[andrewpark-scaleai]

Why

The goal is a faster slow tier on the same 12 runners, with no test weakened. Each slow shard runs one test at a time today, while its runner uses at most 4 GB of its 16 GB.

What changes

The slow step runs pytest -n 2 --dist loadfile. Each shard runs two test files at a time. A file's tests, and its module fixtures and pytest.mark.dependency chains, stay on one worker. No test or assertion changes.

Evidence

The #582 experiment ran this setup on main plus #580:

• The last run passed on all 12 shards. The slowest job took 840s; on main the slowest shards take about 1100s. Peak memory was 2.7 to 3.5 GB of 16 GB, with no OOM kills.
• One earlier run failed because a test counted every container on the machine. test: count and remove only the containers of sandboxes this test's process made #585 fixed that.
• One earlier run failed on shards 4 and 10 in apt-heavy image builds, each after about 11 minutes, on separate runners. An independent review found no state the two workers share in either step: the resolver runs apt in its own container and work folder, and the base image tag carries a uuid. It is most likely an apt mirror flake. Its stderr was not kept, so this is not proven.

Neither experiment run tested this exact commit, so this PR's own local-backends run is the check. I'll report its per-shard times here.

Known limits

• A shard can't finish faster than its biggest file. chromium_app_test.py takes about 550s.
• Host-port choice is serialized per state root, so two workers can in rare cases pick the same free port. The fast tier already runs -n auto with the same exposure.

Confidence Score: 5/5

The PR appears safe to merge; no blocking failures were found.

Summary

Runs two slow-test workers on each of the existing 12 shards.

• Each slow integration shard runs two test files at once.

andrewpark-scaleai acknowledged the known host-port collision exposure. andrewpark-scaleai also disclosed the earlier apt failures and missing stderr; no concrete failure caused by this change was established.

_{Reviews (1) · Last reviewed commit: "ci: run the slow tier's tests two at a t..." · Reviewed by Greptile}