hermes-agent/skills/mlops/torchtitan/references/float8.md

Float8 Training in TorchTitan

Float8 training provides substantial speedups for models where GEMMs are large enough that the FP8 tensorcore speedup outweighs dynamic quantization overhead.

Hardware Requirements

• NVIDIA H100 or newer GPUs (FP8 Tensor Cores)
• Blackwell GPUs for MXFP8 training

Installation

USE_CPP=0 pip install git+https://github.com/pytorch/ao.git

Usage: Tensorwise Scaling

Standard Float8 with tensorwise dynamic scaling:

CONFIG_FILE="./torchtitan/models/llama3/train_configs/llama3_8b.toml" ./run_train.sh \
  --model.converters="quantize.linear.float8" \
  --quantize.linear.float8.enable_fsdp_float8_all_gather \
  --quantize.linear.float8.precompute_float8_dynamic_scale_for_fsdp \
  --compile.enable

Key Arguments

Argument	Description
--model.converters="quantize.linear.float8"	Swap nn.Linear with Float8Linear
--quantize.linear.float8.enable_fsdp_float8_all_gather	Communicate in float8 to save bandwidth
--quantize.linear.float8.precompute_float8_dynamic_scale_for_fsdp	Single all-reduce for all AMAX/scales
--compile.enable	Required - fuses float8 scaling/casting kernels

Usage: Rowwise Scaling

Higher accuracy than tensorwise scaling:

CONFIG_FILE="./torchtitan/models/llama3/train_configs/llama3_8b.toml" ./run_train.sh \
  --model.converters="quantize.linear.float8" \
  --quantize.linear.float8.recipe_name rowwise \
  --compile.enable

Filtering Layers

Not all layers benefit from Float8. Filter small layers:

--quantize.linear.float8.filter_fqns="attention.wk,attention.wv,output"

Auto-filtering

Automatically skip layers too small to benefit:

--quantize.linear.float8.filter_fqns="auto_filter_small_kn"

Thresholds based on H100 microbenchmarks where speedup > overhead.

TOML Configuration

[model]
converters = ["quantize.linear.float8"]

[quantize.linear.float8]
enable_fsdp_float8_all_gather = true
precompute_float8_dynamic_scale_for_fsdp = true
filter_fqns = ["output", "auto_filter_small_kn"]

[compile]
enable = true
components = ["model", "loss"]

How Float8 Works with Distributed Training

Single Device

Cast input and weight to float8 inside forward before calling torch._scaled_mm:

# Float8 matmul requires scales
torch._scaled_mm(input_fp8, weight_fp8, scale_a=scale_input, scale_b=scale_weight)

FSDP + Float8

1. Cast sharded high-precision weights (1/N per rank) to float8
2. Perform float8 all-gather (saves bandwidth vs bf16/fp32)
3. Communicate max(abs) across ranks for scale computation
4. At forward start, have unsharded float8 weights ready

Net benefit: Float8 all-gather + amax communication can beat bf16/fp32 all-gather, depending on world size and message size.

TP + Float8

• Input: Cast sharded input to float8, all-gather in float8
• Weights: Communicate max(abs) for sharded weights
• Matmul: Float8 input (unsharded) x float8 weight (sharded) with global scales

Scaling Strategies

Strategy	Status	Description
Tensorwise dynamic	Stable	Single scale per tensor
Rowwise dynamic	Alpha	Scale per row, higher accuracy

Performance Gains

From benchmarks on H100:

Configuration	TPS/GPU	vs Baseline
FSDP only	5,762	-
FSDP + compile	6,667	+16%
FSDP + compile + Float8	8,532	+48%

Determining Float8 Benefit

Check torchao microbenchmarks for forward+backward pass speedups on "layer norm => linear => sigmoid" for different M,N,K sizes.

Rule of thumb: GEMMs with K,N > 4096 typically benefit from Float8.

MXFP8 Training (Blackwell)

For NVIDIA Blackwell GPUs, TorchTitan supports MXFP8 (Microscaling FP8) for both dense and MoE models. See docs/mxfp8.md for details.