class torch.optim.lr_scheduler.CosineAnnealingLR(optimizer, T_max, eta_min=0, last_epoch=-1, verbose='deprecated')[source]

Set the learning rate of each parameter group using a cosine annealing schedule, where ηmax\eta_{max} is set to the initial lr and TcurT_{cur} is the number of epochs since the last restart in SGDR:

ηt=ηmin+12(ηmaxηmin)(1+cos(TcurTmaxπ)),Tcur(2k+1)Tmax;ηt+1=ηt+12(ηmaxηmin)(1cos(1Tmaxπ)),Tcur=(2k+1)Tmax.\begin{aligned} \eta_t & = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1 + \cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right), & T_{cur} \neq (2k+1)T_{max}; \\ \eta_{t+1} & = \eta_{t} + \frac{1}{2}(\eta_{max} - \eta_{min}) \left(1 - \cos\left(\frac{1}{T_{max}}\pi\right)\right), & T_{cur} = (2k+1)T_{max}. \end{aligned}

When last_epoch=-1, sets initial lr as lr. Notice that because the schedule is defined recursively, the learning rate can be simultaneously modified outside this scheduler by other operators. If the learning rate is set solely by this scheduler, the learning rate at each step becomes:

ηt=ηmin+12(ηmaxηmin)(1+cos(TcurTmaxπ))\eta_t = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1 + \cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right)

It has been proposed in SGDR: Stochastic Gradient Descent with Warm Restarts. Note that this only implements the cosine annealing part of SGDR, and not the restarts.

  • optimizer (Optimizer) – Wrapped optimizer.

  • T_max (int) – Maximum number of iterations.

  • eta_min (float) – Minimum learning rate. Default: 0.

  • last_epoch (int) – The index of last epoch. Default: -1.

  • verbose (bool) –

    If True, prints a message to stdout for each update. Default: False.

    Deprecated since version 2.2: verbose is deprecated. Please use get_last_lr() to access the learning rate.


Return last computed learning rate by current scheduler.


Loads the schedulers state.


state_dict (dict) – scheduler state. Should be an object returned from a call to state_dict().

print_lr(is_verbose, group, lr, epoch=None)

Display the current learning rate.


Returns the state of the scheduler as a dict.

It contains an entry for every variable in self.__dict__ which is not the optimizer.


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