Make the C extension Ractor-safe by removing msgpack_rmem_* slab allocator#392
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Make the C extension Ractor-safe by removing msgpack_rmem_* slab allocator#392ianks wants to merge 1 commit into
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…or_safe) The page-recycling slab was a process-global msgpack_rmem_t mutated through an unsynchronized bitmask, so parallel Ractors (or threads) raced on it. Drop the slab and serve rmem pages straight from xmalloc/xfree (jemalloc and friends recycle these fine), then declare rb_ext_ractor_safe(true). The declaration is guarded by HAVE_RB_EXT_RACTOR_SAFE so the extension still builds on Ruby < 3.0. Add spec/cruby/ractor_spec.rb covering pack/unpack via Factory and Packer/Unpacker from non-main Ractors, including a concurrent multi-Ractor stress over the page-allocation path.
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This makes the extension safe to use from a non-main Ractor. The notable change here is the removal of the
msgpack_rmem_*routines, which serve as a mechanism for efficiently providing chunks of memory for decoding work.Why is it not ractor safe?
The old page-recycling slab is a process-global
msgpack_rmem_tmutated through an unsynchronized bitmask, so parallel Ractors (or threads) race on it.How did you address this?
Drop the global slab entirely and use plain
xmalloc/xfree. Modern arena-based mallocs (i.e. jemalloc) are good at recycling and avoiding thread contention, so maintaining a custom slab allocator is not worth it.Did you try alternatives?
Yes:
No:
malloc(3)Perf (local HTTP requests)
Single-threaded, jemalloc 5.3, decode-heavy workload
On a realistic, "real" HTTP request benchmark, bare-xmalloc is maybe a touch faster, but mostly noise in the diff:
RSS Impact
Memory usage seems fine as well, with the bare-xmalloc (this PR) having a higher peak as decay time increases (this is expected, and not a problem).
dirty_decay_ms:10000(default)dirty_decay_ms:1000closes #390