + copy space + lospace 2 MB nursery per processor active in last cycle 1 survivor cycle for copy space, 0 for lospace Field-logging write barrier Nursery memory range aligned, can be quick XOR check Still has 100% overhead
- 18. BDW Boehm-Demers-Weiser collector Shim behind Whippet API Different safepoint behavior: not cooperative No support for gc_trace_object Not great parallelism Higher memory overhead than MMC
- 19. A practical memory management upgrade for Guile & beyond Embed-only No dependencies C11 Hackable
- 20. Practical testbench: Whiffle Scheme-to-C compiler: https:// github.com/wingo/whiffle Ensure Whippet offers appropriate API for embedders ❧ Allow more test cases to be written before moving to Guile ❧ Handles vs stack maps ❧ Main motivation was testing; shook out many bugs
- 21. A practical memory management upgrade for Guile & beyond The pivot: Whippet API, but BDW collector ❧ MMC collector, with conservative roots ❧ Generational MMC collector (write barriers) ❧ Evacuating nursery? ❧ Shout-out to NLnet foundation for helping us with this work!
- 22. A practical memory management upgrade for Guile & beyond WebAssembly+GC-to-C: Enable standalone Guile compilation via Hoot ? Ocaml, R, etc...
- 23. Results: What do we win with Whippet? Strict throughput improvements: 20- 40% Access to smaller heap sizes: 30-50%
- 28. Results: What do we learn with Whippet? Conservative root-finding is OK Generational GC is complicated, more tuning needed
- 33. Future Guile, finally. This month!!! Your language run-time? Concurrent marking LXR-inspired reference counting of old generation?
- 34. Try it out! https://github.com/wingo/whippet https://github.com/wingo/whiffle https://nlnet.nl/project/Whippet/ [email protected] Thanks!
- 35. Attic
- 36. New since 2023 PCC, generational PCC, precise field- logging write barriers instead of card marking, better parallelism, bug fixes, embeddability, finalizers, dynamic heap sizing (membalancer), less VMM traffic, whiffle, tests, nlnet, platform abstraction, options interface, extern space, stats, HDR histogram, renamings, nofl more eager