add warning about default test order causing problems with MPI - #3192
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kbthomp1 wants to merge 2 commits into
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add warning about default test order causing problems with MPI#3192kbthomp1 wants to merge 2 commits into
kbthomp1 wants to merge 2 commits into
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## devel #3192 +/- ##
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- Coverage 91.27% 91.16% -0.11%
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Files 204 206 +2
Lines 8973 9016 +43
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+ Hits 8190 8219 +29
- Misses 783 797 +14 🚀 New features to boost your workflow:
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horenmar
reviewed
Aug 24, 2026
| > [!WARNING] | ||
| > You should not use this option when launching Catch2 tests on multiples processes that require synchronous communication, | ||
| > (e.g., Message Passing Interface (MPI) code), | ||
| > because the test order of execution will **not** be indentical on all processes. |
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| > because the test order of execution will **not** be indentical on all processes. | |
| > because the test order of execution will **not** be identical on all processes. |
horenmar
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Aug 24, 2026
horenmar
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horenmar
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Warning is fine, but the overall text is wrong; you do want the option with MPI and friends, but using a fixed seed... or use the --order option.
Also I suggest implementing a stronger warning than just in docs, similar to here: 0aeb818
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Summary
This adds a warning in the documentation about the default (random) test order of execution possibly causing issues for MPI programs using Catch2 after version 3.9.1.
We recently update Catch2 from version 3.1.0 to the 3.15.1, and we were surprised that our tests started encountering MPI deadlock due the default test execution being changed to random. The fix is simple (use
--lexor another non-default ordering option), but a google search was unhelpful. I hope that including this warning in the documentation helps improve the visibility of the fix.