lib: load fewer builtins when bootstrapping without a snapshot by codebytere · Pull Request #65329 · nodejs/node

codebytere · 2026-08-16T15:51:27Z

Worker startup gets ~6 % faster and a snapshot-less main-thread bootstrap (embedders that create their own isolate, --no-node-snapshot) ~10 % faster, by not eagerly loading builtins those paths never use. Startup with the snapshot is unchanged.

new Worker() → 'online' → terminate                       21.3 → 20.1 ms   ***   +5.9 %  ±1.4%
misc/startup-core.js mode='worker' script='semicolon.js'                   ***   +5.4 %  ±0.4%
misc/startup-core.js mode='worker' script='empty.mjs'                      ***   +3.1 %  ±0.3%
node --no-node-snapshot semicolon.js (start→exit)         58.9 → 53.4 ms   ***  +10.3 %  ±1.1%
node --no-node-snapshot empty.mjs                                          ***   +2.5 %  ±1.1%
misc/startup-core.js mode='process' (snapshot), all scripts                       ±0.5 %  n.s.

Builtins compiled: bare worker 95 → 83; --no-node-snapshot empty CJS entry 76 → 59, empty ESM entry 76 → 69.

Contexts that aren't deserialized from the snapshot compile every builtin the bootstrap touches, so each eager require there is startup time (~0.15–0.4 ms apiece). Several are eager only so that they land in the snapshot, or for features the bootstrap doesn't use. This makes them lazy without changing the snapshot's contents:

is_main_thread.js: util, url, the ESM loader chain, internal/blob and internal/dns/utils are preloaded only if (isBuildingSnapshot()); otherwise they load on first use.
fs → internal/blob: only for fs.openAsBlob(). internal/url → internal/data_url: only for the Buffer-returning file-URL helpers. execution, cjs loader, esm/translators, esm/load → TypeScript stripper / data: helpers: only for those inputs.
pre_execution → internal/dns/utils: only for an explicit --dns-result-order (still validated at startup) or a snapshot build; 'verbatim' becomes the variable's initializer, so a snapshot-time setDefaultResultOrder() still survives deserialization.
internal/worker → ELU / error_serdes on demand; worker_threads.locks via defineLazyProperties (as util does).

An intermediate version that didn't re-add these to the snapshot regressed node empty.mjs by 2–4 %, which is why the isBuildingSnapshot() block lists them explicitly. test-bootstrap-modules is adjusted for the worker-side list.

Tests: test-bootstrap-modules plus worker, url, fs, dns, process, cli, vm, snapshot, blob, esm, inspector, module, util, test-runner and single-executable suites pass.

Disclosure: the code, test, measurements and this description were written by Claude Code, directed and reviewed by @codebytere.

nodejs-github-bot · 2026-08-16T15:51:32Z

Review requested:

@nodejs/loaders
@nodejs/net
@nodejs/startup
@nodejs/url

codecov · 2026-08-16T17:43:24Z

Codecov Report

❌ Patch coverage is 66.66667% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (30bff4a) to head (088f769).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/bootstrap/switches/is_main_thread.js	0.00%	18 Missing ⚠️
lib/internal/url.js	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65329      +/-   ##
==========================================
- Coverage   90.13%   90.12%   -0.01%     
==========================================
  Files         752      752              
  Lines      251568   251636      +68     
  Branches    47270    47284      +14     
==========================================
+ Hits       226759   226798      +39     
- Misses      16168    16174       +6     
- Partials     8641     8664      +23

Files with missing lines	Coverage Δ
lib/fs.js	`98.36% <100.00%> (-0.01%)`	⬇️
lib/internal/dns/utils.js	`100.00% <100.00%> (+0.55%)`	⬆️
lib/internal/modules/cjs/loader.js	`98.14% <100.00%> (ø)`
lib/internal/modules/esm/load.js	`91.32% <100.00%> (-0.08%)`	⬇️
lib/internal/modules/esm/translators.js	`97.56% <100.00%> (+0.01%)`	⬆️
lib/internal/process/execution.js	`94.76% <100.00%> (+0.21%)`	⬆️
lib/internal/process/pre_execution.js	`97.75% <100.00%> (+0.60%)`	⬆️
lib/internal/worker.js	`96.75% <100.00%> (-0.02%)`	⬇️
lib/worker_threads.js	`100.00% <100.00%> (ø)`
lib/internal/url.js	`93.28% <85.71%> (+0.01%)`	⬆️
... and 1 more

... and 37 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

nodejs-github-bot · 2026-08-17T04:14:12Z

CI: https://ci.nodejs.org/job/node-test-pull-request/75905/

github-actions · 2026-08-17T08:12:18Z

Benchmark GHA (misc / startup-core): https://github.com/nodejs/node/actions/runs/32009241408

Results

[!WARNING]
Do not take GHA benchmark results as face value, always confirm them
using a dedicated machine, e.g. Jenkins CI.

Benchmark results:

                                                                                         confidence improvement accuracy (*)    (**)   (***)
misc/startup-core.js n=30 mode='process' script='benchmark/fixtures/empty.mjs'                           0.95 %      ±16.58% ±21.85% ±28.04%
misc/startup-core.js n=30 mode='process' script='benchmark/fixtures/import-builtins.mjs'                 0.60 %      ±11.57% ±15.25% ±19.56%
misc/startup-core.js n=30 mode='process' script='benchmark/fixtures/require-builtins.js'                 0.19 %      ±13.51% ±17.81% ±22.85%
misc/startup-core.js n=30 mode='process' script='test/fixtures/semicolon.js'                            -0.02 %      ±16.55% ±21.81% ±27.99%
misc/startup-core.js n=30 mode='process' script='test/fixtures/snapshot/typescript.js'                  -0.54 %       ±8.97% ±11.82% ±15.17%
misc/startup-core.js n=30 mode='worker' script='benchmark/fixtures/empty.mjs'                            3.74 %       ±9.02% ±11.89% ±15.26%
misc/startup-core.js n=30 mode='worker' script='benchmark/fixtures/import-builtins.mjs'                  1.08 %       ±8.87% ±11.69% ±15.00%
misc/startup-core.js n=30 mode='worker' script='benchmark/fixtures/require-builtins.js'                  1.37 %       ±8.86% ±11.67% ±14.98%
misc/startup-core.js n=30 mode='worker' script='test/fixtures/semicolon.js'                              4.08 %       ±9.20% ±12.13% ±15.56%
misc/startup-core.js n=30 mode='worker' script='test/fixtures/snapshot/typescript.js'                   -0.84 %       ±8.49% ±11.19% ±14.36%

Be aware that when doing many comparisons the risk of a false-positive
result increases. In this case, there are 10 comparisons, you can thus
expect the following amount of false-positive results:
  0.50 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.10 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)

[!WARNING]
Do not take GHA benchmark results as face value, always confirm them
using a dedicated machine, e.g. Jenkins CI.

mcollina

lgtm

nodejs-github-bot · 2026-08-17T18:57:39Z

CI: https://ci.nodejs.org/job/node-test-pull-request/75945/

nodejs-github-bot · 2026-08-18T08:13:39Z

CI: https://ci.nodejs.org/job/node-test-pull-request/75963/

aduh95 · 2026-08-18T09:13:31Z

The modified test is failing on Windows, indicating this PR is adding flakiness. Can you have a look?

---
duration_ms: 293.997
exitcode: 1
severity: fail
stack: |-
  node:internal/assert/utils:146
    throw error;
    ^

  AssertionError [ERR_ASSERTION]: These builtins are now no longer loaded before pre-execution.
  If this is intentional, remove them from `expected.beforePreExec`.

  --- These could be removed from expected.beforePreExec ---
  [
    'Internal Binding cjs_lexer',
    'Internal Binding encoding_binding',
    'NativeModule internal/blob',
    'NativeModule internal/data_url',
    'NativeModule internal/dns/utils',
    'NativeModule internal/encoding',
    'NativeModule internal/encoding/single-byte',
    'NativeModule internal/encoding/util',
    'NativeModule internal/mime',
    'NativeModule internal/modules/esm/assert',
    'NativeModule internal/modules/esm/get_format',
    'NativeModule internal/modules/esm/load',
    'NativeModule internal/modules/esm/loader',
    'NativeModule internal/modules/esm/module_job',
    'NativeModule internal/modules/esm/module_map',
    'NativeModule internal/modules/esm/resolve',
    'NativeModule internal/modules/esm/translators',
    'NativeModule internal/modules/typescript',
    'NativeModule internal/net',
    'NativeModule url'
  ]

  These builtins are now unexpectedly loaded at run time.
  If this is intentional, add them to `expected.atRunTime`.

  # Note: loading more builtins at run time can lead to startup performance regression.
  - Consider lazy loading builtins that are not used universally.

  --- These could be added to expected.atRunTime ---
  [
    'NativeModule internal/modules/esm/get_format'
  ]


  12 !== 0

      at Object.<anonymous> (d:\workspace\node-test-binary-windows-js-suites\node\test\parallel\test-bootstrap-modules.js:264:8)
      at Module._compile (node:internal/modules/cjs/loader:1937:14)
      at Module._extensions..js (node:internal/modules/cjs/loader:2077:10)
      at Module.load (node:internal/modules/cjs/loader:1659:32)
      at Module._load (node:internal/modules/cjs/loader:1451:12)
      at wrapModuleLoad (node:internal/modules/cjs/loader:261:19)
      at Module.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:154:5)
      at node:internal/main/run_main_module:33:47 {
    generatedMessage: false,
    code: 'ERR_ASSERTION',
    actual: 12,
    expected: 0,
    operator: 'strictEqual',
    diff: 'simple'
  }

  Node.js v27.0.0-pre
...

https://ci.nodejs.org/job/node-test-binary-windows-js-suites/42385/RUN_SUBSET=2,nodes=win11-arm64-COMPILED_BY-vs2022_clang-arm64/testReport/junit/(root)/parallel/test_bootstrap_modules/

Contexts that are not deserialized from the built-in snapshot -- worker threads, and the main context of embedders that create their own isolate or of `node --no-node-snapshot` -- compile (with the code cache at best) every builtin the bootstrap touches, so each eagerly required builtin is startup time (~0.15-0.4 ms apiece). A number of them are only required eagerly so that they end up in the snapshot, or for features the bootstrap path never uses. Load lazily what those paths do not need: - is_main_thread.js: preload util, url, the ESM loader (translators, resolver, module_job/map, source maps, node:module, vm modules, mime, data_url, the TypeScript stripper), internal/blob and internal/dns/utils only while building a snapshot; they load on first use otherwise. - fs: internal/blob (+ internal/encoding and its tables) is only used by fs.openAsBlob(). - internal/url: internal/data_url (+ internal/mime) is only used by the Buffer-returning file URL helpers. - internal/process/execution, the CommonJS loader, esm/translators and esm/load: the TypeScript stripper and data: URL helpers are only needed for TypeScript sources / data: URLs. - pre_execution: internal/dns/utils (+ internal/net) is only needed up front to validate an explicit --dns-result-order or to register the resolver's snapshot serializer; the default order becomes the variable's initializer. - internal/worker: event_loop_utilization and error_serdes are only needed once a sub-worker's ELU is read or it reports an error. - worker_threads: `locks` is defined lazily, like util's lazy exports. Main-thread startup with the snapshot is unchanged (the same modules are preloaded into it; the bootstrap-modules test lists are adjusted). A bare worker compiles 95 -> 83 builtins (cold start -5%); without the snapshot an empty CommonJS entry point compiles 76 -> 59 builtins and an empty ES module entry point 76 -> 69. Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>

codebytere · 2026-08-18T09:21:15Z

@aduh95 fixed!

aduh95 · 2026-08-18T09:33:51Z

+  expected.beforePreExec.delete('NativeModule internal/modules/esm/get_format');
+  expected.atRunTime.add('NativeModule internal/modules/esm/get_format');


nit: can we put this behind a variable so it's easier to read?

Suggested change

expected.beforePreExec.delete('NativeModule internal/modules/esm/get_format');

expected.atRunTime.add('NativeModule internal/modules/esm/get_format');

expected.beforePreExec.delete(getFormatNativeModule);

expected.atRunTime.add(getFormatNativeModule);

nodejs-github-bot added lib / src Issues and PRs related to general changes in the lib or src directory. needs-ci PRs that need a full CI run. labels Aug 16, 2026

codebytere requested review from jasnell and joyeecheung August 16, 2026 16:44

codebytere added request-ci Add this label to start a Jenkins CI on a PR. and removed needs-ci PRs that need a full CI run. labels Aug 16, 2026

jasnell approved these changes Aug 17, 2026

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github-actions Bot removed the request-ci Add this label to start a Jenkins CI on a PR. label Aug 17, 2026

codebytere force-pushed the perf/bootstrap-fewer-builtins-without-snapshot branch from 5d49228 to 088f769 Compare August 17, 2026 08:01

targos added the needs-benchmark-ci PR that need a benchmark CI run. label Aug 17, 2026

aduh95 approved these changes Aug 17, 2026

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mcollina approved these changes Aug 17, 2026

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codebytere added request-ci Add this label to start a Jenkins CI on a PR. and removed needs-benchmark-ci PR that need a benchmark CI run. labels Aug 17, 2026

github-actions Bot removed the request-ci Add this label to start a Jenkins CI on a PR. label Aug 17, 2026

codebytere force-pushed the perf/bootstrap-fewer-builtins-without-snapshot branch from 088f769 to 0d3b8a7 Compare August 18, 2026 09:18

nodejs deleted a comment from codebytere-ant Aug 18, 2026

aduh95 reviewed Aug 18, 2026

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lib: load fewer builtins when bootstrapping without a snapshot - #65329

lib: load fewer builtins when bootstrapping without a snapshot#65329
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		expected.beforePreExec.delete('NativeModule internal/modules/esm/get_format');
		expected.atRunTime.add('NativeModule internal/modules/esm/get_format');

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