[https://nvbugs/5477730][fix] Fix the alltoall case when tp_size larg…

[WeiHaocheng]

…er than ep_size

Summary by CodeRabbit

• Refactor
  • Updated MOE parallel topology and workspace sizing to align with MOE-specific dimensions, improving stability, scalability, and efficiency for distributed MOE workloads.
• Chores
  • Maintains full compatibility with existing configurations; no changes to public APIs or user-facing interfaces.

Description

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📝 Walkthrough

Walkthrough

Updates MOE-related process grouping and workspace sizing in tensorrt_llm/_mnnvl_utils.py: communicator color computation now includes MOE topology; MOE workspace and prepare workspace sizes now use MOE exposure (ep) sizes. No public API signatures changed.

Changes

Cohort / File(s)	Summary
MOE MPI topology and workspace sizing tensorrt_llm/_mnnvl_utils.py	- get_comm: color set to ((pp_rank × cp_size + cp_rank) × moe_tp_size + moe_tp_rank); key remains tp_rank - get_moe_workspaces: workspace_size_per_rank uses moe_ep_size (was tp_size) - get_moe_prepare_workspace: workspace_size_per_rank uses moe_ep_size

Sequence Diagram(s)

sequenceDiagram
  autonumber
  participant Caller
  participant Utils as _mnnvl_utils
  participant Mapping as Mapping (ranks & sizes)
  participant MPI as MPI/Comm

  Note over Caller,Utils: MOE-aware communicator and workspace sizing

  Caller->>Utils: get_comm(mapping)
  Utils->>Mapping: read pp_rank, cp_rank, cp_size, moe_tp_size, moe_tp_rank, tp_rank
  Utils->>Utils: color = ((pp_rank*cp_size + cp_rank)*moe_tp_size + moe_tp_rank)\nkey = tp_rank
  Utils->>MPI: MPI_Comm_split(color, key)
  MPI-->>Utils: comm
  Utils-->>Caller: comm

  Caller->>Utils: get_moe_workspaces(mapping)
  Utils->>Mapping: read moe_ep_size
  Utils->>Utils: workspace_size_per_rank = f(moe_ep_size)
  Utils-->>Caller: sizes

  Caller->>Utils: get_moe_prepare_workspace(mapping)
  Utils->>Mapping: read moe_ep_size
  Utils->>Utils: workspace_size_per_rank = f(moe_ep_size)
  Utils-->>Caller: size

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Pre-merge checks (3 warnings)

❌ Failed checks (3 warnings)

Check name	Status	Explanation	Resolution
Title Check	⚠️ Warning	The current pull request title references fixing the alltoall case when tp_size is larger than ep_size, but the actual changes introduce MOE-specific MPI communicator topology and adjust workspace sizing functions rather than solely addressing an alltoall scenario. As a result, the title does not fully capture the primary modifications and only partially relates to one aspect of the diff. It omits the core enhancements to MOE resource allocation and grouping, making it unclear to readers what the main change is. Therefore, the title fails to accurately reflect the key updates in this PR.	Please update the title to clearly reflect the main changes, for example by referencing the introduction of MOE-specific communicator color mapping and workspace sizing adjustments. Ensure it follows the “[ticket][type] Summary” format and succinctly captures the core modifications, such as “[https://nvbugs/5477730][fix] Add MOE-specific MPI communicator color mapping and update workspace sizing.”
Description Check	⚠️ Warning	The pull request description remains as the unfilled template, containing placeholders for the summary, description, and test coverage without any actual content describing the issue, the solution, or relevant tests. None of the required sections have been populated with details about the changes made or how they are validated. As a result, reviewers have no context to understand the purpose or correctness of the PR.	Populate the Description section with a concise explanation of the problem and the implemented fix, list specific test cases under Test Coverage that verify the new behavior, and confirm each item in the PR Checklist is addressed. Remove the template comments and replace them with real content to provide reviewers with the necessary context and validation steps.
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.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7c2f04f and 03ec047.

📒 Files selected for processing (1)

• tensorrt_llm/_mnnvl_utils.py (3 hunks)

🧰 Additional context used

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🧠 Learnings (2)

📓 Common learnings

Learnt from: djns99
PR: NVIDIA/TensorRT-LLM#6915
File: cpp/tensorrt_llm/kernels/cutlass_kernels/moe_gemm/moe_kernels.cu:4616-4626
Timestamp: 2025-08-19T03:35:20.866Z
Learning: In the MOE profiler TMA workspace preparation (cpp/tensorrt_llm/kernels/cutlass_kernels/moe_gemm/moe_kernels.cu), the overlapping of TMA WS regions for NONE and FINALIZE variants is deliberate design to save memory space, as confirmed by djns99. The comment "reuse the same pointers to save space" reflects this intentional behavior.

📚 Learning: 2025-08-19T03:35:20.866Z

Learnt from: djns99
PR: NVIDIA/TensorRT-LLM#6915
File: cpp/tensorrt_llm/kernels/cutlass_kernels/moe_gemm/moe_kernels.cu:4616-4626
Timestamp: 2025-08-19T03:35:20.866Z
Learning: In the MOE profiler TMA workspace preparation (cpp/tensorrt_llm/kernels/cutlass_kernels/moe_gemm/moe_kernels.cu), the overlapping of TMA WS regions for NONE and FINALIZE variants is deliberate design to save memory space, as confirmed by djns99. The comment "reuse the same pointers to save space" reflects this intentional behavior.

Applied to files:

• tensorrt_llm/_mnnvl_utils.py

🧬 Code graph analysis (1)

tensorrt_llm/_mnnvl_utils.py (2)

tensorrt_llm/_torch/distributed/communicator.py (1)

• cp_size (38-39)

tensorrt_llm/mapping.py (1)

• moe_tp_rank (325-326)

⏰ 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 (3)

tensorrt_llm/_mnnvl_utils.py (3)

101-103: MOE-aware communicator grouping looks correct.

The new color calculation (mapping.pp_rank * mapping.cp_size + mapping.cp_rank) * mapping.moe_tp_size + mapping.moe_tp_rank properly incorporates MOE topology into the MPI communicator grouping. This ensures that processes with different MOE tensor parallel ranks are placed in separate communicators, which is essential for correct MOE alltoall operations when tp_size > ep_size.

---

315-315: Resolved: MOE workspace sizing aligned with moe_ep_size.

The C++ and Python definitions of get_moe_commworkspace_size_per_rank both take an ep_size parameter, confirming that using mapping.moe_ep_size is correct.

---

327-327: Confirm workspace sizing uses MOE EP size
C++ binding get_moe_prepare_workspace_size_per_rank(int ep_size) and existing unit tests invoke it with ep_size, so passing mapping.moe_ep_size is correct.

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[WeiHaocheng]

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