lib: avoid quadratic shift() in startup snapshot callback

Author: watilde

benchmark/v8/startup-snapshot-callbacks.js

@@ -0,0 +1,58 @@
+'use strict';
+
+// Benchmarks startup time when deserialize callbacks are registered in a
+// startup snapshot, measuring real binary execution with --snapshot-blob.
+
+const common = require('../common.js');
+const { spawnSync } = require('child_process');
+const { mkdtempSync, writeFileSync, rmSync } = require('fs');
+const { tmpdir } = require('os');
+const path = require('path');
+
+const bench = common.createBenchmark(main, {
+  count: [10, 100, 1000, 10000],
+  n: [30],
+});
+
+function buildSnapshot(snapshotBlob, fixtureScript, count) {
+  writeFileSync(fixtureScript, `
+'use strict';
+const v8 = require('node:v8');
+for (let i = 0; i < ${count}; i++) {
+  v8.startupSnapshot.addDeserializeCallback(() => {});
+}
+v8.startupSnapshot.setDeserializeMainFunction(() => {});
+`);
+  const result = spawnSync(process.execPath, [
+    '--snapshot-blob', snapshotBlob,
+    '--build-snapshot', fixtureScript,
+  ]);
+  if (result.status !== 0) {
+    throw new Error(`Failed to build snapshot:\n${result.stderr}`);
+  }
+}
+
+function main({ n, count }) {
+  const dir = mkdtempSync(path.join(tmpdir(), 'node-bench-snapshot-'));
+  const snapshotBlob = path.join(dir, 'snapshot.blob');
+  const fixtureScript = path.join(dir, 'build.js');
+
+  try {
+    buildSnapshot(snapshotBlob, fixtureScript, count);
+
+    const warmup = 3;
+    let finished = -warmup;
+
+    while (finished < n) {
+      const child = spawnSync(process.execPath, ['--snapshot-blob', snapshotBlob]);
+      if (child.status !== 0) {
+        throw new Error(`Snapshot run failed:\n${child.stderr}`);
+      }
+      finished++;
+      if (finished === 0) bench.start();
+      if (finished === n) bench.end(n);
+    }
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+}

lib/internal/v8/startup_snapshot.js

@@ -37,10 +37,11 @@ function throwIfBuildingSnapshot(reason) {
 const deserializeCallbacks = [];
 let deserializeCallbackIsSet = false;
 function runDeserializeCallbacks() {
-  while (deserializeCallbacks.length > 0) {
-    const { 0: callback, 1: data } = deserializeCallbacks.shift();
+  for (let i = 0; i < deserializeCallbacks.length; i++) {
+    const { 0: callback, 1: data } = deserializeCallbacks[i];
     callback(data);
   }
+  deserializeCallbacks.length = 0;
 }
 
 function addDeserializeCallback(callback, data) {
@@ -60,14 +61,16 @@ function addDeserializeCallback(callback, data) {
 const serializeCallbacks = [];
 const afterUserSerializeCallbacks = [];  // Callbacks to run after user callbacks
 function runSerializeCallbacks() {
-  while (serializeCallbacks.length > 0) {
-    const { 0: callback, 1: data } = serializeCallbacks.shift();
+  for (let i = 0; i < serializeCallbacks.length; i++) {
+    const { 0: callback, 1: data } = serializeCallbacks[i];
     callback(data);
   }
-  while (afterUserSerializeCallbacks.length > 0) {
-    const { 0: callback, 1: data } = afterUserSerializeCallbacks.shift();
+  serializeCallbacks.length = 0;
+  for (let i = 0; i < afterUserSerializeCallbacks.length; i++) {
+    const { 0: callback, 1: data } = afterUserSerializeCallbacks[i];
     callback(data);
   }
+  afterUserSerializeCallbacks.length = 0;
 }
 
 function addSerializeCallback(callback, data) {