[ci] Pin build shard weights per run to keep worker assignments identical (#37373)
The build workers can restore different weights if they don't restore the cache at the exact same time. The more time difference, the more likely they restore different weights which could lead to some bundles not being built at all (e.g. https://github.com/react/react/actions/runs/32822851267). A new job now restores the latest entry once per run and republishes it as a per-run artifact. The new job sits adds no wall time because it runs in parallel with `runtime_compiler_node_modules_cache`, which already gates the build workers and takes about 30 seconds on a cache hit, while the resolve job does strictly less work (no checkout, no Node setup, a 5KB cache entry instead of the node_modules restore), so it finishes first and the build workers start at the same time as before. Co-authored-by: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>
Sebastian "Sebbie" Silbermann committed
Aug 26, 2026 at 07:58 UTC
373d35eee53e3bcb0be4b31eea70f3b67ce5e4c1
1 file changed
+48
-18
.github/workflows/runtime_build_and_test.yml
+48
-18
@@ -82,6 +82,38 @@ jobs:
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- run: yarn --cwd compiler install --frozen-lockfile
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if: steps.node_modules.outputs.cache-hit != 'true'
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+ # ----- BUILD WEIGHTS -----
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+ # Pins one weights snapshot for the whole run. A cache restore with
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+ # restore-keys returns the newest entry at call time, so 50 workers
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+ # restoring individually can disagree when another run saves a new entry
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+ # mid-fan-out, which scrambles the shard assignment and drops bundles from
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+ # the build.
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+ resolve_build_weights:
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+ name: Resolve build shard weights
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+ runs-on: ubuntu-latest
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+ outputs:
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+ available: ${{ steps.available.outputs.available }}
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+ steps:
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+ - name: Restore latest build shard weights
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+ id: weights
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+ uses: actions/cache/restore@v4
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+ with:
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+ # Must match the save step's path in process_artifacts_combined
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+ # exactly: the cache version is a hash of the path, so a different
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+ # path never matches the key.
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+ path: build-weights.json
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+ key: build-weights-v1-${{ github.run_id }}
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+ restore-keys: build-weights-v1-
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+ - id: available
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+ run: echo "available=${{ steps.weights.outputs.cache-matched-key != '' }}" >> "$GITHUB_OUTPUT"
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+ - name: Publish build shard weights for this run
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+ if: steps.available.outputs.available == 'true'
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+ uses: actions/upload-artifact@v4
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+ with:
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+ name: build-weights
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+ path: build-weights.json
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+ if-no-files-found: error
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+
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# ----- FLOW -----
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discover_flow_inline_configs:
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name: Discover flow inline configs
@@ -269,7 +301,7 @@ jobs:
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# ----- BUILD -----
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build_and_lint:
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name: yarn build and lint
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- needs: [runtime_compiler_node_modules_cache]
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+ needs: [runtime_compiler_node_modules_cache, resolve_build_weights]
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runs-on: ubuntu-latest
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strategy:
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fail-fast: false
@@ -307,15 +339,14 @@ jobs:
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if: steps.node_modules.outputs.cache-hit != 'true'
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- run: yarn --cwd compiler install --frozen-lockfile
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if: steps.node_modules.outputs.cache-hit != 'true'
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- - name: Restore build shard weights
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- uses: actions/cache/restore@v4
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+ # Pinned once per run by resolve_build_weights so every worker computes
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+ # the same shard assignment. Absent on a cold start; the build then
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+ # falls back to round-robin sharding.
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+ - name: Download build shard weights
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+ if: needs.resolve_build_weights.outputs.available == 'true'
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+ uses: actions/download-artifact@v4
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with:
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- # Written by process_artifacts_combined on every push. The
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- # restore-keys prefix picks up the most recent entry. On a miss the
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- # build falls back to round-robin sharding.
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- path: build-weights.json
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- key: build-weights-v1-${{ github.run_id }}
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- restore-keys: build-weights-v1-
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+ name: build-weights
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# setup-java's exports (e.g. JAVA_HOME) only apply to steps after this wait.
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- name: Wait for Java setup
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wait: setup_java
@@ -469,7 +500,7 @@ jobs:
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process_artifacts_combined:
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name: Process artifacts combined
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- needs: [build_and_lint, runtime_node_modules_cache]
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+ needs: [build_and_lint, runtime_node_modules_cache, resolve_build_weights]
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permissions:
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# https://github.com/actions/attest-build-provenance
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id-token: write
@@ -505,15 +536,14 @@ jobs:
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- name: Wait for archived build
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wait: download_build
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# Only used to log weight variance; the new measurement is what gets
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- # saved, so removed bundles drop out instead of accumulating.
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- - name: Restore previous build shard weights
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- uses: actions/cache/restore@v4
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+ # saved, so removed bundles drop out instead of accumulating. Downloads
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+ # the run's pinned snapshot so the diff compares against the weights
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+ # that actually drove this run's assignment.
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+ - name: Download previous build shard weights
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+ if: needs.resolve_build_weights.outputs.available == 'true'
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+ uses: actions/download-artifact@v4
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with:
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- # Must match the save step's path exactly: the cache version is a
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- # hash of the path, so a different path never matches the key.
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- path: build-weights.json
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- key: build-weights-v1-${{ github.run_id }}
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- restore-keys: build-weights-v1-
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+ name: build-weights
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- name: Update build shard weights
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run: node scripts/ci/merge-build-weights.js
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- name: Save build shard weights