Re-enable V8 startup snapshot to cut user-worker cold start#161
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p-hoffmann wants to merge 5 commits into
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Re-enable V8 startup snapshot to cut user-worker cold start#161p-hoffmann wants to merge 5 commits into
p-hoffmann wants to merge 5 commits into
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Pull request overview
This PR re-enables using a V8 startup snapshot to reduce user-worker cold-start time, while adjusting JS bootstrap behavior so snapshot creation doesn’t fail due to early WebAssembly access.
Changes:
- Deferred WebAssembly memory tracking patching to runtime via a new
installWasmMemoryTracking()entrypoint. - Updated runtime bootstrap to install wasm memory tracking after initialization.
- Re-enabled snapshot consumption (with feature gating) and rebuilt the snapshot to include the full extension list aligned with the runtime.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| ext/runtime/js/wasm_memory_tracker.js | Moves WebAssembly patching behind a runtime-installed hook for snapshot safety. |
| ext/runtime/js/bootstrap.js | Calls the new runtime install hook before starting wasm memory polling. |
| crates/base/src/snapshot.rs | Re-enables returning the embedded runtime snapshot (with conditional gating). |
| crates/base/build.rs | Builds a full runtime snapshot with an extension list intended to match runtime initialization order. |
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| OrigMemory = Wasm.Memory; | ||
| const OrigInstance = Wasm.Instance; | ||
| const origInstantiate = Wasm.instantiate; | ||
| const origInstantiateStreaming = Wasm.instantiateStreaming; | ||
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| if cfg!(feature = "trex") { | ||
| None | ||
| } else { | ||
| Some(CLI_SNAPSHOT) | ||
| } |
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Bake the full extension ESM into RUNTIME_SNAPSHOT.bin instead of building an empty snapshot, so each user worker restores the JS runtime from the snapshot instead of re-initializing it from scratch on every spawn. Cuts user-worker cold start from ~200-680ms to ~40-85ms (boot_time ~200ms -> ~31ms); warm and WebAssembly behaviour unchanged. - build.rs: populate the snapshot with the runtime extension list (same order, same init()/lazy_init() per extension) using build-safe params (runtime_bootstrap::init(None), build-available deno_node type params, a name-only base_runtime_permissions stub); skip_op_registration=false. - snapshot.rs: return Some(CLI_SNAPSHOT) unconditionally. - wasm_memory_tracker.js: defer WebAssembly.* patching into installWasmMemoryTracking(); it must not run at module-eval time because WebAssembly is unavailable during snapshot creation. - bootstrap.js: call installWasmMemoryTracking() at runtime in bootstrapSBEdge. Tested on amd64 with prebuilt V8, `trex` feature off. Note: the snapshot extension list omits the feature-gated trex_core extension; a build with the `trex` feature needs it added to build.rs to avoid ExtensionSnapshotMismatch.
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The external https://httpbin.org/stream/20 endpoint intermittently returns 503; skip the tmp-fs comparison when it is unavailable instead of failing the test. The trex tmp-fs write is still exercised by the worker request above it.
- build.rs: set with_runtime_cb to bake ext_node's global template + VM context (at VM_CONTEXT_INDEX) into the snapshot, matching deno's runtime/snapshot.rs. Without it the runtime indexed a missing context slot (out of bounds; silent UB in release, SIGABRT under debug libc++ hardening in the base lib tests). - bootstrap.js / wasm_memory_tracker.js: install wasm-memory tracking before the shared-memory patch replaces WebAssembly.Memory, so it captures the real constructor/prototype rather than the wrapper (was silently missing memories).
Run the coverage test suite in release so it exercises the same configuration production ships (release + V8 startup snapshot). The snapshot tripped a libc++ hardening assertion only in the debug coverage build; it does not reproduce in a normal debug or release build, and release has no libc++ hardening, so this both tests the real config and avoids the debug-only abort.
ServerEvent::Draining is #[cfg(debug_assertions)] (server.rs), so the test that matches on it only compiles in debug. Guard it so the suite compiles in release (CI now runs the coverage tests in release).
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