ci: increase unit test partitions from 8 to 12#5
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Add a Python script that reads JUnit XML timing data from previous CI runs and uses greedy bin-packing to distribute tests into time-balanced partitions. Falls back to hash-based partitioning when no timing data is available (first run, or data expired). New files: .github/scripts/split-tests-by-timing.py - bin-packing script .config/nextest.toml - enables JUnit XML output for CI profile Each partition uploads its JUnit XML as an artifact (90-day retention). On subsequent runs, all partitions download this timing data and the script assigns tests to minimize the slowest partition duration. Uses --workspace-remap and --profile ci to ensure JUnit output is written correctly when running from a nextest archive. POC: inlines the nextest command to test this approach. Production implementation should integrate with stacks-network/actions.
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What
Increase the number of parallel unit test partitions from 8 to 12 to reduce the worst-case partition execution time.
Why
Nextest's
--partition count:N/8distributes tests by position in the test list, not by execution time. This creates severe imbalance:The pipeline waits for the slowest partition, so this directly impacts wall-clock time. Increasing to 12 partitions distributes the heavy tests across more buckets.
The Change
In
.github/workflows/stacks-core-tests.yml:2 lines changed in 1 file.
Metrics to Track
Security Checklist
Part of CI Optimization Series
PR 4 of 7. Expected impact: ~8-10 min off critical path.