Cap maxWorkers and bump heap to bound Fantom Metro memory (#56529)

private/react-native-fantom/config/jest.config.js

@@ -11,10 +11,32 @@
 'use strict';
 
 const baseConfig = require('../../../jest.config');
+const os = require('os');
 const path = require('path');
 
+// Every Fantom worker shares a single Metro server. With unbounded
+// parallelism, concurrent bundle builds pile up faster than the
+// per-test DELETE eviction can free their dependency graphs. Cap
+// concurrency so the number of in-flight graphs stays bounded on
+// high-core machines, while still using all available CPUs on smaller
+// ones. The cap pairs with the Node heap size set in scripts/fantom.sh
+// (16 GB) to leave comfortable headroom over the observed peak.
+function getNumCpus() /*: number */ {
+  if (typeof os.availableParallelism === 'function') {
+    return os.availableParallelism();
+  }
+  const cpus = os.cpus();
+  return cpus != null ? cpus.length : 1;
+}
+
+const FANTOM_MAX_WORKERS /*: number */ = Math.max(
+  1,
+  Math.min(getNumCpus() - 1, 16),
+);
+
 module.exports = {
   displayName: 'fantom',
+  maxWorkers: FANTOM_MAX_WORKERS,
   rootDir: path.resolve(__dirname, '../../..') /*:: as string */,
   roots: [
     '<rootDir>/packages/react-native',

scripts/fantom.sh

@@ -16,7 +16,13 @@ else
   export FANTOM_FORCE_OSS_BUILD=1
 fi
 
-export NODE_OPTIONS='--max-old-space-size=8192'
+# The Fantom Jest process hosts a single shared Metro server. Each test
+# triggers a fresh Metro bundle build whose dependency graph (transformed
+# modules, source maps, inverse-deps, file-watcher subscription) lives in
+# memory until the per-test DELETE evicts it. With the maxWorkers cap in
+# jest.config.js (~16 in-flight bundles at peak), 16 GB gives ~40% heap
+# headroom over the observed steady state of ~6 GB.
+export NODE_OPTIONS='--max-old-space-size=16384'
 
 # Parse arguments to extract custom flags
 ARGS=()