Low-Rank Prune-And-Factorize for Language Model Compression
Siyu Ren Kenny Q. Zhu
The components underpinning PLMs -- large weight matrices -- were shown to
bear considerable redundancy. Matrix factorization, a well-established
technique from matrix theory, has been utilized to reduce the number of
parameters in PLM. However, it fails to retain satisfactory performance under
moderate to high compression rate. In this paper, we identify the
\textit{full-rankness} of fine-tuned PLM as the fundamental bottleneck for the
failure of matrix factorization and explore the use of network pruning to
extract low-rank sparsity pattern desirable to matrix factorization. We find
such low-rank sparsity pattern exclusively exists in models generated by
first-order pruning, which motivates us to unite the two approaches and achieve
more effective model compression. We further propose two techniques:
sparsity-aware SVD and mixed-rank fine-tuning, which improve the initialization
and training of the compression procedure, respectively. Experiments on GLUE
and question-answering tasks show that the proposed method has superior
compression-performance trade-off compared to existing approaches.