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Paper · 2107.05802 · ICLR · 2022

How many degrees of freedom do we need to train deep networks: a loss landscape perspective

Stanislav Fort, Surya Ganguli, Brett Larsen, Nic Becker

arXiv · PDF · Open in the Atlas

Code that ran

We lifted 6 functions out of this paper's own repositories and ran 2 of them in a sandbox. "Ran" means the function executed on a synthesized input and returned a value. It is not a reproduction of the paper's results.

RepositoryRoleRan
ganguli-lab/degrees-of-freedom canonical 2 of 6
FunctionStatusWhere it lives
cross_entropy_loss Ran ganguli-lab/degrees-of-freedom/lottery_subspace.py
pointer only (licence: NONE) · get_code("82620ac387a3de49")
flatten_leaves Ran ganguli-lab/degrees-of-freedom/training_utils.py
pointer only (licence: NONE) · get_code("d75f8e446fd2752b")
generate_projection Not yet run ganguli-lab/degrees-of-freedom/training_utils.py
pointer only (licence: NONE) · get_code("4864ff72334718a3")
normal_accuracy Not yet run ganguli-lab/degrees-of-freedom/lottery_subspace.py
pointer only (licence: NONE) · get_code("b10110cfc5ddad5b")
normal_loss Not yet run ganguli-lab/degrees-of-freedom/lottery_subspace.py
pointer only (licence: NONE) · get_code("1d72592cc37b2066")
reform_leaves Not yet run ganguli-lab/degrees-of-freedom/training_utils.py
pointer only (licence: NONE) · get_code("bcc53283af041484")

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Abstract

A variety of recent works, spanning pruning, lottery tickets, and training within random subspaces, have shown that deep neural networks can be trained using far fewer degrees of freedom than the total number of parameters. We analyze this phenomenon for random subspaces by first examining the success probability of hitting a training loss sublevel set when training within a random subspace of a given training dimensionality. We find a sharp phase transition in the success probability from 0 to 1 as the training dimension surpasses a threshold. This threshold training dimension increases as the desired final loss decreases, but decreases as the initial loss decreases. We then theoretically explain the origin of this phase transition, and its dependence on initialization and final desired loss, in terms of properties of the high dimensional geometry of the loss landscape. In particular, we show via Gordon's escape theorem, that the training dimension plus the Gaussian width of the desired loss sub-level set, projected onto a unit sphere surrounding the initialization, must exceed the total number of parameters for the success probability to be large. In several architectures and datasets, we measure the threshold training dimension as a function of initialization and demonstrate that it is a small fraction of the total parameters, implying by our theory that successful training with so few dimensions is possible precisely because the Gaussian width of low loss sub-level sets is very large. Moreover, we compare this threshold training dimension to more sophisticated ways of reducing training degrees of freedom, including lottery tickets as well as a new, analogous method: lottery subspaces.

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