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@mikeiovine mikeiovine commented Sep 29, 2025

Description

Handle the corner case where speculation is skipped (e.g. because the sequence length went past the draft model's max seq len for every request in the batch). You need to remove all draft tokens and turn speculation off or the ModelEngine will not handle the requests correctly and garbage will be generated.

Test Coverage

Added a new test.

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Summary by CodeRabbit

  • Bug Fixes

    • Speculative decoding now automatically disables when no draft tokens are available, preventing unintended draft usage and improving generation stability.
  • Tests

    • Expanded coverage for speculative decoding with a long-prompt scenario using an Eagle draft model and a Llama target model.
    • Validates outputs against non-speculative runs to ensure exact match.
    • Runs with and without CUDA graph execution to verify consistency across configurations.
    • Includes safeguards for high-memory environments.

@mikeiovine mikeiovine requested a review from a team as a code owner September 29, 2025 20:46
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/bot run --disable-fail-fast

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coderabbitai bot commented Sep 29, 2025

📝 Walkthrough

Walkthrough

Updates speculative decoding control flow to disable it when no draft batch is available, resetting flags and clearing draft tokens. Adds a new long-prompt Eagle3 speculative decoding test for Llama-3.1-8B with optional CUDA graph; the same test appears twice in the file.

Changes

Cohort / File(s) Summary
Speculative decoding control flow
tensorrt_llm/_torch/pyexecutor/py_executor.py
In _handle_speculative_decoding, when no draft run is active, disables speculative decoding (use_spec_decode and engine flag) and clears py_draft_tokens for scheduled requests.
Eagle3 long-prompt tests
tests/unittest/_torch/speculative/test_eagle3.py
Adds parameterized test_llama_eagle3_long_prompt (with/without CUDA graph) comparing speculative vs non-spec outputs; the same test is duplicated later in the file.

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant C as Client
  participant PE as PyExecutor
  participant ME as ModelEngine
  participant DR as Draft Runner

  C->>PE: generate(requests)
  PE->>DR: produce_draft_batch()
  alt Draft batch available
    DR-->>PE: draft_batch
    PE->>ME: enable_spec_decode = true
    PE->>ME: run_with_drafts(draft_batch)
  else No draft batch
    DR-->>PE: none
    note over PE: has_previous_draft_tokens = false
    rect rgba(230,240,255,0.6)
    PE->>PE: use_spec_decode = false
    PE->>ME: enable_spec_decode = false
    PE->>PE: clear scheduled requests' py_draft_tokens = []
    end
    PE->>ME: run_without_drafts()
  end
  ME-->>PE: tokens
  PE-->>C: output
Loading

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)
Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. You can run @coderabbitai generate docstrings to improve docstring coverage.
✅ Passed checks (2 passed)
Check name Status Explanation
Title Check ✅ Passed The pull request title "[https://nvbugs/5549111][fix] Fix 2-model overlap scheduler accuracy on very long prompts" clearly and accurately describes the main change in the changeset. The code modifications handle a corner case in the 2-model overlap (speculation) scheduler when sequence length exceeds the draft model's max sequence length, which is precisely what causes accuracy issues on very long prompts. The title follows the required format with a valid ticket format, type tag, and a concise summary that captures the primary intent of the fix.
Description Check ✅ Passed The pull request description follows the template structure closely and includes all required sections: Description, Test Coverage, and PR Checklist. The Description section clearly explains the issue (corner case where speculation is skipped) and the solution (remove draft tokens and turn speculation off). The Test Coverage section confirms that a new test was added to safeguard the changes. The PR Checklist is complete with the checkbox marked, indicating the author has reviewed all items. The description is specific, technically detailed, and directly addresses the changes being made.
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Actionable comments posted: 1

📜 Review details

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Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between 334e2ca and a831569.

📒 Files selected for processing (2)
  • tensorrt_llm/_torch/pyexecutor/py_executor.py (1 hunks)
  • tests/unittest/_torch/speculative/test_eagle3.py (1 hunks)
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  • tensorrt_llm/_torch/pyexecutor/py_executor.py
  • tests/unittest/_torch/speculative/test_eagle3.py
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Files:

  • tensorrt_llm/_torch/pyexecutor/py_executor.py
  • tests/unittest/_torch/speculative/test_eagle3.py
**/*.{cpp,cxx,cc,h,hpp,hh,hxx,cu,cuh,py}

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Files:

  • tensorrt_llm/_torch/pyexecutor/py_executor.py
  • tests/unittest/_torch/speculative/test_eagle3.py
🧠 Learnings (1)
📚 Learning: 2025-08-18T08:42:02.640Z
Learnt from: samuellees
PR: NVIDIA/TensorRT-LLM#6974
File: tensorrt_llm/serve/scripts/benchmark_dataset.py:558-566
Timestamp: 2025-08-18T08:42:02.640Z
Learning: In TensorRT-LLM's RandomDataset (tensorrt_llm/serve/scripts/benchmark_dataset.py), when using --random-token-ids option, sequence length accuracy is prioritized over semantic correctness for benchmarking purposes. The encode/decode operations should use skip_special_tokens=True and add_special_tokens=False to ensure exact target token lengths.

Applied to files:

  • tensorrt_llm/_torch/pyexecutor/py_executor.py
🧬 Code graph analysis (2)
tensorrt_llm/_torch/pyexecutor/py_executor.py (2)
tensorrt_llm/_torch/pyexecutor/cuda_graph_runner.py (1)
  • enable_spec_decode (90-91)
tensorrt_llm/_torch/pyexecutor/scheduler.py (1)
  • all_requests (38-39)
tests/unittest/_torch/speculative/test_eagle3.py (3)
tests/unittest/_torch/modeling/test_modeling_qwen2_5vl.py (1)
  • llm_models_root (30-36)
tensorrt_llm/llmapi/llm_args.py (3)
  • EagleDecodingConfig (449-562)
  • speculative_model_dir (1520-1521)
  • CudaGraphConfig (109-166)
tensorrt_llm/llmapi/llm.py (4)
  • LLM (1052-1068)
  • prompt (82-83)
  • generate (241-319)
  • shutdown (737-744)
⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (1)
  • GitHub Check: Pre-commit Check
🔇 Additional comments (2)
tensorrt_llm/_torch/pyexecutor/py_executor.py (1)

2107-2112: LGTM! Clean corner-case fix for skipped speculation.

The logic correctly disables speculative decoding when no draft batch is available, preventing ModelEngine from mishandling requests with stale draft tokens. The comment clearly explains the rationale, and the implementation properly resets both the executor and engine flags while clearing draft tokens from all scheduled requests.

tests/unittest/_torch/speculative/test_eagle3.py (1)

179-227: Well-structured test for the long-prompt corner case.

The test correctly exercises the scenario where sequence length exceeds the draft model's max, forcing speculation to be skipped. The exact-match assertion is appropriate since dynamic disabling should produce identical output to non-speculative generation. Proper memory checks and cleanup are in place.

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PR_Github #20298 [ run ] triggered by Bot

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/bot run --disable-fail-fast

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/bot run --disable-fail-fast

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PR_Github #20299 [ run ] triggered by Bot

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PR_Github #20298 [ run ] completed with state ABORTED
LLM/main/L0_MergeRequest_PR #15309 (Blue Ocean) completed with status: ABORTED

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PR_Github #20300 [ run ] triggered by Bot

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PR_Github #20299 [ run ] completed with state ABORTED

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PR_Github #20300 [ run ] completed with state SUCCESS
/LLM/release-1.1/L0_MergeRequest_PR pipeline #23 completed with status: 'FAILURE'

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LGTM. Thanks for catching and fixing this issue.

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/bot run --disable-fail-fast

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PR_Github #20386 [ run ] triggered by Bot

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PR_Github #20386 [ run ] completed with state SUCCESS
/LLM/release-1.1/L0_MergeRequest_PR pipeline #32 completed with status: 'FAILURE'

@mikeiovine mikeiovine changed the base branch from release/1.1 to main October 2, 2025 14:57
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/bot run --disable-fail-fast

Signed-off-by: Mike Iovine <6158008+mikeiovine@users.noreply.github.com>
Signed-off-by: Michael Iovine <miovine@nvidia.com>
Signed-off-by: Mike Iovine <6158008+mikeiovine@users.noreply.github.com>
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/bot run --disable-fail-fast

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PR_Github #20537 [ run ] triggered by Bot

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PR_Github #20538 [ run ] triggered by Bot

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PR_Github #20537 [ run ] completed with state ABORTED

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3 participants