simpo-loss-implementation

SimPO Loss Implementation

Overview

SimPO (Simple Preference Optimization) is a reference-free preference optimization method that trains language models using pairwise preference data. The loss function combines length-normalized rewards with a Bradley-Terry preference model.

Core Mathematical Formulas

1. Length-Normalized Reward

r_SimPO(x, y) = (β/|y|) * log π_θ(y|x)

Where:

• β: Scaling constant (typically 2.0)
• |y|: Response length (number of tokens)
• log π_θ(y|x): Average log probability per token (not summed)

2. Bradley-Terry Preference with Margin

p(y_w ≻ y_l | x) = σ(r(x, y_w) - r(x, y_l) - γ)

Where:

• γ: Target reward margin (calculated as γ = β * gamma_beta_ratio)
• σ(·): Sigmoid function
• y_w: Chosen/winning response
• y_l: Rejected/losing response

3. SimPO Loss Objective

L_SimPO = -log(σ(reward_chosen - reward_rejected - γ))

This is equivalent to binary cross-entropy loss where the target is always 1.

Implementation Pattern

Input Parameters

def simpo_loss(
    self,
    policy_chosen_logps: torch.FloatTensor,  # Shape: (batch_size,)
    policy_rejected_logps: torch.FloatTensor  # Shape: (batch_size,)
) -> Tuple[torch.FloatTensor, torch.FloatTensor, torch.FloatTensor]:

Output

Returns tuple of:

• losses: Individual losses for each pair (batch_size,)
• chosen_rewards: Rewards for chosen responses (batch_size,)
• rejected_rewards: Rewards for rejected responses (batch_size,)

Implementation Steps

Step 1: Extract Configuration Values

Access self.beta and gamma_beta_ratio from the trainer's config to compute:

• gamma = self.beta * self.config.gamma_beta_ratio

Step 2: Calculate Length-Normalized Rewards

For chosen responses:

chosen_rewards = (self.beta / chosen_length) * policy_chosen_logps

For rejected responses:

rejected_rewards = (self.beta / rejected_length) * policy_rejected_logps

Important: The log probabilities passed are already averaged per token (not summed), so division by length has already been applied. Therefore, you multiply by beta only:

chosen_rewards = self.beta * policy_chosen_logps
rejected_rewards = self.beta * policy_rejected_logps

Step 3: Compute Log Differences with Margin

log_odds = chosen_rewards - rejected_rewards - gamma

Step 4: Apply Sigmoid and Compute Loss

Using binary cross-entropy with logits:

losses = -torch.nn.functional.logsigmoid(log_odds)

This computes: -log(σ(x)) which is the negative log-likelihood that the sigmoid evaluates to 1.

Configuration Defaults

From SimPOConfig:

• beta: 2.0 (reward scaling factor)
• gamma_beta_ratio: 0.25 (ratio between margin and beta)
• Default gamma: 0.5

Key Implementation Details

1. Log Probabilities Are Pre-Normalized: The input policy_chosen_logps and policy_rejected_logps are already averaged log probabilities per token. No additional length normalization is needed in the loss function.

2. No Reference Model: Unlike DPO, SimPO uses absolute log-likelihoods, not log-ratios. This eliminates the need for a reference model.

3. Vectorized Operations: All operations should be vectorized across the batch dimension for efficiency.

4. Device Consistency: Ensure all tensors remain on the same device (CPU/GPU).

Common Pitfalls to Avoid

1. Double Length Normalization: Don't divide by length again if log probs are already averaged
2. Dimension Mismatches: Ensure rewards and log_odds maintain shape (batch_size,)
3. Missing Margin: The -gamma term is critical; without it, the model ignores preference margins
4. Wrong Loss Function: Use -logsigmoid() not log_sigmoid() for negative log-likelihood

Testing Pattern

Verify implementation by:

1. Checking output shapes match input batch sizes
2. Confirming losses are positive values
3. Validating that worse log-probability pairs produce higher losses
4. Testing with unit tests using fixed input tensors