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

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Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56529

Fantom runs every test through a single shared Metro server (started in
`globalSetup`). Jest's default `maxWorkers` is `numCpus - 1`, so on a
high-core box (e.g. 176 CPUs) ~175 workers fire bundle requests at Metro
concurrently. Each in-flight request makes Metro materialize a full
dependency `Graph` (transformed modules, source maps, inverse-deps,
file-watcher subscription), which is hundreds of MB.

The per-test `DELETE` eviction added in D101652820 only releases that
memory after the bundle response completes, so the simultaneous in-flight
set still blows past the previous Node `--max-old-space-size=8192` ceiling
in `scripts/fantom.sh` — the Metro process aborts with
`FATAL ERROR: Ineffective mark-compacts near heap limit` after just a
handful of test suites.

Two coordinated changes that balance throughput and safety:

- `scripts/fantom.sh`: bump the Node heap from 8 GB to 16 GB so we have
  headroom over the observed steady-state peak.
- `private/react-native-fantom/config/jest.config.js`: cap `maxWorkers`
  at `min(numCpus - 1, 16)`. With 8 workers the heap peaked at ~3 GB
  (~400 MB / worker), so 16 workers fits comfortably under a 16 GB cap
  with ~40% headroom.

This is intentionally a balance rather than a hard worker cap — bumping
the heap alone would still leave 100+ in-flight graphs racing GC, and
capping workers alone leaves throughput on the table on big machines.

Changelog: [Internal]

Reviewed By: andrewdacenko

Differential Revision: D101791795

fbshipit-source-id: 035c7235c32303f7b7f1eb05698b2a5cba90edc9
This commit is contained in:
Rubén Norte
2026-04-21 06:01:11 -07:00
committed by meta-codesync[bot]
parent b1a8913bb2
commit 0f9b8e4a62
2 changed files with 29 additions and 1 deletions
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scripts/fantom.sh
+7 -1
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@@ -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=()