The author measures gradient subspace overlap across four models to evaluate the standard LoRA default of targeting query and value projections. The analysis indicates that this default may be suboptimal when the goal is retaining previous capabilities rather than maximizing accuracy on a new task.
- On Llama-3-8B fine-tuned for code-to-prose, HumanEval retention improved from 54.1% with the q,v default to 75.6% using a measured placement that excluded v_proj entirely.
- The type ordering of module importance reproduces across Llama-3-8B, Mistral-7B, Qwen2.5-7B, and TinyLlama-1.1B, with o_proj first and v_proj last or absent.
- A cheap rule based on module type and layer depth ranks well (AUROC 0.83 to 0.92) but selects a different set of modules, differing by about a third from the gradient-based selection.
The author provides a Colab notebook and GitHub repository for reproducing these measurements, noting that the implementation is AGPL-licensed.