# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
from . import FairseqLRScheduler, register_lr_scheduler
[docs]@register_lr_scheduler('inverse_sqrt')
class InverseSquareRootSchedule(FairseqLRScheduler):
"""Decay the LR based on the inverse square root of the update number.
We also support a warmup phase where we linearly increase the learning rate
from some initial learning rate (``--warmup-init-lr``) until the configured
learning rate (``--lr``). Thereafter we decay proportional to the number of
updates, with a decay factor set to align with the configured learning rate.
During warmup::
lrs = torch.linspace(args.warmup_init_lr, args.lr, args.warmup_updates)
lr = lrs[update_num]
After warmup::
decay_factor = args.lr * sqrt(args.warmup_updates)
lr = decay_factor / sqrt(update_num)
"""
def __init__(self, args, optimizer):
super().__init__(args, optimizer)
if len(args.lr) > 1:
raise ValueError(
'Cannot use a fixed learning rate schedule with inverse_sqrt.'
' Consider --lr-scheduler=fixed instead.'
)
warmup_end_lr = args.lr[0]
if args.warmup_init_lr < 0:
args.warmup_init_lr = 0 if args.warmup_updates > 0 else warmup_end_lr
# linearly warmup for the first args.warmup_updates
self.lr_step = (warmup_end_lr - args.warmup_init_lr) / args.warmup_updates
# then, decay prop. to the inverse square root of the update number
self.decay_factor = warmup_end_lr * args.warmup_updates**0.5
# initial learning rate
self.lr = args.warmup_init_lr
self.optimizer.set_lr(self.lr)
[docs] @staticmethod
def add_args(parser):
"""Add arguments to the parser for this LR scheduler."""
# fmt: off
parser.add_argument('--warmup-updates', default=4000, type=int, metavar='N',
help='warmup the learning rate linearly for the first N updates')
parser.add_argument('--warmup-init-lr', default=-1, type=float, metavar='LR',
help='initial learning rate during warmup phase; default is args.lr')
# fmt: on
[docs] def step(self, epoch, val_loss=None):
"""Update the learning rate at the end of the given epoch."""
super().step(epoch, val_loss)
# we don't change the learning rate at epoch boundaries
return self.optimizer.get_lr()
[docs] def step_update(self, num_updates):
"""Update the learning rate after each update."""
if num_updates < self.args.warmup_updates:
self.lr = self.args.warmup_init_lr + num_updates*self.lr_step
else:
self.lr = self.decay_factor * num_updates**-0.5
self.optimizer.set_lr(self.lr)
return self.lr