DPO直接偏好优化

RLHF 需要训练奖励模型，再用 PPO 优化——两步流程复杂且不稳定。DPO（Direct Preference Optimization）跳过奖励模型，直接从偏好数据优化策略。更简单、更稳定、效果相当。

类型： 构建 语言： Python 前置条件： Phase 10 Lesson 07（RLHF） 预计时间： ~60 分钟

学习目标

• 理解 DPO 的数学推导：从 RLHF 目标到 DPO 损失
• 实现 DPO 训练循环
• 理解 DPO 与 RLHF 的权衡
• 掌握 DPO 的变体：IPO、KTO、ORPO

DPO 核心思想

RLHF 的目标是最大化奖励同时保持接近参考策略：

$\max_\pi \mathbb{E}_{\pi}[r(x,y)] - \beta \cdot D_{KL}(\pi \| \pi_{ref})$

DPO 的关键洞察：这个优化问题有闭式解。最优策略可以表示为：

$\pi^*(y|x) = \frac{1}{Z(x)} \pi_{ref}(y|x) \exp\left(\frac{1}{\beta} r(x,y)\right)$

将奖励表示为策略和参考策略的函数：

$r(x,y) = \beta \log \frac{\pi^*(y|x)}{\pi_{ref}(y|x)} + \beta \log Z(x)$

代入 Bradley-Terry 偏好模型，配分函数 $Z(x)$ 被消去，得到 DPO 损失：

$\mathcal{L}_{DPO} = -\mathbb{E}\left[\log \sigma\left(\beta \log \frac{\pi(y_w|x)}{\pi_{ref}(y_w|x)} - \beta \log \frac{\pi(y_l|x)}{\pi_{ref}(y_l|x)}\right)\right]$

其中 $y_w$ 是被偏好的回复，$y_l$ 是被拒绝的回复。

DPO 实现

import torch
import torch.nn.functional as F


def dpo_loss(policy_chosen_logps, policy_rejected_logps,
             reference_chosen_logps, reference_rejected_logps,
             beta=0.1):
    """DPO 损失函数"""
    # 计算 log 比率
    chosen_logratios = policy_chosen_logps - reference_chosen_logps
    rejected_logratios = policy_rejected_logps - reference_rejected_logps

    # DPO 损失
    logits = beta * (chosen_logratios - rejected_logratios)
    loss = -F.logsigmoid(logits).mean()

    # 可选：计算指标
    with torch.no_grad():
        chosen_rewards = beta * chosen_logratios
        rejected_rewards = beta * rejected_logratios
        accuracy = (chosen_rewards > rejected_rewards).float().mean()
        margin = (chosen_rewards - rejected_rewards).mean()

    return loss, accuracy, margin


def compute_log_probs(model, input_ids, attention_mask, labels):
    """计算给定序列的 log probability"""
    outputs = model(input_ids, attention_mask=attention_mask)
    logits = outputs.logits

    # 移位：预测下一个 token
    shift_logits = logits[..., :-1, :].contiguous()
    shift_labels = labels[..., 1:].contiguous()

    # 计算 log softmax
    log_probs = F.log_softmax(shift_logits, dim=-1)

    # 选择对应 token 的 log prob
    per_token_logps = log_probs.gather(-1, shift_labels.unsqueeze(-1)).squeeze(-1)

    # 仅计算非填充位置的 log prob
    mask = shift_labels != -100
    per_token_logps = per_token_logps * mask

    return per_token_logps.sum(dim=-1)


def train_dpo(policy_model, ref_model, dataloader, optimizer,
              beta=0.1, epochs=1, device='cuda'):
    """DPO 训练循环"""
    policy_model = policy_model.to(device)
    ref_model = ref_model.to(device)
    ref_model.eval()  # 参考模型不更新

    for epoch in range(epochs):
        total_loss = 0
        total_accuracy = 0

        for batch in dataloader:
            # Chosen 回复
            chosen_ids = batch['chosen_input_ids'].to(device)
            chosen_mask = batch['chosen_attention_mask'].to(device)
            chosen_labels = batch['chosen_labels'].to(device)

            # Rejected 回复
            rejected_ids = batch['rejected_input_ids'].to(device)
            rejected_mask = batch['rejected_attention_mask'].to(device)
            rejected_labels = batch['rejected_labels'].to(device)

            # 策略模型的 log probs
            policy_chosen_logps = compute_log_probs(
                policy_model, chosen_ids, chosen_mask, chosen_labels)
            policy_rejected_logps = compute_log_probs(
                policy_model, rejected_ids, rejected_mask, rejected_labels)

            # 参考模型的 log probs（不计算梯度）
            with torch.no_grad():
                ref_chosen_logps = compute_log_probs(
                    ref_model, chosen_ids, chosen_mask, chosen_labels)
                ref_rejected_logps = compute_log_probs(
                    ref_model, rejected_ids, rejected_mask, rejected_labels)

            # DPO 损失
            loss, accuracy, margin = dpo_loss(
                policy_chosen_logps, policy_rejected_logps,
                ref_chosen_logps, ref_rejected_logps,
                beta=beta,
            )

            loss.backward()
            torch.nn.utils.clip_grad_norm_(policy_model.parameters(), 1.0)
            optimizer.step()
            optimizer.zero_grad()

            total_loss += loss.item()
            total_accuracy += accuracy.item()

        n = len(dataloader)
        print(f"Epoch {epoch + 1} | Loss: {total_loss/n:.4f} | "
              f"Accuracy: {total_accuracy/n:.4f}")

DPO 变体

DPO vs RLHF

关键术语

延伸阅读

• Rafailov et al., 2023 — “Direct Preference Optimization: Your Language Model is Secretly a Reward Model” — DPO 原始论文
• Azar et al., 2023 — “A General Theoretical Paradigm to Understand Learning from Human Preferences” — IPO 论文
• Ethayarajh et al., 2024 — “KTO: Model Alignment as Prospect Theoretic Optimization” — KTO 论文
• Hong et al., 2024 — “ORPO: Monolithic Preference Optimization without Reference Model” — ORPO 论文