post-training-torchforge

torchforge: PyTorch-Native Agentic RL Library

torchforge is Meta's PyTorch-native RL library that separates infrastructure concerns from algorithm concerns. It enables rapid RL research by letting you focus on algorithms while handling distributed training, inference, and weight sync automatically.

When to Use torchforge

Choose torchforge when you need:

• Clean separation between RL algorithms and infrastructure
• PyTorch-native abstractions (no Ray dependency)
• Easy algorithm experimentation (GRPO, DAPO, SAPO in ~100 lines)
• Scalable training with Monarch actor system
• Integration with TorchTitan for model parallelism

Consider alternatives when:

• You need production-ready stability → use miles or verl
• You want Megatron-native training → use slime
• torchforge is experimental and APIs may change

Key Features

• Algorithm isolation: Implement RL algorithms without touching infrastructure
• Scalability: From single GPU to thousands via Monarch
• Modern stack: TorchTitan (training), vLLM (inference), TorchStore (sync)
• Loss functions: GRPO, DAPO, CISPO, GSPO, SAPO built-in

Architecture Overview

┌─────────────────────────────────────────────────────────┐
│ Application Layer (Your Code)                           │
│ - Define reward models, loss functions, sampling        │
└─────────────────────┬───────────────────────────────────┘
                      │
┌─────────────────────▼───────────────────────────────────┐
│ Forge API Layer                                         │
│ - Episode, Group dataclasses                           │
│ - Service interfaces (async/await)                      │
└─────────────────────┬───────────────────────────────────┘
                      │
┌─────────────────────▼───────────────────────────────────┐
│ Distributed Services (Monarch)                          │
│ ├── Trainer (TorchTitan FSDP)                          │
│ ├── Generator (vLLM inference)                          │
│ ├── Reference Model (frozen KL baseline)               │
│ └── Reward Actors (compute rewards)                    │
└─────────────────────────────────────────────────────────┘

Installation

# Create environment
conda create -n forge python=3.12
conda activate forge

# Install (handles PyTorch nightly + dependencies)
./scripts/install.sh

# Verify
python -c "import torch, forge, vllm; print('OK')"

ROCm Installation

./scripts/install_rocm.sh

Quick Start

SFT Training (2+ GPUs)

python -m apps.sft.main --config apps/sft/llama3_8b.yaml

GRPO Training (3+ GPUs)

python -m apps.grpo.main --config apps/grpo/qwen3_1_7b.yaml

---

Workflow 1: GRPO Training for Math Reasoning

Use this workflow for training reasoning models with group-relative advantages.

Prerequisites Checklist

• [ ] 3+ GPUs (GPU0: trainer, GPU1: ref_model, GPU2: generator)
• [ ] Model from HuggingFace Hub
• [ ] Training dataset (GSM8K, MATH, etc.)

Step 1: Create Configuration

# config/grpo_math.yaml
model: "Qwen/Qwen2.5-7B-Instruct"

dataset:
  path: "openai/gsm8k"
  split: "train"
  streaming: true

training:
  batch_size: 4
  learning_rate: 1e-6
  seq_len: 4096
  dtype: bfloat16
  gradient_accumulation_steps: 4

grpo:
  n_samples: 8           # Responses per prompt
  clip_low: 0.2
  clip_high: 0.28
  beta: 0.1              # KL penalty coefficient
  temperature: 0.7

services:
  generator:
    procs: 1
    num_replicas: 1
    with_gpus: true
  trainer:
    procs: 1
    num_replicas: 1
    with_gpus: true
  ref_model:
    procs: 1
    num_replicas: 1
    with_gpus: true

Step 2: Define Reward Function

# rewards.py
# Reward functions are in forge.data.rewards
from forge.data.rewards import MathReward, ThinkingReward
import re

# Or define your own reward function
class CustomMathReward:
    def __call__(self, prompt: str, response: str, target: str) -> float:
        # Extract answer from response
        match = re.search(r'\\boxed{([^}]+)}', response)
        if not match:
            return 0.0

        answer = match.group(1).strip()
        return 1.0 if answer == target else 0.0

Step 3: Launch Training

python -m apps.grpo.main --config config/grpo_math.yaml

Step 4: Monitor Progress

• [ ] Check W&B dashboard for loss curves
• [ ] Verify entropy is decreasing (policy becoming more deterministic)
• [ ] Monitor KL divergence (should stay bounded)

---

Workflow 2: Custom Loss Function

Use this workflow to implement new RL algorithms.

Step 1: Create Loss Class

# src/forge/losses/custom_loss.py
import torch
import torch.nn as nn

class CustomLoss(nn.Module):
    def __init__(self, clip_range: float = 0.2, beta: float = 0.1):
        super().__init__()
        self.clip_range = clip_range
        self.beta = beta

    def forward(
        self,
        logprobs: torch.Tensor,
        ref_logprobs: torch.Tensor,
        advantages: torch.Tensor,
        padding_mask: torch.Tensor,
    ) -> torch.Tensor:
        # Compute importance ratio
        ratio = torch.exp(logprobs - ref_logprobs)

        # Clipped policy gradient
        clipped_ratio = torch.clamp(
            ratio,
            1 - self.clip_range,
            1 + self.clip_range
        )
        pg_loss = -torch.min(ratio * advantages, clipped_ratio * advantages)

        # KL penalty
        kl = ref_logprobs - logprobs

        # Apply mask and aggregate
        masked_loss = (pg_loss + self.beta * kl) * padding_mask
        loss = masked_loss.sum() / padding_mask.sum()

        return loss

Step 2: Integrate into Application

# apps/custom/main.py
from forge.losses.custom_loss import CustomLoss

loss_fn = CustomLoss(clip_range=0.2, beta=0.1)

# In training loop
loss = loss_fn(
    logprobs=logprobs,
    ref_logprobs=ref_logprobs,
    advantages=advantages,
    padding_mask=padding_mask,
)

---

Workflow 3: Multi-GPU Distributed Training

Use this workflow for scaling to multiple GPUs or nodes.

Configuration for Distributed

# config/distributed.yaml
model: "meta-llama/Meta-Llama-3.1-8B-Instruct"

parallelism:
  tensor_parallel_degree: 2    # Split model across GPUs
  pipeline_parallel_degree: 1
  data_parallel_shard_degree: 2

services:
  generator:
    procs: 2                   # 2 processes for TP=2
    num_replicas: 1
    with_gpus: true
  trainer:
    procs: 2
    num_replicas: 1
    with_gpus: true

Launch with SLURM

# Submit job
sbatch --nodes=2 --gpus-per-node=8 run_grpo.sh

Launch Locally (Multi-GPU)

# 8 GPU setup
python -m apps.grpo.main \
    --config config/distributed.yaml \
    --trainer.procs 4 \
    --generator.procs 4

---

Core API Reference

Training Batch Format

torchforge uses dictionary-based batches for training:

# inputs: list of dicts with torch.Tensor values
inputs = [{"tokens": torch.Tensor}]

# targets: list of dicts with training signals
targets = [{
    "response": torch.Tensor,
    "ref_logprobs": torch.Tensor,
    "advantages": torch.Tensor,
    "padding_mask": torch.Tensor
}]

# train_step returns loss as float
loss = trainer.train_step(inputs, targets)

Completion

Generated output from vLLM:

@dataclass
class Completion:
    text: str              # Generated text
    token_ids: list[int]   # Token IDs
    logprobs: list[float]  # Log probabilities
    metadata: dict         # Custom metadata

---

Built-in Loss Functions

Loss Functions

Loss functions are in the forge.losses module:

from forge.losses import SimpleGRPOLoss, ReinforceLoss

# SimpleGRPOLoss for GRPO training
loss_fn = SimpleGRPOLoss(beta=0.1)

# Forward pass
loss = loss_fn(
    logprobs=logprobs,
    ref_logprobs=ref_logprobs,
    advantages=advantages,
    padding_mask=padding_mask
)

ReinforceLoss

from forge.losses.reinforce_loss import ReinforceLoss

# With optional importance ratio clipping
loss_fn = ReinforceLoss(clip_ratio=0.2)

---

Common Issues and Solutions

Issue: Not Enough GPUs

Symptoms: "Insufficient GPU resources" error

Solutions:

# Reduce service requirements
services:
  generator:
    procs: 1
    with_gpus: true
  trainer:
    procs: 1
    with_gpus: true
  # Remove ref_model (uses generator weights)

Or use CPU for reference model:

ref_model:
  with_gpus: false

Issue: OOM During Generation

Symptoms: CUDA OOM in vLLM

Solutions:

# Reduce batch size
grpo:
  n_samples: 4  # Reduce from 8

# Or reduce sequence length
training:
  seq_len: 2048

Issue: Slow Weight Sync

Symptoms: Long pauses between training and generation

Solutions:

# Enable RDMA (if available)
export TORCHSTORE_USE_RDMA=1

# Or reduce sync frequency
training:
  sync_interval: 10  # Sync every 10 steps

Issue: Policy Collapse

Symptoms: Entropy drops to zero, reward stops improving

Solutions:

# Increase KL penalty
grpo:
  beta: 0.2  # Increase from 0.1

# Or add entropy bonus
training:
  entropy_coef: 0.01

---

Resources

• Documentation: https://meta-pytorch.org/torchforge
• GitHub: https://github.com/meta-pytorch/torchforge
• Discord: https://discord.gg/YsTYBh6PD9
• TorchTitan: https://github.com/pytorch/torchtitan
• Monarch: https://github.com/meta-pytorch/monarch