[ci-coach] Rebalance cgo.yml unit-test matrix shards for faster critical path #60846

CI Optimization Proposal

Summary

The cgo.yml unit-test job splits Go tests into 5 alphabetical shards (A-C, D-G, H-M, N-R, S-Z+other) that run in parallel. Real per-test timing data from the current run (/tmp/gh-aw/agent/test-results.json, 10,637 top-level tests) showed the split was badly imbalanced: shard A-C alone took ~64.8s while the other four shards ranged only 23-43s. Since all shards run in parallel, the job's critical path is bounded by the slowest shard — so A-C was the bottleneck every run.

Top Optimization

Rebalance cgo.yml unit-test matrix shards

• Type: Test Suite Restructuring (matrix strategy)
• Impact: Cuts the unit-test job's critical path from ~64.8s (max shard) to ~52.4s — a ~19% reduction in wall-clock time for that job, with no change in total test count or coverage.
• Risk: Low. Only the pattern/name/shard fields of the existing 5-way matrix changed; job structure, concurrency groups, and dependencies are untouched.
• Changes: Applied longest-processing-time (LPT) bin-packing across all 26 letters using real per-letter test durations:
  • C isolated (52.4s — dominated by TestConfig*/TestCompil* tests)
  • A-E-M-O-W (35.8s)
  • B-H-J-K-N-S (35.6s)
  • D-G-L-P-V (35.7s)
  • F-I-Q-R-T-U-X-Y-Z + non-letter test names (35.6s)
• Rationale: The old alphabetical split assumed uniform test-name distribution, but Go test names cluster heavily around common prefixes (TestC... for Config/Compile-related code). LPT bin-packing on measured durations produces near-equal shard times without needing more matrix jobs.

Expected Impact

• Max shard duration: ~64.8s → ~52.4s (~19% faster critical path for the unit-test job)
• No change in job count (still 5 parallel shards), no change in coverage

Validation Results

• ✅ YAML syntax validated (js-yaml)
• ✅ All 5 new regex patterns simulated against all 10,637 real test names: 100% coverage, 0 unmatched, 0 duplicate matches
• ⚠️ make build/make recompile blocked in this sandbox by proxy.golang.org 403 (pre-existing network restriction, unrelated to this YAML-only change — confirmed via pre-flight validation-status.json that build/test-unit already pass on the base commit)

Metrics Baseline

• Source: /tmp/gh-aw/agent/test-results.json (this run's go test -json output, 10,637 top-level tests)
• Old shards: A-C=64.8s, D-G=42.6s, H-M=36.9s, N-R=23.1s, S-Z+other=27.7s
• New shards: C=52.4s, A-E-M-O-W=35.8s, B-H-J-K-N-S=35.6s, D-G-L-P-V=35.7s, F-I-Q-R-T-U-X-Y-Z+other=35.6s

# Download the artifact from the workflow run
gh run download '34848847278' -n agent -D '/tmp/agent-34848847278'

# Resolve the bundle source ref, fetch it into a temporary ref, then create the local branch
bundle_path='/tmp/agent-34848847278/aw-ci-coach-rebalance-cgo-unit-shards-93155.bundle'
temp_ref='refs/bundles/create-pr-ci-coach-rebalance-cgo-unit-shards-93155-efbbce04885d8c34-6d037e4e'
target_ref='refs/heads/ci-coach/rebalance-cgo-unit-shards-93155-efbbce04885d8c34'
bundle_source_ref=$(git bundle list-heads "$bundle_path" | awk '$2 ~ /^refs\/heads\// { print $2 }')
if [ -z "$bundle_source_ref" ]; then
  bundle_source_ref=$(git bundle list-heads "$bundle_path" | awk '$2 == "HEAD" { print $2 }')
fi
if [ "$(printf '%s\n' "$bundle_source_ref" | sed '/^$/d' | wc -l | tr -d ' ')" != "1" ]; then
  echo "Expected exactly one bundle source ref, found: $bundle_source_ref" >&2
  exit 1
fi
git fetch "$bundle_path" "${bundle_source_ref}:${temp_ref}"
git update-ref "$target_ref" "$temp_ref"
git checkout 'ci-coach/rebalance-cgo-unit-shards-93155-efbbce04885d8c34'
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d "$temp_ref"

# Push the branch and create the pull request
git push origin ci-coach/rebalance-cgo-unit-shards-93155-efbbce04885d8c34
gh pr create --title '[ci-coach] Rebalance cgo.yml unit-test matrix shards for faster critical path' --base main --head ci-coach/rebalance-cgo-unit-shards-93155-efbbce04885d8c34 --repo github/gh-aw

network:
  allowed:
    - defaults
    - "proxy.golang.org"

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• expires on Sep 16, 2026, 5:43 AM UTC-08:00