[2/6] Export Megatron fakequant quantizer state and YAML by kinjalpatel27 · Pull Request #2627 · NVIDIA/Model-Optimizer

kinjalpatel27 · 2026-10-01T22:11:26Z

What does this PR do?

Type of change: New feature

Exports quantizer_state.pth and vllm_fq_quantizer_state.yaml alongside Megatron fakequant weights. The YAML records the resolved per-quantizer settings using exported names. Cross-rank collection rejects conflicting recipes for the same name, so the sidecar cannot silently depend on rank order. Quantizer tensors are removed from model shards after the sidecars are collected.

Usage

export_mcore_gpt_to_hf_vllm_fq(model, pretrained_model_name_or_path, export_dir)
# export_dir contains quantizer_state.pth and vllm_fq_quantizer_state.yaml

Testing

Megatron fakequant export tests cover PP=1 and PP=2.
Two-rank YAML merge tests cover matching and conflicting recipes; both cases passed in the prepared Megatron environment.
CI is pending for this draft.

Before your PR is "Ready for review"

Is this change backward compatible?: ✅
If you copied code from any other sources or added a new PIP dependency, did you follow guidance in CONTRIBUTING.md?: N/A (no new runtime dependency or copied code)
Did you write any new necessary tests?: ✅
Did you update Changelog?: N/A (part of the unreleased fakequant export series)
Did you get Claude approval on this PR?: N/A (draft; review not requested yet)

Additional Information

Part of a six-PR stack. Merge in numeric order.

Stack

Merge in this order:

Summary by CodeRabbit

New Features
- vLLM fake-quant exports now include quantizer configuration recipes and quantizer state in sidecar files alongside the exported model.
Bug Fixes
- Quantizer state values retain their original precision in exported files.
- Conflicting quantizer recipes or replicated tensors with differing values are reported instead of silently replacing one another.
Tests
- Export validation now covers pipeline parallelism and verifies saved quantizer state, recipes, and model weights.

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Reviewing files that changed from the base of the PR and between 23b91d3 and 22efe43.

📒 Files selected for processing (2)

modelopt/torch/export/plugins/vllm_fakequant_megatron.py
tests/gpu_megatron/torch/export/test_vllm_fakequant_megatron_export.py

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📝 Walkthrough

Walkthrough

The exporter records quantizer configurations and routes recipes through temporary markers. It saves merged recipes and quantizer tensors as sidecars. Distributed merging rejects conflicting same-name recipes or tensors.

Changes

Fake-quant export

Layer / File(s)	Summary
Capture and route quantizer recipes `modelopt/torch/export/plugins/vllm_fakequant_megatron.py`	The exporter records quantizer configurations and adds temporary markers to route recipes through export mappings. It resolves and removes markers from layer shards.
Merge and save quantizer sidecars `modelopt/torch/export/plugins/vllm_fakequant_megatron.py`	The exporter merges recipes and quantizer tensors across ranks. It raises `ValueError` for conflicting same-name recipes or tensors and saves both sidecars. Quantizer tensors retain their dtype when copied to CPU.
Validate export and distributed merges `tests/gpu_megatron/torch/export/test_vllm_fakequant_megatron_export.py`	Tests cover pipeline-parallel sizes 1 and 2, direct exporter saving, sidecar contents, marker removal, model shards, quantizer hook counts, and matching or conflicting distributed values.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Quantizer
  participant VllmFqGPTModelExporter
  participant ExportMappings
  participant RecipeGather
  participant TensorGather
  participant MasterRank
  Quantizer->>VllmFqGPTModelExporter: provide quantizer configuration
  VllmFqGPTModelExporter->>ExportMappings: add recipe marker
  ExportMappings->>VllmFqGPTModelExporter: return layer shards
  VllmFqGPTModelExporter->>VllmFqGPTModelExporter: resolve recipes and remove markers
  VllmFqGPTModelExporter->>RecipeGather: provide rank-local recipes
  VllmFqGPTModelExporter->>TensorGather: provide quantizer tensors
  RecipeGather->>MasterRank: write quant_recipe.yaml
  TensorGather->>MasterRank: save quantizer_state.pth

Merge Risk: ⚪ Minimal · up to 22efe

The previously identified pre-save export failure is fixed. No actionable merge-blocking risk remains after normal checks.

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 71.43% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: exporting Megatron fakequant quantizer state and YAML sidecar data.
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.
Security Anti-Patterns	✅ Passed	No listed security anti-pattern is introduced. The only new torch.load calls use weights_only=True. trust_remote_code remains caller-configurable with a default of False. The patch adds no eval(), exe…

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kinjalpatel27 · 2026-10-02T19:47:16Z

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Inline comments:
Review comments at @modelopt/torch/export/plugins/vllm_fakequant_megatron.py:
- Around line 211-215: Initialize `_quantizer_state_for_recipe` and
`_quantizer_recipe_markers` in `VllmFqGPTModelExporter.__init__` before lazy
shard access can reach `_get_quantized_state`. In the extraction flow around
`_extract_quantizer_recipe_markers`, reset the marker list only when layer
shards have not already been constructed, preserving markers when shards are
cached.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7a2344f and 23b91d3.

📒 Files selected for processing (2)

modelopt/torch/export/plugins/vllm_fakequant_megatron.py
tests/gpu_megatron/torch/export/test_vllm_fakequant_megatron_export.py

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codecov · 2026-10-02T21:03:02Z

Codecov Report

❌ Patch coverage is 92.85714% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.38%. Comparing base (ef7eda7) to head (23d2713).

Files with missing lines	Patch %	Lines
...pt/torch/export/plugins/vllm_fakequant_megatron.py	92.85%	5 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           kinjal/vllm-fq-cli    #2627      +/-   ##
======================================================
- Coverage               78.38%   78.38%   -0.01%     
======================================================
  Files                     619      619              
  Lines                   69539    69601      +62     
======================================================
+ Hits                    54509    54557      +48     
- Misses                  15030    15044      +14

Flag	Coverage Δ
examples-diffusers	`21.08% <12.85%> (-0.26%)`	⬇️
examples-gpt-oss	`13.46% <12.85%> (-0.15%)`	⬇️
examples-hf_ptq	`22.89% <12.85%> (-0.28%)`	⬇️
examples-llm_distill	`13.52% <12.85%> (+<0.01%)`	⬆️
examples-llm_eval	`17.42% <12.85%> (-0.20%)`	⬇️
examples-llm_qat	`17.57% <12.85%> (-0.01%)`	⬇️
examples-llm_sparsity	`15.86% <12.85%> (-0.01%)`	⬇️
examples-megatron_bridge	`26.71% <12.85%> (+0.03%)`	⬆️
examples-specdec_bench	`13.23% <12.85%> (+<0.01%)`	⬆️
examples-speculative_decoding	`17.76% <12.85%> (-0.01%)`	⬇️
examples-torch_onnx	`21.63% <12.85%> (-0.30%)`	⬇️
examples-torch_trt	`15.25% <12.85%> (-0.16%)`	⬇️
examples-vllm_serve	`13.70% <12.85%> (+<0.01%)`	⬆️
gpu	`58.32% <92.85%> (+0.03%)`	⬆️
regression	`15.11% <12.85%> (-0.19%)`	⬇️

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kinjalpatel27 · 2026-10-02T21:55:24Z

/claude review

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

cjluo-nv

Bot review (gpt-6.1-sol) — DM the bot to share feedback.

Changes requested: sidecars can be overwritten, and quantizers exported later by the base exporter bypass recipe collection and cleanup.

Needs action:

Fix live MTP export ordering in vllm_fakequant_megatron.py so its quantizers reach both sidecars and no markers reach safetensors; add a regression test.
Protect export-owned sidecars from the base exporter's source-file copy; test a local source containing stale quant_recipe.yaml and quantizer_state.pth.
Preserve nondefault forward settings in _quantizer_configs, or explicitly reject unsupported settings; test rotation and unsigned quantization.
Explain in the PR body why the new YAML schema is preferable to existing quantizer_state()/get_modelopt_state() metadata used by HF fakequant export.

No action needed:

Existing test edits are justified by the added pipeline-parallel export coverage; assertions are strengthened.
Tests inspected, not executed.

cjluo-nv · 2026-10-02T23:15:16Z

            self._pop_quantizer_keys(_layer_sd)
+        self._pop_quantizer_keys(self._state_dict)

        super().save_pretrained(save_directory, pretrained_model_name_or_path)


Bot comment.

The base save_pretrained() invokes _get_mtp_state_dict() and merges live MTP tensors into the final layer shard after this subclass collects sidecars and removes quantizer keys. Those MTP mappings invoke this subclass's _get_quantized_state(), introducing fresh recipe markers and quantizer tensors which are then written directly to safetensors. Their recipes/tensors never reach the sidecars. Build and collect the live MTP state before sidecar extraction/cleanup, or add a hook at the base exporter's post-collection boundary. Please cover a quantized live MTP module.

cjluo-nv · 2026-10-02T23:15:16Z

+        self._extract_quantizer_recipe_markers(layer_state_dicts)
+
+        gather_mcore_vllm_fq_quantized_state_dict(self.model, layer_state_dicts, save_dir)
+        gather_mcore_vllm_fq_quantizer_recipe(self._quantizer_state_for_recipe, save_dir)


Bot comment.

Both export-owned sidecars are written before super().save_pretrained(). For a local source, the base exporter calls copy_non_safetensor_files_from_ckpt() without exclusions, which copies YAML and PTH files too. A source with an existing quant_recipe.yaml or quantizer_state.pth therefore silently overwrites the freshly collected output with stale settings/state. Exclude these names from source copying or arrange to publish the sidecars after that copy, and add a regression using stale source sidecars.

cjluo-nv · 2026-10-02T23:15:16Z

+    """
+    return {
+        get_unwrapped_name(name, module): {
+            "_num_bits": m.num_bits,


Bot comment.

These four fields are insufficient to reproduce resolved fakequant behavior. For example, enabled input quantizers can have rotation configured, or integer quantizers can use unsigned=True/nondefault narrow_range; their tensors plus this YAML cannot distinguish those settings from defaults. They also evade the cross-rank conflict check because the distinguishing fields are omitted. Preserve supported forward-affecting attributes (using existing quantizer metadata/config facilities where practical), or fail explicitly on unsupported settings. Add nondefault-setting and conflict coverage.

kinjalpatel27 force-pushed the kinjal/vllm-fq-megatron-sidecars branch 2 times, most recently from 2bb481f to 23b91d3 Compare October 2, 2026 00:21

kinjalpatel27 marked this pull request as ready for review October 2, 2026 19:49

kinjalpatel27 requested review from a team as code owners October 2, 2026 19:49

kinjalpatel27 requested review from cjluo-nv and removed request for a team October 2, 2026 19:49

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Comment thread modelopt/torch/export/plugins/vllm_fakequant_megatron.py

kinjalpatel27 force-pushed the kinjal/vllm-fq-cli branch from 5583299 to 2195aaf Compare October 2, 2026 19:52

kinjalpatel27 requested a review from a team as a code owner October 2, 2026 19:52

kinjalpatel27 force-pushed the kinjal/vllm-fq-megatron-sidecars branch from 23b91d3 to ba88e5d Compare October 2, 2026 20:50

kinjalpatel27 force-pushed the kinjal/vllm-fq-megatron-sidecars branch from ba88e5d to 22efe43 Compare October 2, 2026 21:17

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kinjalpatel27 force-pushed the kinjal/vllm-fq-cli branch from 725919d to ef7eda7 Compare October 2, 2026 22:27

kinjalpatel27 added 6 commits October 2, 2026 22:38

[2/5] Export Megatron fakequant quantizer state and YAML

0084db7

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

Reject conflicting cross-rank quantizer tensors

a4994d8

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

Rename vLLM fakequant recipe sidecar to quant_recipe.yaml

697921a

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

Explain cross-rank quantizer tensor merge guard

e12b9da

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

Name temporary quant recipe marker consistently

76dd482

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

Name quantizer recipe gather helper for its output

1c10419

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

Initialize recipe markers before lazy shard construction

23d2713

Signed-off-by: Kinjal Patel <kinjalpravin@nvidia.com>

kinjalpatel27 force-pushed the kinjal/vllm-fq-megatron-sidecars branch from 22efe43 to 23d2713 Compare October 2, 2026 22:41

cjluo-nv reviewed Oct 2, 2026

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[2/6] Export Megatron fakequant quantizer state and YAML - #2627

[2/6] Export Megatron fakequant quantizer state and YAML#2627
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