Fix qwen3.5 mtp eplb related issues#19767
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Summary of ChangesHello, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed! This pull request enhances the SGLang framework by integrating Expert Parallelism Load Balancing (EPLB) capabilities for Qwen3.5 and Qwen3-Next models, particularly in Model Tensor Parallelism (MTP) environments. The changes involve introducing a specific flag to differentiate model behaviors, optimizing expert weight loading, and carefully managing expert distribution recording during the model's forward pass to ensure compatibility and efficiency within the distributed inference system. Highlights
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Code Review
This pull request introduces support for EPLB (Expert Placement and Load Balancing) with MTP (Multi-Turn Prefill) for Qwen3.5 and Qwen3-Next models. The changes primarily involve adding an is_nextn flag to differentiate model variants and control expert location dispatching. Additionally, expert distribution recording is disabled for MTP models to avoid unwanted side effects.
The implementation is consistent across the affected model files. I have one suggestion to refactor a duplicated helper method to improve code maintainability. Overall, the changes look good and align with the PR's objectives.
| @classmethod | ||
| def get_model_config_for_expert_location(cls, config): | ||
| text_config = getattr(config, "text_config", config) | ||
| return ModelConfigForExpertLocation( | ||
| num_layers=text_config.num_hidden_layers, | ||
| num_logical_experts=text_config.num_experts, | ||
| num_groups=None, | ||
| ) |
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This get_model_config_for_expert_location method is identical to the one in Qwen3_5MoeForConditionalGeneration (in python/sglang/srt/models/qwen3_5.py). To improve maintainability and reduce code duplication, consider extracting this logic into a shared helper function or a mixin class.
For example, you could create a helper function in a utility module:
from sglang.srt.eplb.expert_location import ModelConfigForExpertLocation
def get_model_config_for_expert_location_with_text_config(config):
text_config = getattr(config, "text_config", config)
return ModelConfigForExpertLocation(
num_layers=text_config.num_hidden_layers,
num_logical_experts=text_config.num_experts,
num_groups=None,
)And then call this helper from both places.
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**Upstream status** as of 2026-04-06: - Qwen3.5: fixed via [PR #19767](sgl-project/sglang#19767) (merged 2026-03-09, included in v0.5.10) - Qwen3: [PR #21461](sgl-project/sglang#21461) — closed without merge 2026-03-30 (CI failure), superseded by #21822 - Qwen3: [PR #21822](sgl-project/sglang#21822) — new fix opened 2026-03-26, addresses `AttributeError: 'LazyValue' object has no attribute 'keys'` in `eplb_manager.py` for Qwen3 MoE. Code review 2026-04-04 by `Fridge003` and `Evgueni-Petrov-aka-espetrov`. Alternative `LazyValue.__getattr__` approach proposed (avoids modifying the model class). **Approved** by `Fridge003` on 2026-04-06, CI rerun triggered — awaiting merge. (Duplicate [PR #21820](sgl-project/sglang#21820) was closed same day in favour of #21822.) Not in v0.5.10 When `--enable-eplb` is active with EP, the `EPLBManager` crashes after its first rebalance interval (default: 1000 forward passes): - SGLang PR #17137 — non-Marlin WNA16MoE port (does not fix EP bug) - SGLang #14158 — update_weights_from_tensor for WNA16MoE (unrelated) - SGLang [PR #13715](sgl-project/sglang#13715) — fix EPLB + FP4 weight tensor filtering (merged, different issue) - SGLang [PR #20963](sgl-project/sglang#20963) — Nvidia modelopt refactoring (1/N). Under active review: reviewer `Edwardf0t1` asked for end-to-end verification 2026-03-31, author `wenscarl` responded 2026-04-01 and posted 3 further inline review responses 2026-04-06. Not stalled but awaiting approval. Migrates the NVFP4 code as-is — expected vehicle for EP-awareness fixes (#20869, #21630). Watch this PR for resolution of the NVFP4 input_scale and CutlassMoEParams bugs - SGLang [PR #21822](sgl-project/sglang#21822) — new EPLB/Qwen3 fix (opened 2026-03-26). Addresses `LazyValue.keys()` AttributeError. Code review 2026-04-04 by `Fridge003` and `Evgueni-Petrov-aka-espetrov`. Alternative `LazyValue.__getattr__` approach proposed. **Approved** by `Fridge003` on 2026-04-06, CI rerun triggered — awaiting merge "Good code is like humor: when you have to explain it, it’s bad." - Cory House P.S.: Code reviews and approvals are crucial for maintaining high-quality software.
Motivation
support eplb with mtp for Qwen3.5 & Qwen3-Next
Modifications
Accuracy Tests
Benchmarking and Profiling
Checklist
Review Process
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