Ross Wightman
3 years ago
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f4fb068b11
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""" PyTorch LARS / LARC Optimizer
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An implementation of LARS (SGD) + LARC in PyTorch
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Based on:
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* PyTorch SGD: https://github.com/pytorch/pytorch/blob/1.7/torch/optim/sgd.py#L100
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* NVIDIA APEX LARC: https://github.com/NVIDIA/apex/blob/master/apex/parallel/LARC.py
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Additional cleanup and modifications to properly support PyTorch XLA.
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Copyright 2021 Ross Wightman
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"""
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import torch
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from torch.optim.optimizer import Optimizer, required
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class Lars(Optimizer):
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""" LARS for PyTorch
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Paper: `Large batch training of Convolutional Networks` - https://arxiv.org/pdf/1708.03888.pdf
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Args:
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params (iterable): iterable of parameters to optimize or dicts defining parameter groups.
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lr (float, optional): learning rate. (default: 1e-3)
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momentum (float, optional): momentum factor (default: 0)
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weight_decay (float, optional): weight decay (L2 penalty) (default: 0)
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dampening (float, optional): dampening for momentum (default: 0)
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nesterov (bool, optional): enables Nesterov momentum (default: False)
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trust_coeff (float): trust coefficient for computing adaptive lr / trust_ratio (default: 0.001)
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eps (float): eps for division denominator (default: 1e-8)
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larc (bool): enable LARC clipping (default: False)
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always_scale (bool): always apply LARS scaling, otherwise only when group weight_decay != 0 (default: False)
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"""
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def __init__(
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self,
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params,
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lr=required,
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momentum=0,
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dampening=0,
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weight_decay=0,
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nesterov=False,
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trust_coeff=0.001,
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eps=1e-8,
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larc=False,
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always_scale=False,
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):
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if lr is not required and lr < 0.0:
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raise ValueError(f"Invalid learning rate: {lr}")
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if momentum < 0.0:
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raise ValueError(f"Invalid momentum value: {momentum}")
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if weight_decay < 0.0:
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raise ValueError(f"Invalid weight_decay value: {weight_decay}")
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if nesterov and (momentum <= 0 or dampening != 0):
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raise ValueError("Nesterov momentum requires a momentum and zero dampening")
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defaults = dict(
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lr=lr,
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momentum=momentum,
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dampening=dampening,
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weight_decay=weight_decay,
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nesterov=nesterov,
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trust_coeff=trust_coeff,
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eps=eps,
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larc=larc,
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always_scale=always_scale,
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)
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super().__init__(params, defaults)
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def __setstate__(self, state):
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super().__setstate__(state)
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for group in self.param_groups:
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group.setdefault("nesterov", False)
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@torch.no_grad()
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def step(self, closure=None):
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"""Performs a single optimization step.
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Args:
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closure (callable, optional): A closure that reevaluates the model and returns the loss.
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"""
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loss = None
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if closure is not None:
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with torch.enable_grad():
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loss = closure()
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device = self.param_groups[0]["params"][0].device
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one_tensor = torch.tensor(1.0, device=device) # because torch.where doesn't handle scalars correctly
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# exclude scaling for params with 0 weight decay
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for group in self.param_groups:
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weight_decay = group['weight_decay']
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momentum = group['momentum']
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dampening = group['dampening']
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nesterov = group['nesterov']
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trust_coeff = group['trust_coeff']
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eps = group['eps']
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for p in group['params']:
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if p.grad is None:
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continue
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grad = p.grad
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# apply LARS scaling, LARC clipping, weight decay
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# ref: https://github.com/NVIDIA/apex/blob/master/apex/parallel/LARC.py
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if weight_decay != 0 or group['always_scale']:
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w_norm = p.norm(2.0)
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g_norm = grad.norm(2.0)
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trust_ratio = trust_coeff * w_norm / (g_norm + w_norm * weight_decay + eps)
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# FIXME nested where required since logical and/or not working in PT XLA
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trust_ratio = torch.where(
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w_norm > 0,
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torch.where(g_norm > 0, trust_ratio, one_tensor),
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one_tensor,
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)
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if group['larc']:
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trust_ratio = torch.minimum(trust_ratio / group['lr'], one_tensor)
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grad.add(p, alpha=weight_decay)
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grad.mul_(trust_ratio)
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# apply SGD update https://github.com/pytorch/pytorch/blob/1.7/torch/optim/sgd.py#L100
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if momentum != 0:
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param_state = self.state[p]
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if 'momentum_buffer' not in param_state:
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buf = param_state['momentum_buffer'] = torch.clone(grad).detach()
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else:
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buf = param_state['momentum_buffer']
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buf.mul_(momentum).add_(grad, alpha=1. - dampening)
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if nesterov:
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grad = grad.add(buf, alpha=momentum)
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else:
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grad = buf
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p.add_(grad, alpha=-group['lr'])
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return loss
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