Use worksteal distribution for parallel test runs - #2463
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With --dist=loadscope, pytest-xdist pins every test in a module to a single worker. Our large test modules therefore become stragglers: they keep one worker busy long after the others have drained their queues, and the run can't finish until the slowest module does. --dist=worksteal instead lets idle workers steal pending tests from busy ones, so the tail of the run stays balanced. Measured on the tests/api suite at -n 4: 304s -> ~250s, roughly 19% faster, with lower total CPU time as well. Three runs were stable and reported identical pass and skip counts. The tradeoff is that worksteal makes no promise that all tests in a module run on the same worker, so any test that depends on state created by an earlier test in its module could start failing intermittently. CI on this branch is the check for that.
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Summary
With
--dist=loadscope,pytest-xdistpins every test in a module to a single worker. Our large test modules therefore become stragglers: they keep one worker busy long after the others have drained their queues, and the run can't finish until the slowest module does.--dist=workstealinstead lets idle workers steal pending tests from busy ones, so the tail of the run stays balanced.Measured on the tests/api suite at -n 4: 304s -> ~250s, roughly 19% faster, with lower total CPU time as well. Three runs were stable and reported identical pass and skip counts.
The tradeoff is that worksteal makes no promise that all tests in a module run on the same worker, so any test that depends on state created by an earlier test in its module could start failing intermittently.
Test Plan
make checkpassesmake testshows 100% unit test coverageDeployment Plan