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IterIS: Iterative Inference-Solving Alignment for LoRA Merging
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Proposes IterIS, an iterative inference-solving, closed-form LoRA merging method that refines the merging objective using actual unified-adapter input features, with regularization and adaptive weights, evaluated on diffusion, VLM, and LLM tasks.
Problem
Merging multiple task-specific LoRAs into one unified adapter without training data or gradient-based training; existing real-distribution merging methods approximate unified-adapter inputs with each LoRA's inputs, need many samples, and produce unbalanced objectives.
Approach
Replace per-LoRA input features with the features actually seen by the unified adapter, recomputed iteratively via inference and closed-form solves; add a Frobenius-scaled identity regularizer so only a few dozen samples are needed, and adaptive per-task weights (inverse squared norms) to balance objective terms.
Key results
- On multi-style captioning (POS+NEG), IterIS reaches 83.1% POS and 78.1% NEG style accuracy versus 62.4%/69.2% for RegMean.
- On seven diffusion concept pairs, IterIS beats linear merging on average by 0.78% (image alignment 1), 1.61% (image alignment 2), and 2.53% (text alignment).
- On GLUE multi-task integration (21 pairs, FLAN-T5-base), IterIS averages 5.811 vs 5.764 for RegMean and 5.625 for linear merging.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use IterIS instead of linear merging for multi-concept customization | Stable Diffusion v1.5 LoRA merging · Multi-concept customization (mean) | Text-alignment: 0.6547 -> 0.6800 | Helped |
| Use IterIS instead of custom diffusion for multi-concept customization | Stable Diffusion v1.5 LoRA merging · Multi-concept customization (mean) | Image-alignment1: 0.6881 -> 0.6889 | No clear effect |
| Use IterIS instead of RegMean for multi-style caption generation | BLIP-image-captioning-base LoRA merging · Positive and negative caption styles | POS style accuracy: 0.624 -> 0.831 | Helped |
| Use IterIS instead of RegMean for multi-style caption generation | BLIP-image-captioning-base LoRA merging · Positive and negative caption styles | NEG style accuracy: 0.692 -> 0.781 | Helped |
| Use IterIS instead of RegMean for multi-style caption generation | BLIP-image-captioning-base LoRA merging · Positive and negative caption styles | CIDEr: 0.790 -> 0.794 | Helped |
| Use IterIS instead of RegMean for multi-style caption generation | BLIP-image-captioning-base LoRA merging · Positive and negative caption styles | B-4: 0.150 -> 0.152 | Helped |
| Use IterIS instead of RegMean for in-domain emotion task integration | FLAN-t5-large LoRA merging · In-domain emotion integration | SUM: 2.854 -> 2.940 | Helped |
| Use IterIS instead of RegMean for GLUE multi-task integration | FLAN-t5-base LoRA merging · GLUE multi-task integration | SUM: 5.764 -> 5.811 | Helped |
| Use IterIS instead of linear merging on COLA | FLAN-t5-base LoRA merging · GLUE multi-task integration (COLA) | MCC (normalized): 0.678 -> 0.666 | Hurt |
Surprising findings
- Linear merging of the POS and NEG caption LoRAs is close to chance on style accuracy (52.2% and 55.7%), and task arithmetic yields the same accuracy as linear merging.
- On COLA, IterIS slightly underperforms linear merging (0.666 vs 0.678).
Limitations
- Iteration count is capped at 20 to prevent overfitting, with convergence bound only derived for self-attention encoders with Q, K, V LoRAs.
- Evaluation is on small models (Stable Diffusion v1.5, BLIP base, FLAN-T5) and a limited set of task pairs.
- Only the authors' own two-to-four-task combinations are evaluated; the method is not shown to generalize to other PEFT methods (acknowledged as future work).
Experimental setup
Datasets: CustomConcept101 DreamBooth SentiCap Emotion datasets (Emoint, EC, TEC, ISEAR) GLUE
Benchmarks: CustomConcept101 DreamBooth SentiCap Emotion datasets GLUE
Metrics: CLIP image alignment CLIP text alignment style accuracy CIDEr BLEU-1 to BLEU-4 F1 accuracy MCC
Baselines: Linear merging Textual inversion Custom Diffusion Task arithmetic Ties merging Hyper-linear RegMean
Builds on: RegMean Mix-of-Show Custom Diffusion Linear merging (PEMs) DreamBooth-LoRA
Closest papers by meaning
- Merging LoRAs like Playing LEGO: Pushing the Modularity of LoRA to Extremes Through Rank-Wise Clustering (ICLR 2025 · similarity 0.90)
- LoRA-GA: Low-Rank Adaptation with Gradient Approximation (NeurIPS 2024 · similarity 0.90)
- Make LoRA Great Again: Boosting LoRA with Adaptive Singular Values and Mixture-of-Experts Optimization Alignment (ICML 2025 · similarity 0.90)
- Flat-LoRA: Low-Rank Adaptation over a Flat Loss Landscape (ICML 2025 · similarity 0.89)
- SDS-LoRA: Overcoming Anisotropic Gradient Scaling in Low-Rank Adaptation (NeurIPS 2026 · similarity 0.89)
- LoRAtorio: An intrinsic approach to LoRA Skill Composition (NeurIPS 2026 · similarity 0.89)
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