Add an upper bound on the number of test cases we run in parallel. #1390
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This PR adds an upper bound, `NCORES * 2`, on the number of test cases we run in parallel. Depending on the exact nature of your tests, this can significantly reduce the maximum amount of dirty memory needed, but does not generally impact execution time. You can configure a different upper bound by setting the `"SWT_EXPERIMENTAL_MAXIMUM_PARALLELIZATION_WIDTH"` environment variable (look, naming is hard okay?)
grynspan
requested review from
briancroom,
jerryjrchen,
stmontgomery and
suzannaratcliff
as code owners
grynspan
added
enhancement
stmontgomery
reviewed
Nov 7, 2025
stmontgomery
reviewed
Nov 7, 2025
stmontgomery
reviewed
Nov 7, 2025
jerryjrchen
reviewed
Nov 7, 2025
| private var _continuations = [CheckedContinuation<Void, Never>]() | ||
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| init(maximumWidth: Int = 1) { | ||
| self.maximumWidth = maximumWidth |
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Should we precondition on maximumWidth > 0 here? I think run blocks forever if the user sets a width of zero, so it doesn't really make sense to allow.
jerryjrchen
approved these changes
Nov 7, 2025
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Overall looks good. The only concern I have is over the naming for Serializer
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This PR adds an upper bound,
NCORES * 2, on the number of test cases we run in parallel. Depending on the exact nature of your tests, this can significantly reduce the maximum amount of dirty memory needed, but does not generally impact execution time.As an example, Swift Testing's own tests go from > 300MB max memory usage to around 60MB.
You can configure a different upper bound by setting the
"SWT_EXPERIMENTAL_MAXIMUM_PARALLELIZATION_WIDTH"environment variable (look, naming is hard okay?)Checklist: