[None][fix] Fix KV cache size estimation with Qwen3.5 + EAGLE3 by AlessioNetti · Pull Request #19018 · NVIDIA/TensorRT-LLM

AlessioNetti · 2026-09-10T13:34:29Z

Dev Engineer Review

Draft KV-cache paths now pass _cache_transceiver_config when selecting the draft manager.
This fixes the ValueError for Qwen3.5 linear attention with an EAGLE3 llama-style drafter.
No public API changes are shown.
CI reported failed or unstable pipelines. Test results remain unavailable.

QA Engineer Review

Updated test_kv_cache_budget_split.py to cover manager selection across speculative modes, including MTP Eagle.
Expanded fixtures for transceiver, disaggregation, sparse attention, beam width, and KV connector settings.
Updated test_kv_cache_estimation.py to initialize _cache_transceiver_config.
The tests cover draft manager selection and KV-cache estimation.
No integration test-list files changed. These unit tests do not require test-list entries.
Coverage verdict: needs follow-up because supplied CI results include failed or unstable pipelines.

Per-File QA Perspective

tensorrt_llm/_torch/pyexecutor/_util.py: Verify manager selection for EAGLE3 and external drafters with the effective transceiver configuration.
tests/unittest/_torch/executor/kv_cache/test_kv_cache_budget_split.py: Covers manager selection across speculative modes with expanded cache fixtures. No integration test-list entry applies.
tests/unittest/_torch/executor/kv_cache/test_kv_cache_estimation.py: Covers one-model draft manager creation with initialized transceiver configuration. No integration test-list entry applies.

Description

This PR fixes the bug reported in issue #18449. In particular, TRT-LLM crashes during the KV cache size estimation phase when using a Qwen3.5 model (i.e., linear attention) combined with an EAGLE3 drafter (llama-style); this happens because, during size estimation, the draft model shares the base model's KV cache manager class, leading to a ValueError and a crash.

This PR applies the fix suggested in the issue itself, having the EAGLE3 branch also request a specific class for the KV cache manager like in the external drafter case.

Test Coverage

Tests have been updated to ensure that the EAGLE3 path also requests its own KV cache manager class.

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Walkthrough

The draft KV-cache estimation and one-model draft construction paths now pass the configured transceiver settings when resolving the draft manager. Tests initialize related configuration fields and validate manager resolution for all speculative modes.

Changes

Draft KV-cache costing

Layer / File(s)	Summary
Draft manager resolution and costing `tensorrt_llm/_torch/pyexecutor/_util.py`	Both draft KV-cache paths pass `self._cache_transceiver_config` when resolving the draft manager.
Parameterized draft-manager validation `tests/unittest/_torch/executor/kv_cache/test_kv_cache_budget_split.py`, `tests/unittest/_torch/executor/kv_cache/test_kv_cache_estimation.py`	Test fixtures initialize disaggregation, sparse-attention, beam-width, connector, and transceiver attributes. Assertions cover every parameterized speculative mode.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: bowenfu

Merge Risk: 🔵 Low · up to 47745

The implementation currently forwards transceiver settings correctly, but the tests could miss a future regression in this path. The remaining risk is limited to test coverage.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows the required [None][fix] format and clearly identifies the main change: fixing KV-cache size estimation for Qwen3.5 with EAGLE3.
Description check	✅ Passed	The description explains the issue, affected models, root cause, implemented fix, and test coverage. It does not include the PR checklist, but the main required content is present.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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AlessioNetti · 2026-09-10T13:57:06Z

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VALLIS-NERIA

lgtm

Signed-off-by: Alessio Netti <26897207+AlessioNetti@users.noreply.github.com>

AlessioNetti · 2026-09-15T08:49:53Z

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In `@tensorrt_llm/_torch/pyexecutor/_util.py`:
- Around line 873-876: Update both _get_draft_cache_cost and
_create_one_model_draft_kv_cache_manager calls to get_kv_cache_manager_cls by
passing the resolved self._cache_transceiver_config as cache_transceiver_config,
preserving the existing behavior while ensuring draft cache estimation and
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Inline comments:
In `@tests/unittest/_torch/executor/kv_cache/test_kv_cache_budget_split.py`:
- Line 225: Update the draft-manager tests to verify transceiver configuration
propagation: in
tests/unittest/_torch/executor/kv_cache/test_kv_cache_budget_split.py lines
225-225, replace the None fixture value with a sentinel and assert
get_kv_cache_manager_cls() receives it; in
tests/unittest/_torch/executor/kv_cache/test_kv_cache_estimation.py lines
1117-1117, capture the patched resolver and assert it receives the same
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…budget split test NVIDIA#19018 made the separate-draft arm of KvCacheCreator._get_draft_cache_cost resolve the draft manager class through get_kv_cache_manager_cls (reading _cache_transceiver_config) and the V2 fallback gate (reading _max_beam_width and model_config.sparse_attention_config). The test added in NVIDIA#18697, test_target_cost_uses_derived_layer_type_windows, builds the creator via object.__new__ without those fields, so main fails with AttributeError at the NVIDIA#19018 merge commit. Set the fields like the neighbouring one-model draft test does, and patch get_kv_cache_manager_cls back to the recording manager so the draft windows assertion still observes the draft costing. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Signed-off-by: HuiGao <huig@nvidia.com>

AlessioNetti requested review from a team as code owners September 10, 2026 13:34

AlessioNetti requested review from eopXD, lori-ren, tongyuantongyu and yizhang-nv September 10, 2026 13:34

github-actions Bot assigned AlessioNetti Sep 10, 2026

VALLIS-NERIA approved these changes Sep 12, 2026

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lori-ren approved these changes Sep 15, 2026

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AlessioNetti added 2 commits September 15, 2026 01:44

Select own KV cache manager class for EAGLE3

3865ff5

Signed-off-by: Alessio Netti <26897207+AlessioNetti@users.noreply.github.com>

Fix test for KV cache budget calculation

9c09e8c

Signed-off-by: Alessio Netti <26897207+AlessioNetti@users.noreply.github.com>

AlessioNetti force-pushed the fix-qwen3.5-eagle3-kv-cache branch from 53686da to 9c09e8c Compare September 15, 2026 08:46

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Comment thread tensorrt_llm/_torch/pyexecutor/_util.py Outdated

Fix mocks, pass _cache_transceiver_config to get_kv_cache_manager_cls()

73044d1

Signed-off-by: Alessio Netti <26897207+AlessioNetti@users.noreply.github.com>

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Comment thread tests/unittest/_torch/executor/kv_cache/test_kv_cache_budget_split.py

Fix one more unit test mock

4774555

Signed-off-by: Alessio Netti <26897207+AlessioNetti@users.noreply.github.com>

trtllm-agent added the ci: full pre-merge approved label Sep 16, 2026

MartinMarciniszyn merged commit 0732d9f into NVIDIA:main Sep 17, 2026
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moraxu mentioned this pull request Oct 13, 2026

[https://nvbugs/6800150][test] Unwaive the KV budget split tests and fix their stale mocks #20243

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lowsfer mentioned this pull request Oct 15, 2026

[https://nvbugs/6800223][fix] Fix stale KvCacheCreator fixtures in budget split tests #20399

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