What Matters for Model Merging at Scale?
Authors: name: Prateek Yadav, name: Tu Vu, name: Jonathan Lai, name: Alexandra Chronopoulou, name: Manaal Faruqui, name: Mohit Bansal, name: Tsendsuren Munkhdalai

Model merging aims to combine multiple expert models into a more capable
single model, offering benefits such as reduced storage and serving costs,
improved generalization, and support for decentralized model development.
Despite its promise, previous studies have primarily focused on merging a few
small models. This leaves many unanswered questions about the effect of scaling
model size and how it interplays with other key factors -- like the base model
quality and number of expert models -- , to affect the merged model's
performance. This work systematically evaluates the utility of model merging at
scale, examining the impact of these different factors. We experiment with
merging fully fine-tuned models using 4 popular merging methods -- Averaging,
Task~Arithmetic, Dare, and TIES -- across model sizes ranging from 1B-64B
parameters and merging up to 8 different expert models. We evaluate the merged
models on both held-in tasks, i.e., the expert's training tasks, and zero-shot
generalization to unseen held-out tasks. Our experiments provide several new
insights about model merging at scale and the interplay between different
factors. First, we find that merging is more effective when experts are created
from strong base models, i.e., models with good zero-shot performance. Second,
larger models facilitate easier merging. Third merging consistently improves
generalization capabilities. Notably, when merging 8 large expert models, the
merged models often generalize better compared to the multitask trained models.
Fourth, we can better merge more expert models when working with larger models.
Fifth, different merging methods behave very similarly at larger scales.
Overall, our findings shed light on some interesting properties of model
merging while also highlighting some limitations. We hope that this study will
serve as a reference point on large-scale merging for upcoming research.