What Is Test Parallelism?
Test parallelism is the execution of multiple tests concurrently rather than one after another, either across threads and processes on a single machine or across many machines. It reduces wall-clock time for large suites. Effective parallelism requires tests to be independent and isolated so they do not interfere with each other.
Why it matters
Serial tests waste idle CPU and runner time; parallelism reclaims it. The catch is that tests must not share mutable state, or parallel runs become flaky. Clean, isolated environments and proper fixtures are what let parallelism scale safely.
Related concepts
- Test sharding is parallelism across machines
- Requires independent, isolated tests
- Limited by available runner concurrency
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