distributed-llm-pretraining-torchtitan✓ Pass

SKILL.md Content

---
name: distributed-llm-pretraining-torchtitan
description: Provides PyTorch-native distributed LLM pretraining using torchtitan with 4D parallelism (FSDP2, TP, PP, CP). Use when pretraining Llama 3.1, DeepSeek V3, or custom models at scale from 8 to 512+ GPUs with Float8, torch.compile, and distributed checkpointing.
version: 1.0.0
author: Orchestra Research
license: MIT
dependencies: [torch>=2.6.0, torchtitan>=0.2.0, torchao>=0.5.0]
platforms: [linux, macos]
metadata:
  hermes:
    tags: [Model Architecture, Distributed Training, TorchTitan, FSDP2, Tensor Parallel, Pipeline Parallel, Context Parallel, Float8, Llama, Pretraining]

---

# TorchTitan - PyTorch Native Distributed LLM Pretraining

## Quick start

TorchTitan is PyTorch's official platform for large-scale LLM pretraining with composable 4D parallelism (FSDP2, TP, PP, CP), achieving 65%+ speedups over baselines on H100 GPUs.

**Installation**:
```bash
# From PyPI (stable)
pip install torchtitan

# From source (latest features, requires PyTorch nightly)
git clone https://github.com/pytorch/torchtitan
cd torchtitan
pip install -r requirements.txt
```

**Download tokenizer**:
```bash
# Get HF token from https://huggingface.co/settings/tokens
python scripts/download_hf_assets.py --repo_id meta-llama/Llama-3.1-8B --assets tokenizer --hf_token=...
```

**Start training on 8 GPUs**:
```bash
CONFIG_FILE="./torchtitan/models/llama3/train_configs/llama3_8b.toml" ./run_train.sh
```

## Common workflows

### Workflow 1: Pretrain Llama 3.1 8B on single node

Copy this checklist:

```
Single Node Pretraining:
- [ ] Step 1: Download tokenizer
- [ ] Step 2: Configure training
- [ ] Step 3: Launch training
- [ ] Step 4: Monitor and checkpoint
```

**Step 1: Download tokenizer**

```bash
python scripts/download_hf_assets.py \
  --repo_id meta-llama/Llama-3.1-8B \
  --assets tokenizer \
  --hf_token=YOUR_HF_TOKEN
```

**Step 2: Configure training**

Edit or create a TOML config file:

```toml
# llama3_8b_custom.toml
[job]
dump_folder = "./outputs"
description = "Llama 3.1 8B training"

[model]
name = "llama3"
flavor = "8B"
hf_assets_path = "./assets/hf/Llama-3.1-8B"

[optimizer]
name = "AdamW"
lr = 3e-4

[lr_scheduler]
warmup_steps = 200

[training]
local_batch_size = 2
seq_len = 8192
max_norm = 1.0
steps = 1000
dataset = "c4"

[parallelism]
data_parallel_shard_degree = -1  # Use all GPUs for FSDP

[activation_checkpoint]
mode = "selective"
selective_ac_option = "op"

[checkpoint]
enable = true
folder = "checkpoint"
interval = 500
```

**Step 3: Launch training**

```bash
# 8 GPUs on single node
CONFIG_FILE="./llama3_8b_custom.toml" ./run_train.sh

# Or explicitly with torchrun
torchrun --nproc_per_node=8 \
  -m torchtitan.train \
  --job.config_file ./llama3_8b_custom.toml
```

**Step 4: Monitor and checkpoint**

TensorBoard logs are saved to `./outputs/tb/`:
```bash
tensorboard --logdir ./outputs/tb
```

### Workflow 2: Multi-node training with SLURM

```
Multi-Node Training:
- [ ] Step 1: Configure parallelism for scale
- [ ] Step 2: Set up SLURM script
- [ ] Step 3: Submit job
- [ ] Step 4: Resume from checkpoint
```

**Step 1: Configure parallelism for scale**

For 70B model on 256 GPUs (32 nodes):
```toml
[parallelism]
data_parallel_shard_degree = 32  # FSDP across 32 ranks
tensor_parallel_degree = 8        # TP within node
pipeline_parallel_degree = 1      # No PP for 70B
context_parallel_degree = 1       # Increase for long sequences
```

**Step 2: Set up SLURM script**

```bash
#!/bin/bash
#SBATCH --job-name=llama70b
#SBATCH --nodes=32
#SBATCH --ntasks-per-node=8
#SBATCH --gpus-per-node=8

srun torchrun \
  --nnodes=32 \
  --nproc_per_node=8 \
  --rdzv_backend=c10d \
  --rdzv_endpoint=$MASTER_ADDR:$MASTER_PORT \
  -m torchtitan.train \
  --job.config_file ./llama3_70b.toml
```

**Step 3: Submit job**

```bash
sbatch multinode_trainer.slurm
```

**Step 4: Resume from checkpoint**

Training auto-resumes if checkpoint exists in configured folder.

### Workflow 3: Enable Float8 training for H100s

Float8 provides 30-50% speedup on H100 GPUs.

```
Float8 Training:
- [ ] Step 1: Install torchao
- [ ] Step 2: Configure Float8
- [ ] Step 3: Launch with compile
```

**Step 1: Install torchao**

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

**Step 2: Configure Float8**

Add to your TOML config:
```toml
[model]
converters = ["quantize.linear.float8"]

[quantize.linear.float8]
enable_fsdp_float8_all_gather = true
precompute_float8_dynamic_scale_for_fsdp = true
filter_fqns = ["output"]  # Exclude output layer

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

**Step 3: Launch with compile**

```bash
CONFIG_FILE="./llama3_8b.toml" ./run_train.sh \
  --model.converters="quantize.linear.float8" \
  --quantize.linear.float8.enable_fsdp_float8_all_gather \
  --compile.enable
```

### Workflow 4: 4D parallelism for 405B models

```
4D Parallelism (FSDP + TP + PP + CP):
- [ ] Step 1: Create seed checkpoint
- [ ] Step 2: Configure 4D parallelism
- [ ] Step 3: Launch on 512 GPUs
```

**Step 1: Create seed checkpoint**

Required for consistent initialization across PP stages:
```bash
NGPU=1 CONFIG_FILE=./llama3_405b.toml ./run_train.sh \
  --checkpoint.enable \
  --checkpoint.create_seed_checkpoint \
  --parallelism.data_parallel_shard_degree 1 \
  --parallelism.tensor_parallel_degree 1 \
  --parallelism.pipeline_parallel_degree 1
```

**Step 2: Configure 4D parallelism**

```toml
[parallelism]
data_parallel_shard_degree = 8   # FSDP
tensor_parallel_degree = 8       # TP within node
pipeline_parallel_degree = 8     # PP across nodes
context_parallel_degree = 1      # CP for long sequences

[training]
local_batch_size = 32
seq_len = 8192
```

**Step 3: Launch on 512 GPUs**

```bash
# 64 nodes x 8 GPUs = 512 GPUs
srun torchrun --nnodes=64 --nproc_per_node=8 \
  -m torchtitan.train \
  --job.config_file ./llama3_405b.toml
```

## When to use vs alternatives

**Use TorchTitan when:**
- Pretraining LLMs from scratch (8B to 405B+)
- Need PyTorch-native solution without third-party dependencies
- Require composable 4D parallelism (FSDP2, TP, PP, CP)
- Training on H100s with Float8 support
- Want interoperable checkpoints with torchtune/HuggingFace

**Use alternatives instead:**
- **Megatron-LM**: Maximum performance for NVIDIA-only deployments
- **DeepSpeed**: Broader ZeRO optimization ecosystem, inference support
- **Axolotl/TRL**: Fine-tuning rather than pretraining
- **LitGPT**: Educational, smaller-scale training

## Common issues

**Issue: Out of memory on large models**

Enable activation checkpointing and reduce batch size:
```toml
[activation_checkpoint]
mode = "full"  # Instead of "selective"

[training]
local_batch_size = 1
```

Or use gradient accumulation:
```toml
[training]
local_batch_size = 1
global_batch_size = 32  # Accumulates gradients
```

**Issue: TP causes high memory with async collectives**

Set environment variable:
```bash
export TORCH_NCCL_AVOID_RECORD_STREAMS=1
```

**Issue: Float8 training not faster**

Float8 only benefits large GEMMs. Filter small layers:
```toml
[quantize.linear.float8]
filter_fqns = ["attention.wk", "attention.wv", "output", "auto_filter_small_kn"]
```

**Issue: Checkpoint loading fails after parallelism change**

Use DCP's resharding capability:
```bash
# Convert sharded checkpoint to single file
python -m torch.distributed.checkpoint.format_utils \
  dcp_to_torch checkpoint/step-1000 checkpoint.pt
```

**Issue: Pipeline parallelism initialization**

Create seed checkpoint first (see Workflow 4, Step 1).

## Supported models

| Model | Sizes | Status |
|-------|-------|--------|
| Llama 3.1 | 8B, 70B, 405B | Production |
| Llama 4 | Various | Experimental |
| DeepSeek V3 | 16B, 236B, 671B (MoE) | Experimental |
| GPT-OSS | 20B, 120B (MoE) | Experimental |
| Qwen 3 | Various | Experimental |
| Flux | Diffusion | Experimental |

## Performance benchmarks (H100)

| Model | GPUs | Parallelism | TPS/GPU | Techniques |
|-------|------|-------------|---------|------------|
| Llama 8B | 8 | FSDP | 5,762 | Baseline |
| Llama 8B | 8 | FSDP+compile+FP8 | 8,532 | +48% |
| Llama 70B | 256 | FSDP+TP+AsyncTP | 876 | 2D parallel |
| Llama 405B | 512 | FSDP+TP+PP | 128 | 3D parallel |

## Advanced topics

**FSDP2 configuration**: See [references/fsdp.md](references/fsdp.md) for detailed FSDP2 vs FSDP1 comparison and ZeRO equivalents.

**Float8 training**: See [references/float8.md](references/float8.md) for tensorwise vs rowwise scaling recipes.

**Checkpointing**: See [references/checkpoint.md](references/checkpoint.md) for HuggingFace conversion and async checkpointing.

**Adding custom models**: See [references/custom-models.md](references/custom-models.md) for TrainSpec protocol.

## Resources

- GitHub: https://github.com/pytorch/torchtitan
- Paper: https://arxiv.org/abs/2410.06511
- ICLR 2025: https://iclr.cc/virtual/2025/poster/29620
- PyTorch Forum: https://discuss.pytorch.org/c/distributed/torchtitan/44

License

Declared license: MIT

MIT License

Copyright (c) 2025 Orchestra Research

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

View the license in the source repository — the version published there is authoritative.