maxParallelThreads was unset everywhere and only 3 of ~20 test projects carried
an xunit.runner.json at all, so xunit ran Environment.ProcessorCount collections
at once (14 on the dev box) in every suite. That is the general load multiplier
behind the row-82/91/95 flake family.
Policy: per-suite caps, NOT a global one. Pure unit suites stay uncapped for
speed; a suite gets a config only when a test CLASS costs a real external or
OS-level resource, so ProcessorCount of them at once is a resource question
rather than a CPU question. Four suites qualify, all capped at
maxParallelThreads: 4.
- ConfigurationDatabase.Tests, AuditLog.Tests, SiteCallAudit.Tests — verified as
the three that actually take MsSqlMigrationFixture (the latter two by project
reference). Every MSSQL-backed test class means a CREATE DATABASE plus the
~100-migration chain against the one shared dev SQL Server. 4 is chosen to
MATCH the default slot count of row 82's cross-process sp_getapplock migrate
semaphore, so the in-process and cross-process limits agree rather than
fighting.
- Host.Tests — classes boot real ASP.NET Core hosts and real ActorSystems;
Akka.Remote binds real sockets, Kestrel binds real ports, gRPC dials them.
Threads are capped, collections are NOT serialized. IntegrationTests
(parallelizeAssembly/parallelizeTestCollections false) and
CentralUI.PlaywrightTests (maxParallelThreads 1) already carry stricter
pre-existing settings and are left alone — the Playwright suite drives a browser
against a live rig, where shared UI state, not CPU, is the constraint.
Each capped csproj gains
<Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />;
the SDK does not copy it automatically and without it the file is silently
ignored. Verified by moving the copied file out of bin/ to take an uncapped
baseline: ConfigurationDatabase.Tests ran at 216 % CPU uncapped vs 145 % capped,
so the cap demonstrably takes effect.
Cost is close to nil — these suites are latency-bound on an external resource,
not CPU-bound (14-25 % utilisation for the two slowest). Standalone, uncapped ->
capped: AuditLog 383 tests 1m52s -> 1m51s; Host 520 tests 4m05s -> 4m08s;
ConfigurationDatabase 396 tests 10s -> 12s; SiteCallAudit 66 tests 8s -> 8s. All
green.
docs/test_infra/test_infra.md gains a Test Parallelism Policy section: the two
axes (in-process collection threads = this cap; cross-process solution-level
dotnet test = unbounded in general, bounded on the DB-migration path by the
sp_getapplock semaphore), the per-suite table with reasons, the rules (cap
threads rather than serialize; a cap is not a substitute for test isolation; the
Content copy is required), and the default for new projects (no config until the
project acquires a per-class external resource). test_infra_db.md cross-links it
from the semaphore bullet.
Joseph Doherty
d2d4db492b