INT4 Quantization-Aware Training

When the model is large enough that even FP8 will not fit on one node, the options are spreading across more nodes (and paying cross-node bandwidth) or quantizing further. Miles ships an INT4 W4A16 quant-aware-training pipeline. On an 8 × 141 GB H200 node, this is the path used to fit very large models in a single box. The recipe is inspired by the Kimi K2-Thinking team’s report.

What W4A16 means

Term	Bits	Notes
W4	4-bit weights	Group-quantized (typical group size 32–128)
A16	16-bit activations	BF16 activation pathway

The combination keeps the weights small (memory-bound) while activations stay in BF16 (math-bound). With QAT the model trains with the quantization in the loop, so the weights round well during inference.

Calibration

Convert a BF16 HuggingFace checkpoint to INT4 with tools/convert_hf_to_int4.py (GPTQ via llmcompressor):

python tools/convert_hf_to_int4.py \
   --input-dir  /root/MyModel \
   --output-dir /root/MyModel-INT4 \
   --data-dir   /root/calibration_dataset \
   --quant-type W4A16 \
   --num-calibration-samples 256 \
   --quant-group-size 128

Flag	Default	Notes
`--quant-type`	`W4A16`	Also accepts `W8A16`.
`--num-calibration-samples`	`256`	Calibration set size.
`--quant-group-size`	`32`	GPTQ group size; `128` is also common.
`--max-sequence-length`	`2048`	Calibration sequence length.
`--dampening-frac`	`0.01`	GPTQ damping.
`--trust-remote-code`	off	Pass when the HF config requires custom code.

The output is a HuggingFace directory with per-group INT4 weights and scales. Point --hf-checkpoint at it; SGLang autodetects the quantization at load time.

Enabling QAT

QAT is currently driven by environment variables passed through Ray’s runtime env rather than CLI flags. The canonical recipe is examples/low_precision/run-qwen3-30B-A3B-int4.sh:

RUNTIME_ENV_JSON='{
  "env_vars": {
    "OPEN_TRAINING_INT4_FAKE_QAT_FLAG": "1",
    "OPEN_TRAINING_INT4_GROUP_SIZE": "128"
  }
}'

ray job submit --address="http://127.0.0.1:8265" \
   --runtime-env-json="${RUNTIME_ENV_JSON}" \
   -- python3 train.py ...

Pair the INT4 --hf-checkpoint with a BF16 --ref-load torch_dist directory so the KL anchor stays full-precision.

Tuning

Symptom	Try
Eval reward drops noticeably vs BF16	Lower `OPEN_TRAINING_INT4_GROUP_SIZE` (e.g. 64), or recalibrate with more samples.
Slower than BF16	Confirm `--sglang-cuda-graph-bs` covers your batch sizes.

Pairs with

R3. Keeps MoE routing stable across the quantized forward.
P2P weight transfer. INT4 weights are 4× smaller, so weight sync transfers less data.
Speculative decoding. Compounds for end-to-end rollout speedup.

When QAT is not appropriate

The model fits comfortably without it.
The model architecture is still in development; introduce QAT after a BF16 baseline.
Tasks that are highly precision-sensitive (some math and safety eval suites).

Documentation Index

​What W4A16 means

​Calibration

​Enabling QAT

​Tuning

​Pairs with

​When QAT is not appropriate