Bi-Phase Training: Learning Efficiently From Scratch

Abel Gurung, Joseph Campbell


Abstract
Pre-training foundational models is computationally expensive as models scale. In this work, we introduce Bi-Phase Training (BPT), a parameter-efficient pre-training method that preserves the high-rank learning dynamics observed in full-parameter optimization with substantially reduced trainable parameters. BPT parameterizes each weight update as a composition of constrained high-rank left/right transformations via trainable diagonal matrices and low-rank adapters whose updates are periodically merged into the frozen base weights, resulting in a compounding expansion of accessible directions during training. Across language-model pre-training from scratch (Qwen2.5 models from 100M to 1.5B parameters and OLMo-2-1B) on FineWeb-Edu (10B and 100B tokens), BPT achieves validation perplexity close to full-parameter baselines while using far fewer trainable parameters—e.g., at the 1.5B scale, BPT uses 66% fewer trainable parameters with only a small validation-perplexity gap. Additionally, a comprehensive evaluation across diverse downstream tasks shows that BPT preserves much of the full-parameterized model’s downstream performance.
Anthology ID:
2026.findings-aacl.111
Volume:
Findings of the Association for Computational Linguistics: IJCNLP-AACL 2026
Month:
November
Year:
2026
Address:
Hengqin, China
Editors:
Yue Zhang, Ruifeng Xu, Ayu Purwarianti, Tong Rong, Jun Suzuki
Venue:
Findings
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
2189–2202
Language:
URL:
https://aclanthology.org/2026.findings-aacl.111/
DOI:
Bibkey:
Cite (ACL):
Abel Gurung and Joseph Campbell. 2026. Bi-Phase Training: Learning Efficiently From Scratch. In Findings of the Association for Computational Linguistics: IJCNLP-AACL 2026, pages 2189–2202, Hengqin, China. Association for Computational Linguistics.
Cite (Informal):
Bi-Phase Training: Learning Efficiently From Scratch (Gurung & Campbell, Findings 2026)
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PDF:
https://aclanthology.org/2026.findings-aacl.111.pdf