To optimize fine-tuning for Sentence Transformers, focus on learning rate scheduling, layer freezing, and balancing batch sizes with optimizer choices. These adjustments help balance training speed and model performance while preventing overfitting. Start by experimenting with these parameters systematically, using validation metrics to guide decisions.
First, learning rate scheduling is critical. Begin with a higher initial learning rate (e.g., 2e-5) to allow the model to adjust quickly, then gradually reduce it. For example, use a linear warmup for the first 10% of training steps to stabilize gradients early, followed by cosine decay to smoothly lower the rate. This approach prevents overshooting optimal weights early while refining them later. If training stalls, try cyclical learning rates that oscillate between higher and lower values to escape local minima. For instance, in a 10,000-step training run, warm up over 1,000 steps, then apply cosine decay. Always validate these choices by comparing loss curves—if the training loss plateaus too soon, the initial rate might be too low.
Second, selectively freezing layers can speed up convergence. Pre-trained models like BERT or RoBERTa have lower layers that capture general language features, which may not need retraining for similar tasks. For example, freeze the first 6 of 12 transformer layers in BERT-base when adapting to a semantic similarity task. This reduces trainable parameters by ~50%, cutting computation time. However, if your task differs significantly from the model’s pretraining data (e.g., medical text), unfreeze more layers to allow adaptation. Test this by comparing performance when freezing 0%, 33%, or 50% of layers. Use tools like PyTorch’s requires_grad_(False) to freeze layers efficiently. If validation accuracy drops, unfreeze additional layers incrementally.
Finally, optimize batch size and optimizer settings. Larger batches (e.g., 32-64) stabilize gradient estimates but require more memory. If GPU memory is limited, use gradient accumulation (e.g., effective batch size 32 via 4 steps of size 8). Pair this with AdamW (weight decay 0.01) to prevent overfitting. For tasks with small datasets, smaller batches (16) with higher learning rates (3e-5) often work better. Monitor validation metrics like cosine similarity or retrieval accuracy every 500 steps—if performance plateaus, reduce the learning rate by a factor of 2-5. Early stopping (e.g., halting if no improvement in 3 evaluations) prevents unnecessary computation. Tools like Weights & Biases or TensorBoard can track these experiments, making it easier to compare hyperparameter combinations.