This document summarizes the bootstrap process for Node.js core. It describes how Node.js initializes V8, parses command line arguments, loads built-in modules and the main script, sets up global objects and environments, and eventually runs the event loop. It also discusses ongoing work to optimize startup time through refactoring and integrating with V8's snapshot feature to deserialize context objects instead of re-executing initialization code.

1. Bootstrap of Node.js Core
Joyee Cheung
Slides: https://github.com/nodejs/summit/issues/147
OpenJS collaboration summit, May 2019

2. A Node.js Process
Main Instance (JS thread)
Worker (if any)
(JS thread)
Task Scheduler
V8 Platform
thread pool
Inspector I/O
libuv
thread pool
Environment
Main V8
Context
VM V8
Context
V8 Isolate
Event
loop

3. Bootstrap (2019.05)
node::Start()

4. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，OpenSSL, ICU, signal handler…
node::Start()

5. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，OpenSSL, ICU, signal handler…
node::Start()
NodeMainInstance() / Worker()

6. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，OpenSSL, ICU, signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
JS heap, JS exceptions,
Microtask queue...

7. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，OpenSSL, ICU, signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
JS heap, JS exceptions,
Microtask queue...
global proxy, JS builtins

8. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，OpenSSL, ICU, signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins

9. Primordials
u JavaScript builtins like Object, Object.prototype are cloned onto
an object and frozen for internal use
u Users can delete Function.prototype.call
u WIP to transition all internal usage of these

10. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
node::Environment
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins
Stuff that does not
have a better place to
go

11. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
node::Environment
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins
Stuff that does not
have a better place to
go
libuv handles

12. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
node::Environment
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins
Stuff that does not
have a better place to
go
libuv handles
inspector agent

13. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
node::Environment
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins
Stuff that does not
have a better place to
go
bootstrap/*.js
global, process, task
queues, ESM/CJS
loaders ...
libuv handles
inspector agent

14. lib/internal/bootstrap/loaders.js
u Internal module loaders
u C++ binding loaders
u process.binding()
u process._linkedBinding()
u internalBinding()
u require() for loading other internal JavaScript modules

15. Built-in Modules (Native Modules)
lib/*.js
tools/js2c.py
NativeModuleLoader::LoadJavaScriptSource()
“use strict”;
...
static const uint16_t timers_raw[] = {
39,117,115,101...
};
JavaScript code
static data array
containing the source

16. Built-in Modules (Native Modules)
lib/*.js
tools/mkcodecache
NativeModuleEnv::InitializeCodeCache()
“use strict”;
...
static const uint8_t assert[] = {
165,3,222,192,132, ...
};
JavaScript code
static data array
containing the code cache

17. Built-in Modules (Native Modules)
Compiled with a special wrapper
that include access to more internals
function (exports, require, module, process,
internalBinding, primordials) {
require(‘internal/fs/utils’);
module.exports = {…};
}

18. lib/internal/bootstrap/node.js
u Set up most stuff on process and global
u C++ passes isMainThread, ownsProcessState into the script
u false for workers, true for the main thread

19. lib/internal/bootstrap/node.js
u Set up most stuff on process and global
u C++ passes isMainThread, ownsProcessState into the script
u false for workers, true for the main thread
u Set up JavaScript callbacks that will be added as v8::Persistent to the
Environment
u Async hook callbacks
u Timers & process.nextTick() schedulers
u Must not run async operations (not snapshottable)
u Must not depend on any CLI arguments or environment variables

20. lib/internal/bootstrap/pre_execution.js
u Not actively run. Required by main scripts (explained later)
u Bootstrap things that depend on CLI arguments and environment
variables
u Not included in the snapshot

21. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
node::Environment
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins
Stuff that does not
have a better place to
go
bootstrap/*.js
main/?.js e.g.
run_main_module.js
global, process, task
queues, ESM/CJS
loaders ...
libuv handles
inspector agent

22. Main scripts
u lib/internal/main/*.js
u Main thread
uStartMainThreadExecution()
uSelect a script based on CLI arguments, etc.
u Worker threads
uworker_thread.js
u Runs lib/internal/bootstrap/pre_execution.js
first to bootstrap the parts that depend on run time states

23. Main scripts
u check_syntax.js: node –c test.js
u eval_stdin.js: cat test.js | node –e
u eval_string.js: node –e ‘1’
u inspect.js: node inspect ...
u print_bash_completion.js: node --completion-bash
u print_help.js: node --help
u prof_process.js: node --prof-process v8.log

24. Main scripts
u run_third_party_main.js
u Run lib/_third_party_main.js embedders
u environment.js
u For C++ test fixtures
Requested
u bundled cli tool entry point?
u better entry point for embedders?

25. Main scripts
u repl.js: node
u worker_thread.js: for workers
u run_main_module.js
u node index.js
u node --experimental-modules index.mjs

26. Main scripts
u repl.js: node
u run_main_module.js
u node index.js
u node --experimental-modules index.mjs
u worker_thread.js: for workers

27. lib/internal/bootstrap/pre_execution.js
u Bootstrap that depend on run time states
ue.g. CLI arguments, environment variables
uIncluding CJS & ESM loader initilization
if (!getOptionValue('--no-warnings') &&
process.env.NODE_NO_WARNINGS !== '1') {
process.on('warning', onWarning);
}

28. User land CommonJS Modules
function (exports, require, module, __filename, __dirname) {
require(‘fs’);
}
u Loader implemented in lib/internal/modules/cjs/
Wrap user code with objects initialized by Node.js

29. User land ECMAScript Modules
u Loader implementation in lib/internal/modules/esm/
u Does not mess with the context except things added to the
global proxy
uBuffer, process, etc.

30. User land ECMAScript Modules
u An internal WeakMap holding ModuleWrap -> Options
uOptions includes dynamic import() callback and
import.meta data
uPer-isolate
uHostImportModuleDynamicallyCallback
uHostInitializeImportMetaObjectCallback

31. Bootstrap (2019.05)
InitializeOncePerProcess()
Parse the CLI arguments, Initialize the V8
Platform，signal handler…
node::Start()
NodeMainInstance() / Worker()
v8::Isolate
v8::Context
node::Environment
do {
uv_run(...)
} while (...)
Event Loop
JS heap, JS exceptions,
Microtask queue...
per_context/*.js Node.js primordials
global proxy, JS builtins
Stuff that does not
have a better place to
go
bootstrap/*.js
main/?.js e.g.
run_main_module.js
global, process, task
queues, ESM/CJS
loaders ...
libuv handles
inspector agent

32. Snapshot Integration
out/Release/node --cpu-prof-interval=100 --cpu-prof -e "{}"

33. Snapshot Integration
out/Release/node --cpu-prof-interval=100 --cpu-prof -e "{}"
Can be snapshotted Depend on run time states

34. Snapshot Integration
d8 with default snapshot
node master without snapshot

35. Snapshot Integration v8::Isolate
v8::Context
node::Environment
per_context/*.js
bootstrap/*.js
main/?.js
loop & inspector
Original
SetIsolateUpForNode()

36. Snapshot Integration v8::Isolate
Context::FromSnapshot()
node::Environment
bootstrap/*.js
main/?.js
loop & inspector
SetIsolateUpForNode()
Snapshotted (2019.05)
Re-install callbacks

37. Snapshot Integration
Environment:: FromSnapshot()
Goal
Deserialize from snapshot instead of
executing per_context/*.js &
bootstrap/*.js
v8::Isolate
Context::FromSnapshot()
SetIsolateUpForNode()
Re-install callbacks
main/?.js
loop & inspector

38. Snapshot Integration
Environment:: FromSnapshot()
The bootstrap process must be
independent of run time states before
the snapshot is captured.
v8::Isolate
Context::FromSnapshot()
SetIsolateUpForNode()
Re-install callbacks
main/?.js
loop & inspector
Refactoring

39. Snapshot Integration
Used to be spaghetti mixed together

40. Snapshot Integration
Environment:: FromSnapshot()
v8::Isolate
Context::FromSnapshot()
SetIsolateUpForNode()
Re-install callbacks
main/?.js
loop & inspector
Refactoring
if (!getOptionValue('--no-warnings') &&
process.env.NODE_NO_WARNINGS !== '1') {
process.on('warning', onWarning);
}
lib/internal/bootstrap/pre_execution.js

41. Snapshot Integration
Environment:: FromSnapshot()
v8::Isolate
Context::FromSnapshot()
SetIsolateUpForNode()
Re-install callbacks
main/?.js
loop & inspector
Refactoring
Reorganize so that we can reinstall C++
states

42. Current state
u v12.3.1 v.s. v10.16.0
u ~60% faster child process startup
u ~120% faster worker startup
u Some refactoring has also been backported to v10 so the
actual speed up is higher

43. Current state
u v12.3.1 v.s. v10.16.0
u ~60% faster child process startup
u ~120% faster worker startup
u Some refactoring has also been backported to v10 so the
actual speed up is higher
u Mostly from lazy-loading and embedded code cache
u Further speedup anticipated from snapshot integration
u 4x in https://github.com/nodejs/node/issues/17058

44. Challenges
u Lack of reviews
u https://github.com/nodejs/node/pull/27539 27 days without
reviews
u Incrementally refactoring the spaghetti code + 7-day wait = slow

45. Challenges
u Lack of reviews
u https://github.com/nodejs/node/pull/27539 27 days without
reviews
u Incrementally refactoring the spaghetti code + 7-day wait = slow
u Fixed hash seed
u Rehashing maps & sets
u https://bugs.chromium.org/p/v8/issues/detail?id=9187
u Snapshot is currently still disabled on master behind a build time
flag

46. Future plans
u Finish integration before v12 LTS
u Explore user-land snapshot builder & loader

47. Thank you