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nodejs/tools/snapshot/README.md
Joyee Cheung 30e8b5e2a2
tools: refactor snapshot builder 
This patch:

- Moves the snapshot building code to src/ so that we can reuse it
  later when generating custom snapshots from an entry point accepted
  by the node binary.
- Create a SnapshotData struct that incorporates all the data useful
  for a snapshot blob, including both the V8 data and the Node.js
  data.

PR-URL: https://github.com/nodejs/node/pull/38902
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
2021-06-10 14:17:49 +08:00

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Node.js startup snapshot builder

This is the V8 startup snapshot builder of Node.js. Not to be confused with V8's own snapshot builder, which builds a snapshot containing JavaScript builtins, this builds a snapshot containing Node.js builtins that can be deserialized on top of V8's own startup snapshot. When Node.js is launched, instead of executing code to bootstrap, it can deserialize the context from an embedded snapshot, which readily contains the result of the bootstrap, so that Node.js can start up faster.

Currently only the main context of the main Node.js instance supports snapshot deserialization, and the snapshot does not yet cover the entire bootstrap process. Work is being done to expand the support.

How it's built and used

The snapshot builder is built with the node_mksnapshot target in node.gyp when node_use_node_snapshot is set to true, which is currently done by default.

In the default build of the Node.js executable, to embed a V8 startup snapshot into the Node.js executable, libnode is first built with these unresolved symbols:

  • node::NodeMainInstance::GetEmbeddedSnapshotBlob
  • node::NodeMainInstance::GetIsolateDataIndices

Then the node_mksnapshot executable is built with C++ files in this directory, as well as src/node_snapshot_stub.cc which defines the unresolved symbols.

node_mksnapshot is run to generate a C++ file <(SHARED_INTERMEDIATE_DIR)/node_snapshot.cc that is similar to src/node_snapshot_stub.cc in structure, but contains the snapshot data written as static char array literals. Then libnode is built with node_snapshot.cc to produce the final Node.js executable with the snapshot data embedded.

For debugging, Node.js can be built without Node.js's own snapshot if --without-node-snapshot is passed to configure. A Node.js executable with Node.js snapshot embedded can also be launched without deserializing from it if the command line argument --no-node-snapshot is passed.