Speed up TaskGroup.topological_sort with int-indexed projected sweep - #67288
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Pull request overview
This PR optimizes TaskGroup.topological_sort (and the serialized equivalent) by pre-projecting upstream dependencies to sibling indices once per call, then performing a sweep using an efficient emitted-flag structure. This aims to reduce repeated per-edge work during sorting while preserving the existing emission order.
Changes:
- Replaced the previous modified-Kahn loop in
TaskGroup.topological_sortwith a “project then sweep” implementation, including explicit cycle detection. - Mirrored the same algorithm into
SerializedTaskGroup.topological_sortfor consistency/performance. - Added Task SDK tests that validate topological correctness across multiple DAG shapes, plus a newsfragment entry.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| task-sdk/tests/task_sdk/definitions/test_taskgroup.py | Adds cross-shape correctness tests for TaskGroup.topological_sort. |
| task-sdk/src/airflow/sdk/definitions/taskgroup.py | Implements projected dependency computation and sweep-based topo sort in Task SDK TaskGroup. |
| airflow-core/src/airflow/serialization/definitions/taskgroup.py | Mirrors the new topo sort algorithm for serialized task groups and adds cycle detection behavior. |
| airflow-core/newsfragments/67288.improvement.rst | Documents the TaskGroup.topological_sort performance improvement. |
The previous modified-Kahn implementation re-derived each child's upstream edges (materializing ``upstream_list`` and walking ``parent_group``) on every outer-loop pass. Project per-task upstream IDs onto sibling-level integer indices once up front, then run a greedy multi-pass sweep against that projection with a ``bytearray`` emission flag. Emission order is identical to the previous implementation; existing order-sensitive tests cover the contract. Same change is mirrored in SerializedTaskGroup.
Adds a round-trip test for SerializedTaskGroup.topological_sort (the serialization variant was previously untested), rewrites the newsfragment in user-facing terms, and cleans up a stale reference and type annotation in the task-sdk shape tests.
Per Copilot review feedback, the previous wording said cycles are caught at "deserialization time" — but DAG.check_cycle runs at DAG parse time (via dagbag loading), not during from_dict/from_json. Reword to describe cycles reaching this code path as malformed serialized data, with the defensive ValueError still raised on detection.
Tighten _project_child_deps and the topological_sort body: drop the per-edge child_idx comparison in favour of a single set.discard at the end, inline the get_task/.task_group chain, and replace the pre-alloc loop with a list comprehension. Net -15 lines per file with a small (~10%) speedup on the layered shape and no regression elsewhere. The hot _sweep_projection inner loop is left intact — earlier attempts to extract its duplicated body into a closure cost 10-30% on independent / layered shapes and were reverted.
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Drop n=500 from the parametrize grid. The algorithm has no n-dependent branches, so n=100 covers every code path; n=500 only re-runs the same loops with more iterations and added ~0.7s to the file without exercising new correctness behaviour (per Copilot review feedback on PR apache#67288).
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…ected sweep (#67288) (#67687) * Speed up TaskGroup.topological_sort with int-indexed projected sweep (backport #67288) * Remove __getitem__ and NodeNotFound tests not available in v3-2-test These tests depend on TaskGroup.__getitem__ and NodeNotFound exception which don't exist in v3-2-test. Keep topological_sort shape correctness tests which are for the actual optimization. Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com> --------- Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>
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…ected sweep (#67288) (#67687) * Speed up TaskGroup.topological_sort with int-indexed projected sweep (backport #67288) * Remove __getitem__ and NodeNotFound tests not available in v3-2-test These tests depend on TaskGroup.__getitem__ and NodeNotFound exception which don't exist in v3-2-test. Keep topological_sort shape correctness tests which are for the actual optimization. Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com> --------- Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>
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…deps TaskGroup._project_child_deps only looked at a group's own upstream_task_ids, which stays empty for a direct group-to-group dependency (list or individual `>>`) and for a task-level dependency that crosses into another group's entry task. Both cases sorted the group as if it had no upstream at all. Now also pulls in the group's upstream_group_ids and its root tasks' upstream task ids before projecting sibling dependencies. Applied to both the serialization-layer sort and the mirrored design-time sort in task-sdk. closes: apache#65291 Related: apache#67964 (closed for inactivity, written against the topological_sort implementation before PR apache#67288/apache#67688 rewrote it) and apache#65639 (draft, same issue, also predates the rewrite).
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…dependencies (#69933) * Fix grid/graph view topological sort for group-level and cross-group deps TaskGroup._project_child_deps only looked at a group's own upstream_task_ids, which stays empty for a direct group-to-group dependency (list or individual `>>`) and for a task-level dependency that crosses into another group's entry task. Both cases sorted the group as if it had no upstream at all. Now also pulls in the group's upstream_group_ids and its root tasks' upstream task ids before projecting sibling dependencies. Applied to both the serialization-layer sort and the mirrored design-time sort in task-sdk. closes: #65291 Related: #67964 (closed for inactivity, written against the topological_sort implementation before PR #67288/#67688 rewrote it) and #65639 (draft, same issue, also predates the rewrite). * Address review feedback: cache get_task_group_dict, describe test intent not issue numbers viiccwen pointed out that fetching the group map inside topological_sort() rebuilds the whole DAG's group tree on every nested group's own call, turning a render with G groups into an O(G^2) cost. get_task_group_dict() is now memoized per DAG instance (kept behind a small private helper since methodtools.lru_cache has no type stubs and would otherwise widen the public method's return type to Any for every caller). Also reworded test comments/docstrings that cited issue numbers to describe what's actually being verified instead. * Hoist common logic into shared lib * Remove caching on get_task_group_dict * Remove stale get_task_group_dict cache tests The cache these tests asserted was removed in the previous commit, so the identity check and the _get_task_group_dict_cached.cache_info() assertions no longer apply. * Add call-level task group memo to reduce calc * Tidy Typy * Add test for task group memoing --------- Co-authored-by: TP <uranusjr@apache.org> Co-authored-by: LIU ZHE YOU <zhu424.dev@gmail.com>
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…dependencies (#69933) (#70591) * Fix grid/graph view topological sort for group-level and cross-group deps TaskGroup._project_child_deps only looked at a group's own upstream_task_ids, which stays empty for a direct group-to-group dependency (list or individual `>>`) and for a task-level dependency that crosses into another group's entry task. Both cases sorted the group as if it had no upstream at all. Now also pulls in the group's upstream_group_ids and its root tasks' upstream task ids before projecting sibling dependencies. Applied to both the serialization-layer sort and the mirrored design-time sort in task-sdk. closes: #65291 Related: #67964 (closed for inactivity, written against the topological_sort implementation before PR #67288/#67688 rewrote it) and #65639 (draft, same issue, also predates the rewrite). * Address review feedback: cache get_task_group_dict, describe test intent not issue numbers viiccwen pointed out that fetching the group map inside topological_sort() rebuilds the whole DAG's group tree on every nested group's own call, turning a render with G groups into an O(G^2) cost. get_task_group_dict() is now memoized per DAG instance (kept behind a small private helper since methodtools.lru_cache has no type stubs and would otherwise widen the public method's return type to Any for every caller). Also reworded test comments/docstrings that cited issue numbers to describe what's actually being verified instead. * Hoist common logic into shared lib * Remove caching on get_task_group_dict * Remove stale get_task_group_dict cache tests The cache these tests asserted was removed in the previous commit, so the identity check and the _get_task_group_dict_cached.cache_info() assertions no longer apply. * Add call-level task group memo to reduce calc * Tidy Typy * Add test for task group memoing --------- Co-authored-by: TP <uranusjr@apache.org> Co-authored-by: LIU ZHE YOU <zhu424.dev@gmail.com> (cherry picked from commit d7aa929) # Conflicts: # airflow-core/src/airflow/api_fastapi/core_api/services/ui/task_group.py Co-authored-by: Hemkumar Chheda <95332229+hkc-8010@users.noreply.github.com>
vatsrahul1001
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…dependencies (#69933) (#70591) * Fix grid/graph view topological sort for group-level and cross-group deps TaskGroup._project_child_deps only looked at a group's own upstream_task_ids, which stays empty for a direct group-to-group dependency (list or individual `>>`) and for a task-level dependency that crosses into another group's entry task. Both cases sorted the group as if it had no upstream at all. Now also pulls in the group's upstream_group_ids and its root tasks' upstream task ids before projecting sibling dependencies. Applied to both the serialization-layer sort and the mirrored design-time sort in task-sdk. closes: #65291 Related: #67964 (closed for inactivity, written against the topological_sort implementation before PR #67288/#67688 rewrote it) and #65639 (draft, same issue, also predates the rewrite). * Address review feedback: cache get_task_group_dict, describe test intent not issue numbers viiccwen pointed out that fetching the group map inside topological_sort() rebuilds the whole DAG's group tree on every nested group's own call, turning a render with G groups into an O(G^2) cost. get_task_group_dict() is now memoized per DAG instance (kept behind a small private helper since methodtools.lru_cache has no type stubs and would otherwise widen the public method's return type to Any for every caller). Also reworded test comments/docstrings that cited issue numbers to describe what's actually being verified instead. * Hoist common logic into shared lib * Remove caching on get_task_group_dict * Remove stale get_task_group_dict cache tests The cache these tests asserted was removed in the previous commit, so the identity check and the _get_task_group_dict_cached.cache_info() assertions no longer apply. * Add call-level task group memo to reduce calc * Tidy Typy * Add test for task group memoing --------- Co-authored-by: TP <uranusjr@apache.org> Co-authored-by: LIU ZHE YOU <zhu424.dev@gmail.com> (cherry picked from commit d7aa929) # Conflicts: # airflow-core/src/airflow/api_fastapi/core_api/services/ui/task_group.py Co-authored-by: Hemkumar Chheda <95332229+hkc-8010@users.noreply.github.com>
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Summary
Replaces the modified-Kahn body of
TaskGroup.topological_sort(and the mirror inSerializedTaskGroup) with a projected sweep:upstream_task_idsis projected onto sibling-level integer indices once up front (_project_child_deps)._sweep_projection) then runs against the projection with abytearray-backed emission flag - per-edge work happens once per sort instead of once per outer-loop pass.Emission order is identical to the previous implementation. The existing order-sensitive tests (
test_topological_sort1/2,test_topological_nested_groups,test_topological_group_dep) cover the contract.Benchmark
Min of five runs, N=2000 children, ms per call. Source: https://gist.github.com/shahar1/9c61dc9f34f7e77cd29cfb9d67af7ceb
The 7.8x on rev-chain comes from the projection precompute alone (one projection vs N projections); algorithmic complexity is unchanged and remains O(N²) on that shape until the follow-up PR.
The nested row times a recursive walk across all 39 nested groups (depth=3, ~222 tasks each); the root-only timing was dominated by setup overhead and was previously labelled
~noisein this table.Test plan
task-sdk/tests/task_sdk/definitions/test_taskgroup.py+test_dag.py— 145 passed (18 new shape-correctness cases)airflow-core/tests/unit/utils/test_task_group.py— 20 passed (1 new serialization round-trip case)airflow-core/tests/unit/serialization/(task-group / topological filter) — 4 passedprek run mypy-task-sdk— passedprek run mypy-airflow-core— passedprek run --from-ref upstream/main --stage pre-commit— passedWas generative AI tooling used to co-author this PR?
Generated-by: Claude Code (Opus 4.7) following the guidelines