[None][fix] Preserve configured Mamba snapshot placement by liji-nv · Pull Request #18724 · NVIDIA/TensorRT-LLM

liji-nv · 2026-09-04T09:44:50Z

Dev Engineer Review

Preserves configured Mamba snapshot placement.
Adds prompt_len only when no snapshot placement is configured.
Updates enable_branch_snapshot documentation to match the behavior.
No public API changes or unrelated scope changes are evident.

QA Engineer Review

Modified the prompt-end branch snapshot test.
Added coverage for preserving a configured nonzero end offset.
No matching entries were found in tests/integration/test_lists/.
Verdict: needs follow-up.

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coderabbitai · 2026-09-04T09:58:26Z

No actionable comments were generated in the recent review. 🎉

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tensorrt_llm/_torch/pyexecutor/kv_cache/mamba_cache_manager.py
tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py

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Walkthrough

Mamba cache managers now support KDA replay configuration, replay-buffer lifecycle management, accepted-token bookkeeping, slot relocation, and disaggregated-generation seeding. Branch snapshot handling preserves configured placements and uses the prompt end only as a fallback.

Changes

Mamba cache updates

Layer / File(s)	Summary
KDA replay configuration and allocation `tensorrt_llm/_torch/pyexecutor/kv_cache/mamba_cache_manager.py`, `tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py`	The managers expose KDA replay capability. V2 validates replay settings and allocates per-slot replay buffers. Tests cover factory routing, configuration validation, allocation, and cache geometry.
KDA replay state lifecycle `tensorrt_llm/_torch/pyexecutor/kv_cache/mamba_cache_manager.py`, `tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py`	The manager resets and relocates replay state, records accepted draft counts, seeds caches for disaggregated generation, exposes layer caches, and releases buffers. Tests cover state updates, slot relocation, context resets, and cache seeding.
Branch snapshot placement and reuse `tensorrt_llm/_torch/pyexecutor/kv_cache/mamba_cache_manager.py`, `tensorrt_llm/llmapi/llm_args.py`, `tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py`	Branch snapshot handling preserves configured placements, retries pending points after failed lookup, uses authoritative reuse depth, and adds the prompt end only when no placement exists. Documentation and tests cover stale cursors, retries, and nonzero end offsets.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to 10095

The change preserves configured Mamba snapshot placement and adds tested KDA replay handling. No actionable merge-blocking risk is currently identified.

Sequence Diagram(s)

sequenceDiagram
  participant DraftGeneration
  participant MambaHybridCacheManagerV2
  participant KDAReplayCaches
  participant Verification
  DraftGeneration->>MambaHybridCacheManagerV2: submit drafted tokens and resource updates
  MambaHybridCacheManagerV2->>KDAReplayCaches: record accepted draft counts
  MambaHybridCacheManagerV2->>KDAReplayCaches: reset, relocate, or seed replay state
  Verification->>MambaHybridCacheManagerV2: request replay layer caches
  MambaHybridCacheManagerV2-->>Verification: return KDA replay caches

Suggested reviewers: bowenfu

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description contains only the unfilled template. It does not explain the issue, solution, or relevant test coverage, although the checklist is marked complete.	Add a concise Description section that explains the issue and implementation. Add a Test Coverage section that lists the relevant KDA replay and branch-snapshot tests. Confirm any API-change label and checklist items as applicable.
Docstring Coverage	⚠️ Warning	Docstring coverage is 41.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows the required format and clearly describes the snapshot-placement fix, which is a central change in the pull request.
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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SimengLiu-nv

Approve as the functional changes LGTM. Better to fix the documentation misalignment.

sylvesterkaczmarek

This restores the right precedence between explicit placement policy and the branch-snapshot fallback. prompt_len should be synthesized only when there is no configured placement; otherwise it changes which recurrent checkpoint survives within a block and effectively overrides user policy. The repeat-call regression is useful because it also proves prepare_expect_snapshot_points() is idempotent after the branch point is recorded. The functional boundary looks sound.

coderabbitai · 2026-09-07T02:42:04Z

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coderabbitai · 2026-09-07T02:51:26Z

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Actionable comments posted: 1

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Inline comments:
In `@tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py`:
- Line 3018: Add
tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py to an
appropriate CI test-database entry, covering both
test_branch_snapshot_adds_prompt_end_without_configured_placement and
test_branch_snapshot_preserves_configured_nonzero_end_offset; no QA registration
is needed.

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📒 Files selected for processing (3)

tensorrt_llm/_torch/pyexecutor/kv_cache/mamba_cache_manager.py
tensorrt_llm/llmapi/llm_args.py
tests/unittest/_torch/executor/kv_cache/test_mamba_cache_manager.py

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liji-nv · 2026-09-07T05:04:42Z

/bot run --disable-fail-fast

tensorrt-cicd · 2026-09-07T05:12:09Z

PR_Github #71859 [ run ] triggered by Bot. Commit: 4b379a8 Link to invocation

tensorrt-cicd · 2026-09-07T08:25:42Z

PR_Github #71859 [ run ] completed with state SUCCESS. Commit: 4b379a8
/LLM/main/L0_MergeRequest_PR pipeline #58935 completed with status: 'FAILURE'

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liji-nv · 2026-09-07T08:27:25Z

/bot run --disable-fail-fast

tensorrt-cicd · 2026-09-07T08:34:45Z

PR_Github #71906 [ run ] triggered by Bot. Commit: 4b379a8 Link to invocation

tensorrt-cicd · 2026-09-07T10:26:10Z

PR_Github #71906 [ run ] completed with state SUCCESS. Commit: 4b379a8
/LLM/main/L0_MergeRequest_PR pipeline #58977 completed with status: 'SUCCESS'

CI Report

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Signed-off-by: Jin Li <59594262+liji-nv@users.noreply.github.com>

liji-nv · 2026-09-07T11:34:59Z

/bot run --disable-fail-fast

tensorrt-cicd · 2026-09-07T11:41:42Z

PR_Github #71944 [ run ] triggered by Bot. Commit: 1009560 Link to invocation

tensorrt-cicd · 2026-09-07T14:26:04Z

PR_Github #71944 [ run ] completed with state SUCCESS. Commit: 1009560
/LLM/main/L0_MergeRequest_PR pipeline #59009 completed with status: 'SUCCESS'

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liji-nv requested a review from a team as a code owner September 4, 2026 09:44

liji-nv requested review from VALLIS-NERIA and eopXD September 4, 2026 09:44

github-actions Bot assigned liji-nv Sep 4, 2026

Tabrizian approved these changes Sep 4, 2026

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SimengLiu-nv approved these changes Sep 4, 2026

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Comment thread tensorrt_llm/_torch/pyexecutor/kv_cache/mamba_cache_manager.py Outdated

sylvesterkaczmarek reviewed Sep 4, 2026

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athena-nv approved these changes Sep 4, 2026

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liji-nv force-pushed the codex/fix-branch-snapshot-offset-preservation branch from 10e7f2d to 7fbb077 Compare September 7, 2026 02:41

liji-nv requested a review from a team as a code owner September 7, 2026 02:47

liji-nv requested review from allisonlim-nv and zhaoyangwang-nvidia September 7, 2026 02:47

coderabbitai Bot reviewed Sep 7, 2026

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

liji-nv added the api-compatible Accepted LLM API contract change that is backwards-compatible label Sep 7, 2026

liji-nv added 2 commits September 7, 2026 03:37

[None][fix] Preserve configured Mamba snapshot placement

2c9d336

Signed-off-by: Jin Li <59594262+liji-nv@users.noreply.github.com>

[None][docs] Clarify branch snapshot fallback behavior

1009560

Signed-off-by: Jin Li <59594262+liji-nv@users.noreply.github.com>

liji-nv force-pushed the codex/fix-branch-snapshot-offset-preservation branch from 4b379a8 to 1009560 Compare September 7, 2026 10:58

liji-nv enabled auto-merge (squash) September 7, 2026 11:35

liji-nv merged commit 41e6c8f into NVIDIA:main Sep 7, 2026
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