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Initialize E8M0 FP8 scale parameters#1

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jasl merged 1 commit intojasl:ds4-sm120from
sniper35:fix-e8m0-scale-init
Apr 27, 2026
Merged

Initialize E8M0 FP8 scale parameters#1
jasl merged 1 commit intojasl:ds4-sm120from
sniper35:fix-e8m0-scale-init

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@sniper35 sniper35 commented Apr 27, 2026 •
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Purpose

This is to add a missing initialization of E8M0 FP8 scale parameters. It doesn't fix any reported issue on the original branch, it's just a defensive patch to avoid a random scale value is read when it's suppoesed to be 0 in the following cases:

  • a loader edge case leaves padded scale elements untouched;
  • a missing or mismatched checkpoint tensor does not fully populate the parameter;
  • a sharded/partitioned load path copies only part of the parameter;
  • a test or future code path reads the parameter before load completion;
  • a fallback path uses allocated scale tensors as workspace/defaults.

Test Plan

//test-e8m0.py

import torch
import vllm.model_executor.parameter as parameter
from vllm.model_executor.layers.quantization.utils.fp8_utils import create_fp8_scale_parameter
from vllm.model_executor.parameter import BlockQuantScaleParameter

parameter.get_tensor_model_parallel_rank = lambda: 0
parameter.get_tensor_model_parallel_world_size = lambda: 1

scale = create_fp8_scale_parameter(
  BlockQuantScaleParameter,
  output_partition_sizes=[128],
  input_size_per_partition=128,
  block_size=[128, 128],
  weight_loader=None,
  scale_dtype=torch.float8_e8m0fnu,
)

print("raw:", scale.data.view(torch.uint8))
print("fp32:", scale.data.to(torch.float32))

Test Result

After the fix, the raw byte is always:

tensor([[0]], dtype=torch.uint8)
fp32: tensor([[5.8775e-39]])

Before the fix, the raw byte can be arbitrary, i.e:

raw: tensor([[159]], dtype=torch.uint8)
fp32: tensor([[4.2950e+09]])

or

raw: tensor([[125]], dtype=torch.uint8)
fp32: tensor([[0.2500]])
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  • (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

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github-actions Bot commented Apr 27, 2026

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@sniper35 sniper35 mentioned this pull request Apr 27, 2026
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jasl commented Apr 27, 2026

Thank you

@jasl jasl merged commit 8bd4332 into jasl:ds4-sm120 Apr 27, 2026
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2 participants

@sniper35 @jasl