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[Prototype] Node 24: run await continuations from fibers in hidden fibers (port of #6 + #8) - #9
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Prototype, needs the patched node (v8-microtask-dispatch.h). While a fiber runs, its context's CPED is a marker, so every promise reaction registered inside it (the code after `await`, `.then` callbacks) is handed to a dispatcher that runs it in a new fiber. Those fibers are hidden: `Fiber.current` reports undefined there, as it does after an `await` today, so Fiber.current-based mode switches (e.g. cursor.observeChanges returning a Promise) are unchanged. Future.wait uses the new `Fiber.currentIncludingHidden`, so blocking calls work. The dispatch API is resolved with dlsym, so this binary still loads (with the feature off) on stock node. FIBERS_AWAIT_DISPATCH=0 disables it; Fiber.microtasksDispatched counts dispatched jobs. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Instead of a new hidden fiber per dispatched job, keep one long-lived hidden dispatch fiber. After each job it runs the dispatchable jobs queued right behind it (Fiber.__runNextDispatchable, needs the matching node patch), so a chain of awaits in fibered code costs one switch onto the fiber and one back instead of two per await. A job that blocks keeps the fiber and the next dispatch starts a new one; a fiber that was parked and later resumed never pulls in other jobs. FIBERS_AWAIT_REUSE=0 restores one new fiber per job. Fiber.microtasksBatched counts jobs run without a switch. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
- V8 13 moved the continuation-preserved embedder data (CPED) slot from the context to the isolate; use Isolate::Get/SetContinuationPreserved EmbedderData there and keep the Context API on older V8. - Tag with `true` instead of a heap object. On V8 13 the CPED slot is a GC root visited once per archived thread state (one per fiber), and a young object visited twice in one scavenge corrupts the heap. `true` lives in read-only space and never moves. - Don't install the dispatcher when node 24's AsyncContextFrame is on: it keeps AsyncLocalStorage state in the same slot. Detected with Fiber.__cpedInUse() inside an AsyncLocalStorage.run(); fibers needs --no-async-context-frame on node 24 anyway. - Keep Fiber.microtasksBatched on the shared native module, so it reads correctly when an app loads more than one copy of fibers_async.js. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
…tly requested Helper processes (build tools, child processes) load fibers on node 24 without --no-async-context-frame; a warning on every load is noise. Fiber.awaitDispatchEnabled reports whether dispatch is on. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Without AsyncContextFrame an AsyncLocalStorage enables promise hooks for the whole process, which made every await ~5x slower in processes that use no AsyncLocalStorage of their own. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This was referenced Oct 2, 2026
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Summary
Port of the node 18 fiber-await prototype (#6 + #8) to node 24, stacked on #5 (
node24-pthread; merge #5 first). Needs qualialabs/node#10. Code after anawait(or a.thencallback) registered inside a fiber runs on a hidden fiber, reused and batched across consecutive jobs;Fiber.currentstill readsundefinedthere, soFiber.current-based mode switches are unchanged, whileFuture.wait(andPromise.awaitwith qualialabs/shared-packages#265) can block.f8fdefdfibers_async.js)b632621dcf9992trueinstead of a heap object (the CPED slot is a GC root visited once per archived thread, i.e. per fiber, and a young object visited twice in a scavenge corrupts the heap); don't install the dispatcher when AsyncContextFrame is on (it keeps AsyncLocalStorage state in the same slot; detected withFiber.__cpedInUse());Fiber.microtasksBatchedkept on the shared native module1eacecdFIBERS_AWAIT_DISPATCH=1asks for dispatch (helper processes load fibers without the flag);Fiber.awaitDispatchEnabledreports the stated0ed894AsyncLocalStorageotherwise turns on promise hooks for the whole process and made fiberless awaits ~5x slowerMeasurement tooling for node 24 (counters,
fiber-await/{test,bench}.js,app_stats.mjs, README) is onjbaczuk/fiber-await-v24-stats, which is not meant to merge.Testing
node 24.21.0 from node#10, arm64 Docker,
--no-async-context-frame:fiber-await/test.js: 11/11 with reuse on and off;FIBERS_AWAIT_DISPATCH=0, stock [Prototype] Run await continuations from fibers in hidden fibers (needs node#7) #6 node, or AsyncContextFrame on: the expected 7 failures, andFiber.awaitDispatchEnabled === falsewith AsyncContextFrame on.test/: all pass (includingpool.js/cleanup.js, thanks to Node 24 / V8 13 compatibility with CORO_PTHREAD coroutines #5'scoro_destroyfix). znewsham's scenarios (Yield after await #7, running fiber): 9/9. meteor-promise suite: 15/15.--stress-scavenge=20,--stress-compaction): behaviour checks pass.jbaczuk/node-24-gdc-fiber-await, on top of qualia#56473, vendoring this branch with a prebuilt node 24 binary): boots; a fibered startup hook that awaits and then callsMeteor.users.find().count()works on a hidden fiber; fiberless code still throws. Two hand-patches needed there: meteor-lite#41's tarball guard, and shared-packages#265's one-liner in both meteor-promise copies (GDC loads the one nested under@meteor/promise/node_modules).🤖 Generated with Claude Code