[GitHub] [tvm-rfcs] driazati commented on a change in pull request #55: @slow test RFC

[GitBox <gi...@apache.org>]

driazati commented on a change in pull request #55:
URL: https://github.com/apache/tvm-rfcs/pull/55#discussion_r801939469



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File path: rfcs/0055-slow-tests.md
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@@ -0,0 +1,110 @@
+- Feature Name: `slow_tests_decorator`
+- Start Date: 2022-01-26
+- RFC PR: [apache/tvm-rfcs#55](https://github.com/apache/tvm-rfcs/pull/55)
+
+# Summary
+
+[summary]: #summary
+
+Add a Python decorator to skip tests on PRs but run them on branches (e.g. `main`).
+
+# Motivation
+
+[motivation]: #motivation
+
+A small subset of tests take up a large portion of the total test runtime. This RFC proposes that we skip these tests on PRs where CI runtime is critical and run them only on main.
+
+# Guide-level explanation
+
+[guide-level-explanation]: #guide-level-explanation
+
+CI runtime constantly plagues TVM developers with long iteration times, exacerbated by flakiness and difficult-to-reproduce steps. To reduce runtime, we can execute more work concurrently and increase usage of available resources, usually by parallelization within (apache/tvm#9834) or between (apache/tvm#9733) CI jobs. Another way is to do less work, which is what this RFC proposes. By running some tests on `main` only, we still get some measure of coverage provided by these tests without burdening PR developers.
+
+The runtime savings of this change are potentially significant, as this gives us a black-box knob which we can manually tune over time to trade off between PR test coverage and PR test runtime. [This gist](https://gist.github.com/driazati/e009f09ff44c6bc91c4d95a8e17fd6f1) shows a listing of TVM test runtime in descending order. And this list the potential time savings (across all jobs) in CI of cutting off tests above an arbitrary runtime (not to propose use a cutoff, but just to demonstrate in broad strokes the potential impact of this change):
+
+```
+[cutoff=10.0s] Total savings (m): 419.95m by skipping 695 tests
+[cutoff=20.0s] Total savings (m): 338.27m by skipping 320 tests
+[cutoff=30.0s] Total savings (m): 291.5m by skipping 205 tests
+[cutoff=40.0s] Total savings (m): 251.59m by skipping 135 tests
+[cutoff=50.0s] Total savings (m): 222.56m by skipping 96 tests
+[cutoff=60.0s] Total savings (m): 203.3m by skipping 75 tests
+[cutoff=70.0s] Total savings (m): 192.68m by skipping 65 tests
+[cutoff=80.0s] Total savings (m): 181.56m by skipping 56 tests
+[cutoff=90.0s] Total savings (m): 171.45m by skipping 49 tests
+[cutoff=100.0s] Total savings (m): 160.36m by skipping 42 tests
+```
+
+# Reference-level explanation
+
+[reference-level-explanation]: #reference-level-explanation
+
+A decorator `@tvm.testing.slow` will be added (see apache/tvm#10057) that implements
+the above behavior. Skipping slow tests would be an opt-in, rather than opt-out.
+This way developers who don't read this RFC won't have to adjust their workflows
+at all to run these tests locally. There is also a need to run slow tests on PRs
+in some cases, such as fixing reverted commits or if a developer suspects their
+change would have a wide reaching impact. In this case, `@ci run slow tests` can
+be added to the PR body before tests are run in order to disable skipping slow tests.
+
+Using a decorator has the advantage of being explicit compared to an automated system to detect and skip slow tests. Clicking through to the decorator’s short definition makes it clear what is going on so this shouldn’t add too much of a development burden. Slow tests run by default to minimize disruption but can be controlled by setting `SKIP_SLOW_TESTS=1`.
+
+An environment variable `SKIP_SLOW_TESTS=1` will be set in Jenkins on PRs. Branches,
+including `main` and `ci-docker-staging` will not have this flag set and will
+always run the full set of tests.
+
+# Drawbacks
+
+[drawbacks]: #drawbacks
+
+The primary caveat is that tests that run on `main` may now fail due to PRs that were green when they were merged, so this will require some buy-in from all TVM developers. However, the runtime savings are significant (see below) enough to make this worth it. Developments like apache/tvm#9554 will make the revert process much smoother as well to minimize disruptions.
+
+# Rationale and alternatives
+
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+This isn't a complete solution. Most PRs end up running lots of tests that the
+PR didn't affect at all. Ideally we would have a way a-la Bazel or Buck to
+walk the actual dependency graph of PRs (ignoring / banning some of Python's
+dynamism e.g. `importlib.import_module`) to determine what tests to run on a PR.
+This could also be implemented at the human level, with developers tagging their
+PRs based on what they think should run, though this has a higher potential to
+miss certain tests. However, this run-what-changed future would be difficult to
+achieve.
+
+Other efforts involve looking into tests themselves to determine why they are slow.
+Often TVM's tests are running much more work than they actually intend to test in
+more of an integration test than a unit test. Replacing these types of test with
+a framework that makes it easier to test TVM passes and functionality in smaller
+chunks is related but orthogonal to this work. It still has the same issue (coverage

Review comment:
       Yeah I see what you mean, it all comes down to implementation/how it's used, but either way it's pretty immaterial to this RFC so I removed that language




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