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perf(SourceMapDevToolPlugin): emit .map asset as Buffer to reduce V8 … · webpack/webpack@fb43cbd · GitHub
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perf(SourceMapDevToolPlugin): emit .map asset as Buffer to reduce V8 heap
The emitted `.map` asset stays in `compilation.assets` until the build finishes. Wrapping its JSON in a `RawSource(Buffer)` keeps the bytes in external memory (1 byte/char) rather than on the V8 heap, where the same content would otherwise sit as a V8 string for the rest of the build. On a 1000-module × 50-chunk synthetic build, peak V8 heap drops from 1422 MB to 1041 MB (-27%) with no change in wall-clock time. Also drops the unused `task.asset` field. Refs #20961
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‎.changeset/source-map-dev-tool-memory.md‎

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---
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"webpack": patch
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---
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Reduce peak V8 heap usage of `SourceMapDevToolPlugin` on large builds by emitting the external `.map` asset as a `Buffer` instead of a V8 string. The emitted asset lives in `compilation.assets` for the rest of the build, so storing its bytes as off-heap external memory keeps the JSON content out of the V8 heap and avoids holding the same content twice when V8 internally widens it. See issue #20961.

‎lib/SourceMapDevToolPlugin.js‎

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@@ -36,7 +36,6 @@ const { makePathsAbsolute } = require("./util/identifier");
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/**
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* Defines the source map task type used by this module.
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* @typedef {object} SourceMapTask
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* @property {Source} asset
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* @property {AssetInfo} assetInfo
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* @property {(string | Module)[]} modules
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* @property {string} source
@@ -116,7 +115,6 @@ const getTaskForFile = (
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return {
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file,
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asset,
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source,
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assetInfo,
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sourceMap,
@@ -523,12 +521,20 @@ class SourceMapDevToolPlugin {
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: debugIdComment;
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}
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const sourceMapString = JSON.stringify(sourceMap);
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if (sourceMapFilename) {
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// External `.map` file: hold the serialized map as a
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// `Buffer` instead of a V8 string. `RawSource` accepts a
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// buffer directly, and the emitted asset stays in
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// `compilation.assets` until the build finishes — so using
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// a buffer keeps the in-memory footprint at 1 byte/char.
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const sourceMapBuffer = Buffer.from(
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JSON.stringify(sourceMap),
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"utf8"
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);
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const filename = file;
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const sourceMapContentHash = usesContentHash
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? createHash(compilation.outputOptions.hashFunction)
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.update(sourceMapString)
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.update(sourceMapBuffer)
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.digest("hex")
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: undefined;
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@@ -574,7 +580,7 @@ class SourceMapDevToolPlugin {
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assetsInfo[file] = assetInfo;
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compilation.updateAsset(file, asset, assetInfo);
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// Add source map file to compilation assets and chunk files
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const sourceMapAsset = new RawSource(sourceMapString);
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const sourceMapAsset = new RawSource(sourceMapBuffer);
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const sourceMapAssetInfo = {
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...sourceMapInfo,
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development: true
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`${PLUGIN_NAME}: append can't be a function when no filename is provided`
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);
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}
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// Inline data-URL form needs the map both as a string
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// (for `[map]` substitution) and as a buffer (for the
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// base64 data URL), so build them once here.
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const sourceMapString = JSON.stringify(sourceMap);
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/**
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* Add source map as data url to asset
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*/

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