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Loss Surface Simplexes

This repository contains the code accompanying our paper Loss Surface Simplexes for Mode Connecting Volumes and Fast Ensembling (ICML 2021) by Greg Benton, Wesley Maddox, Sanae Lotfi, and Andrew Gordon Wilson.

Introduction

The repository holds the implementation for Simplicial Pointwise Random Optimization (SPRO) as a method of finding simplicial complexes of low loss in the parameter space of neural networks. This work extends the approach of Loss Surfaces, Mode Connectivity, and Fast Ensembling of DNNs by Garipov et al. allowing us to find not just mode connecting paths but large intricate volumes of low loss that connect independently trained models in parameter space.

The left plot above shows loss surface projections along the edges and faces of a simplicial complex of low loss where and are independently trained VGG16 models on CIFAR-10, and and are connecting points trained with SPRO. The right plot shows a 3 dimensional projection of a simplicial complex containing 7 independently trained modes (orange) and 9 connecting points (blue) forming a total of 12 interconnected simplexes (blue shade) in the parameter space of a VGG16 network trained on CIFAR-100.

Beyond providing the ability to find multidimensional simplexes of low loss in parameter space our method introduces a practical method for improving accuracy and calibration over standard deep ensembles.

For any fixed training budget or any fixed number of ensemble components we are able to outperform standard deep ensembles.

Dependencies

Usage and General Repository Structure

There are 2 main directories

  • experiments contains the main directories to reproduce the core experiments of the paper,
  • simplex contains the core code including mode definitions and utilities like training and evaluation functions.

Experiments

vgg-cifar10 and vgg-cifar100

These directories operate very similarly. Each contains several scripts: base_trainer.py for training VGG style networks on the corresponding dataset, simplex_trainer.py to train a simplex starting from one of the pre-trained models, and complex_iterative.py which takes in a set of pre-trained models and computes a mode connecting simplicial complex containing them. Finally, these directories contain ensemble_trainer.py which can be used to train an ensemble of simplexes in a single command.

The specific commands to reproduce the results in the paper can be found in the READMEs in the corresponding directories.

Simplex

The core class definition here is SimplexNet contained in simplex/models/simplex_models.py, which is an adaptation of the CurveNet class used in Loss Surfaces, Mode Connectivity, and Fast Ensembling of DNNs with the accompanying codebase here. The SimplexNet class serves as a wrapper for a network such as VGG16Simplex in simplex/models/vgg_noBN.py, which will be stored as the attribute SimplexNet.net.

The networks that have simplex base components (such as VGG16Simplex) hold each of the vertices of the simplex as their parameters and keep a list of fix_points that determine which of these vertices should recieve gradients for training.

Forward calls to a SimplexNet instance by default samples a set of parameters from within the simplex defined by the weight vectors in SimplexNet.net, and computes an output based on those parameters. Practically this means that if you call SimplexNet(input) repeatedly, you will get different results each time.

Relevant Papers

Loss Surfaces, Mode Connectivity, and Fast Ensembling of DNNs by Timur Garipov, Pavel Izmailov, Dmitrii Podoprikhin, Dmitry Vetrov, Andrew Gordon Wilson

Essentially No Barriers in Neural Network Energy Landscape by Felix Draxler, Kambis Veschgini, Manfred Salmhofer, Fred A. Hamprecht

Large Scale Structure of Neural Network Loss Landscapes by Stanislav Fort, Stanislaw Jastrzebski

Citation

If you use this work, please cite it as

@inproceedings{benton2021loss,
  title={Loss Surface Simplexes for Mode Connecting Volumes and Fast Ensembling},
  author={Benton, Gregory W and Maddox, Wesley J and Lotfi, Sanae and Wilson, Andrew Gordon},
  booktitle={International Conference on Machine Learning},
  year={2021}
}

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