Low Precision RL

A common failure mode in MoE RL is precision drift between training and inference. Pipelines that train in BF16 and serve in FP8 accumulate per-layer numerical disagreement, which compounds into divergent log-probabilities and gradients pointing in unintended directions. Miles supports a unified low-precision path where rollout and training share the same quantization logic on the forward pass. Backward passes and master weights stay in BF16. The same path is wired up for three formats today — block-wise FP8, MXFP8, and (experimental) NVFP4 — plus the lower-friction “BF16 train + FP8 inference” mode that’s useful when standing up a new model architecture.

Choose a precision

Format	Block layout	Hardware	Models tested	Maturity
BF16	—	All NVIDIA + AMD MI300X / MI325 / MI350 / MI355X	All	Baseline
FP8 block-wise (DeepSeek-style)	128×128, FP32 scales	Hopper (H100 / H200), Blackwell (B200+)	Qwen3-4B, Qwen3-30B-A3B, DeepSeek-V3 / R1	Generally available
MXFP8	1×32, UE8M0 scales	Blackwell only (B200, B300, GB200, GB300)	Qwen3-30B-A3B	Beta
NVFP4 (E2M1)	1×16, two-level (FP8 + FP32) scales, MoE experts only	Blackwell, following the TransformerEngine NVFP4 reference	—	Experimental

Rollout × training compatibility

Each row is a rollout (inference) precision; each column is the trainer’s forward precision. ✅ = supported; ✗ = not supported.

Rollout \ Train	BF16	FP8 block-wise	MXFP8	NVFP4
BF16	✅ baseline	✗	✗	✗
FP8 block-wise	✅	✅ Hopper + Blackwell	✗	✗
MXFP8	✅	✗	✅ Blackwell	✗
NVFP4	✗	✗	✗	🚧 coming soon

Two rules enforced in the reference script (scripts/run_qwen3_30b_a3b.py):

--rollout-mxfp8 and --rollout-fp8 are mutually exclusive.
--train-mxfp8 requires --rollout-mxfp8 (no MXFP8-train + FP8-rollout combo).

Unified training recipe

Stage	Typical pipeline	Miles unified low-precision
Rollout (forward)	FP8 / MXFP8 GEMM	FP8 / MXFP8 GEMM
Trainer (forward)	BF16 GEMM	FP8 / MXFP8 GEMM with matching quant config
Trainer (backward)	BF16 grads	BF16 backward (master weights in BF16)
Optimizer	BF16 master	BF16 master

The forward pass in training matches rollout. The backward pass and master weights remain BF16, which keeps the gradient signal stable. Weights are re-quantized on each weight-update sync to sglang.

Modes

1. BF16 train + FP8 inference

The lowest-friction path. SGLang loads FP8 weights while the trainer keeps a BF16 torch_dist checkpoint. There is precision drift between the two paths; on MoE workloads, pair this with R3 (and optionally TIS).

hf download Qwen/Qwen3-30B-A3B-FP8 --local-dir /root/Qwen3-30B-A3B-FP8

CKPT_ARGS=(
   --hf-checkpoint /root/Qwen3-30B-A3B-FP8        # FP8 weights for SGLang
   --ref-load      /root/Qwen3-30B-A3B_torch_dist  # BF16 torch_dist for trainer
)

2. Unified block-wise FP8 (DeepSeek-style)

Rollout and training share the same block-wise FP8 quantization. This is the recipe to use on Hopper, and the recipe DeepSeek-V3 / DeepSeek-R1 ship in. Block layout is 128×128 with FP32 scales.

--transformer-impl transformer_engine
--bf16
--fp8-format e4m3
--fp8-recipe blockwise

# Optional, for MoE numerical stability
--use-tis

Flag	Effect
`--transformer-impl transformer_engine`	Routes Megatron’s forward through TransformerEngine so FP8 GEMM is engaged.
`--fp8-format e4m3`	Forward FP8 format used by TransformerEngine.
`--fp8-recipe blockwise`	128×128 block-wise quantization; sglang must serve weights in the matching layout.
`--use-tis`	Truncated Importance Sampling for residual precision drift.

Set NVTE_FP8_BLOCK_SCALING_FP32_SCALES=1 in the Ray runtime env to use FP32 scales (miles/ray/actor_group.py already sets this in the actor env). For models that already ship 128×128 block-wise FP8 weights (DeepSeek-V3, DeepSeek-R1, Qwen/Qwen3-30B-A3B-FP8), point --hf-checkpoint at the block-wise FP8 directory and let SGLang autodetect. Otherwise convert with tools/convert_hf_to_fp8.py. For MoE workloads, also consider --use-rollout-routing-replay (R3). The canonical recipe leaves it commented out by default but the flag is available. Reference recipes:

examples/low_precision/run-qwen3-4b-fp8.sh — single-node Qwen3-4B.
examples/low_precision/run-qwen3-30b-a3b-fp8-two-nodes.sh — two-node Qwen3-30B-A3B.

3. Unified MXFP8 (Blackwell)

MXFP8 uses a finer block layout (1×32) with UE8M0 (power-of-two) scales packed as uint8. Weights are stored as float8_e4m3fn. This is the format wired into the Blackwell path of the Qwen3-30B-A3B reference script. Hardware: Blackwell only — B200, B300, GB200, GB300. The reference script asserts the GPU class on enable (scripts/run_qwen3_30b_a3b.py). Train flags — same Megatron knobs as FP8, with mxfp8 recipe:

--transformer-impl transformer_engine
--bf16
--fp8-format e4m3
--fp8-recipe mxfp8

Rollout flags — sglang side:

--sglang-fp8-gemm-backend triton
--sglang-moe-runner-backend cutlass
# DeepEP / DeepGEMM are not yet supported with MXFP8 in sglang;
# do not set --sglang-moe-a2a-backend deepep here.

Conversion. There is no HF-shipped MXFP8 checkpoint for Qwen3 today, so convert from BF16 first:

python tools/convert_hf_to_mxfp8.py \
   --model-dir /root/models/Qwen3-30B-A3B \
   --save-dir  /root/models/Qwen3-30B-A3B-MXFP8

The tool quantizes every 2D *.weight tensor whose last dim is divisible by 32, except layernorm, embed, router, mlp.gate., norm, lm_head, eh_proj, weights_proj (tools/convert_hf_to_mxfp8.py). The HF config is rewritten with:

{
  "quant_method": "mxfp8",
  "fmt": "e4m3",
  "weight_block_size": [1, 32],
  "scale_fmt": "ue8m0",
  "activation_scheme": "dynamic"
}

Caveats:

No DeepEP / DeepGEMM yet — MoE all-to-all uses the cutlass MoE runner, which does not currently support EP. Plan EP/PP accordingly.
--train-mxfp8 requires --rollout-mxfp8 (the script enforces this).

Reference recipe: scripts/run_qwen3_30b_a3b.py with --rollout-mxfp8 --train-mxfp8 --hardware B200. There is no dedicated shell script under examples/low_precision/ yet.

4. NVFP4 (experimental)

NVFP4 is FP4 E2M1 with 1D block scaling (group size 16) and a two-level scale (per-block FP8 + per-tensor FP32), following the TransformerEngine NVFP4 reference. Today only MoE expert GEMMs are quantized; dense layers stay in their original precision. The full unified NVFP4 recipe is in development.

Hardware support

GPU	BF16	FP8 block-wise	MXFP8	NVFP4
NVIDIA H100 / H200	✅	✅	✗	✗
NVIDIA B200 / B300 / GB200 / GB300	✅	✅	✅	🚧 in development
NVIDIA A100	✅	✗	✗	✗
AMD MI300X / MI325 / MI350 / MI355X	✅	✗	✗	✗

When BF16 is enough

Dense models below ~30 B.
A100 hardware (no FP8 GEMM).
AMD hardware today.
Bring-up of a new model architecture, where clean BF16 numerics simplify debugging.

​Choose a precision

​Rollout × training compatibility

​Unified training recipe

​Modes

​1. BF16 train + FP8 inference

​2. Unified block-wise FP8 (DeepSeek-style)

​3. Unified MXFP8 (Blackwell)

​4. NVFP4 (experimental)

​Hardware support

​When BF16 is enough