L-bfgs-b and line search methods for l-bfgs
The current version of lbfgs does not support line search, so simple box constrained is not available. <SCODE>import torch
from functools import reduce
from .optimizer import Optimizer
from math import isinf

class LBFGSB(Optimizer):
    """Implements L-BFGS algorithm.

.. warning::
    This optimizer doesn't support per-parameter options and parameter
    groups (there can be only one).

.. warning::
    Right now all parameters have to be on a single device. This will be
    improved in the future.

.. note::
    This is a very memory intensive optimizer (it requires additional
    ``param_bytes * (history_size + 1)`` bytes). If it doesn't fit in memory
    try reducing the history size, or use a different algorithm.

Arguments:
    lr (float): learning rate (default: 1)
    max_iter (int): maximal number of iterations per optimization step
        (default: 20)
    max_eval (int): maximal number of function evaluations per optimization
        step (default: max_iter * 1.25).
    tolerance_grad (float): termination tolerance on first order optimality
        (default: 1e-5).
    tolerance_change (float): termination tolerance on function value/parameter
        changes (default: 1e-9).
    line_search_fn (str): line search methods, currently available
        ['backtracking', 'goldstein', 'weak_wolfe']
    bounds (list of tuples of tensor): bounds[i][0], bounds[i][1] are elementwise
        lowerbound and upperbound of param[i], respectively
    history_size (int): update history size (default: 100).
"""

def __init__(self, params, lr=1, max_iter=20, max_eval=None,
             tolerance_grad=1e-5, tolerance_change=1e-9, history_size=100,
             line_search_fn=None, bounds=None):
    if max_eval is None:
        max_eval = max_iter * 5 // 4
    defaults = dict(lr=lr, max_iter=max_iter, max_eval=max_eval,
                    tolerance_grad=tolerance_grad, tolerance_change=tolerance_change,
                    history_size=history_size, line_search_fn=line_search_fn, bounds=bounds)
    super(LBFGS, self).__init__(params, defaults)

    if len(self.param_groups) != 1:
        raise ValueError("LBFGS doesn't support per-parameter options "
                         "(parameter groups)")

    self._params = self.param_groups[0]['params']
    self._bounds = [(None, None)] * len(self._params) if bounds is None else bounds
    self._numel_cache = None

def _numel(self):
    if self._numel_cache is None:
        self._numel_cache = reduce(lambda total, p: total + p.numel(), self._params, 0)
    return self._numel_cache

def _gather_flat_grad(self):
    return torch.cat(
        tuple(param.grad.data.view(-1) for param in self._params), 0)

def _add_grad(self, step_size, update):
    offset = 0
    for p in self._params:
        numel = p.numel()
        p.data.add_(step_size, update[offset:offset + numel].resize_(p.size()))
        offset += numel
    assert offset == self._numel()

def step(self, closure):
    """Performs a single optimization step.

    Arguments:
        closure (callable): A closure that reevaluates the model
            and returns the loss.
    """
    assert len(self.param_groups) == 1

    group = self.param_groups[0]
    lr = group['lr']
    max_iter = group['max_iter']
    max_eval = group['max_eval']
    tolerance_grad = group['tolerance_grad']
    tolerance_change = group['tolerance_change']
    line_search_fn = group['line_search_fn']
    history_size = group['history_size']

    state = self.state['global_state']
    state.setdefault('func_evals', 0)
    state.setdefault('n_iter', 0)

    # evaluate initial f(x) and df/dx
    orig_loss = closure()
    loss = orig_loss.data[0]
    current_evals = 1
    state['func_evals'] += 1

    flat_grad = self._gather_flat_grad()
    abs_grad_sum = flat_grad.abs().sum()

    if abs_grad_sum <= tolerance_grad:
        return loss

    # variables cached in state (for tracing)
    d = state.get('d')
    t = state.get('t')
    old_dirs = state.get('old_dirs')
    old_stps = state.get('old_stps')
    H_diag = state.get('H_diag')
    prev_flat_grad = state.get('prev_flat_grad')
    prev_loss = state.get('prev_loss')

    n_iter = 0
    # optimize for a max of max_iter iterations
    while n_iter < max_iter:
        # keep track of nb of iterations
        n_iter += 1
        state['n_iter'] += 1

        ############################################################
        # compute gradient descent direction
        ############################################################
        if state['n_iter'] == 1:
            d = flat_grad.neg()
            old_dirs = []
            old_stps = []
            H_diag = 1
        else:
            # do lbfgs update (update memory)
            y = flat_grad.sub(prev_flat_grad)
            s = d.mul(t)
            ys = y.dot(s)  # y*s
            if ys > 1e-10:
                # updating memory
                if len(old_dirs) == history_size:
                    # shift history by one (limited-memory)
                    old_dirs.pop(0)
                    old_stps.pop(0)

                # store new direction/step
                old_dirs.append(s)
                old_stps.append(y)

                # update scale of initial Hessian approximation
                H_diag = ys / y.dot(y)  # (y*y)

            # compute the approximate (L-BFGS) inverse Hessian
            # multiplied by the gradient
            num_old = len(old_dirs)

            if 'ro' not in state:
                state['ro'] = [None] * history_size
                state['al'] = [None] * history_size
            ro = state['ro']
            al = state['al']

            for i in range(num_old):
                ro[i] = 1. / old_stps[i].dot(old_dirs[i])

            # iteration in L-BFGS loop collapsed to use just one buffer
            q = flat_grad.neg()
            for i in range(num_old - 1, -1, -1):
                al[i] = old_dirs[i].dot(q) * ro[i]
                q.add_(-al[i], old_stps[i])

            # multiply by initial Hessian
            # r/d is the final direction
            d = r = torch.mul(q, H_diag)
            for i in range(num_old):
                be_i = old_stps[i].dot(r) * ro[i]
                r.add_(al[i] - be_i, old_dirs[i])

        if prev_flat_grad is None:
            prev_flat_grad = flat_grad.clone()
        else:
            prev_flat_grad.copy_(flat_grad)
        prev_loss = loss

        ############################################################
        # compute step length
        ############################################################
        # directional derivative
        gtd = flat_grad.dot(d)  # g * d

        # check that progress can be made along that direction
        if gtd > -tolerance_change:
            break

        # reset initial guess for step size
        if state['n_iter'] == 1:
            t = min(1., 1. / abs_grad_sum) * lr
        else:
            t = lr

        # optional line search: user function
        ls_func_evals = 0
        if line_search_fn is not None:
            # perform line search, using user function
            # raise RuntimeError("line search function is not supported yet")
            if line_search_fn == 'weak_wolfe':
                t = self._weak_wolfe(closure, d)
            elif line_search_fn == 'goldstein':
                t = self._goldstein(closure, d)
            elif line_search_fn == 'backtracking':
                t = self._backtracking(closure, d)
            self._add_grad(t, d)
        else:
            # no line search, simply move with fixed-step
            self._add_grad(t, d)
        if n_iter != max_iter:
            # re-evaluate function only if not in last iteration
            # the reason we do this: in a stochastic setting,
            # no use to re-evaluate that function here
            loss = closure().data[0]
            flat_grad = self._gather_flat_grad()
            abs_grad_sum = flat_grad.abs().sum()
            ls_func_evals = 1

        # update func eval
        current_evals += ls_func_evals
        state['func_evals'] += ls_func_evals

        ############################################################
        # check conditions
        ############################################################
        if n_iter == max_iter:
            break

        if current_evals >= max_eval:
            break

        if abs_grad_sum <= tolerance_grad:
            break

        if d.mul(t).abs_().sum() <= tolerance_change:
            break

        if abs(loss - prev_loss) < tolerance_change:
            break

    state['d'] = d
    state['t'] = t
    state['old_dirs'] = old_dirs
    state['old_stps'] = old_stps
    state['H_diag'] = H_diag
    state['prev_flat_grad'] = prev_flat_grad
    state['prev_loss'] = prev_loss

    return orig_loss

def _copy_param(self):
    original_param_data_list = []
    for p in self._params:
        param_data = p.data.new(p.size())
        param_data.copy_(p.data)
        original_param_data_list.append(param_data)
    return original_param_data_list

def _set_param(self, param_data_list):
    for i in range(len(param_data_list)):
        self._params[i].data.copy_(param_data_list[i])

def _set_param_incremental(self, alpha, d):
    offset = 0
    for p in self._params:
        numel = p.numel()
        p.data.copy_(p.data + alpha*d[offset:offset + numel].resize_(p.size()))
        offset += numel
    assert offset == self._numel()

def _directional_derivative(self, d):
    deriv = 0.0
    offset = 0
    for p in self._params:
        numel = p.numel()
        deriv += torch.sum(p.grad.data * d[offset:offset + numel].resize_(p.size()))
        offset += numel
    assert offset == self._numel()
    return deriv

def _max_alpha(self, d):
    offset = 0
    max_alpha = float('inf')
    for p, bnd in zip(self._params, self._bounds):
        numel = p.numel()
        l_bnd, u_bnd = bnd
        p_grad = d[offset:offset + numel].resize_(p.size())
        if l_bnd is not None:
            from_l_bnd = ((l_bnd-p.data)/p_grad)[p_grad<0]
            min_l_bnd = torch.min(from_l_bnd) if from_l_bnd.numel() > 0 else max_alpha
        if u_bnd is not None:
            from_u_bnd = ((u_bnd-p.data)/p_grad)[p_grad>0]
            min_u_bnd = torch.min(from_u_bnd) if from_u_bnd.numel() > 0 else max_alpha
        max_alpha = min(max_alpha, min_l_bnd, min_u_bnd)
    return max_alpha


def _backtracking(self, closure, d):
    # 0 < rho < 0.5 and 0 < w < 1
    rho = 1e-4
    w = 0.5

    original_param_data_list = self._copy_param()
    phi_0 = closure().data[0]
    phi_0_prime = self._directional_derivative(d)
    alpha_k = 1.0
    while True:
        self._set_param_incremental(alpha_k, d)
        phi_k = closure().data[0]
        self._set_param(original_param_data_list)
        if phi_k <= phi_0 + rho * alpha_k * phi_0_prime:
            break
        else:
            alpha_k *= w
    return alpha_k


def _goldstein(self, closure, d):
    # 0 < rho < 0.5 and t > 1
    rho = 1e-4
    t = 2.0

    original_param_data_list = self._copy_param()
    phi_0 = closure().data[0]
    phi_0_prime = self._directional_derivative(d)
    a_k = 0.0
    b_k = self._max_alpha(d)
    alpha_k = min(1e4, (a_k + b_k) / 2.0)
    while True:
        self._set_param_incremental(alpha_k, d)
        phi_k = closure().data[0]
        self._set_param(original_param_data_list)
        if phi_k <= phi_0 + rho*alpha_k*phi_0_prime:
            if phi_k >= phi_0 + (1-rho)*alpha_k*phi_0_prime:
                break
            else:
                a_k = alpha_k
                alpha_k = t*alpha_k if isinf(b_k) else (a_k + b_k) / 2.0
        else:
            b_k = alpha_k
            alpha_k = (a_k + b_k)/2.0
        if torch.sum(torch.abs(alpha_k * d)) < self.param_groups[0]['tolerance_grad']:
            break
        if abs(b_k-a_k) < 1e-6:
            break
    return alpha_k


def _weak_wolfe(self, closure, d):
    # 0 < rho < 0.5 and rho < sigma < 1
    rho = 1e-4
    sigma = 0.9

    original_param_data_list = self._copy_param()
    phi_0 = closure().data[0]
    phi_0_prime = self._directional_derivative(d)
    a_k = 0.0
    b_k = self._max_alpha(d)
    alpha_k = min(1e4, (a_k + b_k) / 2.0)
    while True:
        self._set_param_incremental(alpha_k, d)
        phi_k = closure().data[0]
        phi_k_prime = self._directional_derivative(d)
        self._set_param(original_param_data_list)
        if phi_k <= phi_0 + rho*alpha_k*phi_0_prime:
            if phi_k_prime >= sigma*phi_0_prime:
                break
            else:
                alpha_hat = alpha_k + (alpha_k - a_k) * phi_k_prime / (phi_0_prime - phi_k_prime)
                a_k = alpha_k
                phi_0 = phi_k
                phi_0_prime = phi_k_prime
                alpha_k = alpha_hat
        else:
            alpha_hat = a_k + 0.5*(alpha_k-a_k)/(1+(phi_0-phi_k)/((alpha_k-a_k)*phi_0_prime))
            b_k = alpha_k
            alpha_k = alpha_hat
        if torch.sum(torch.abs(alpha_k * d)) < self.param_groups[0]['tolerance_grad']:
            break
        if abs(b_k-a_k) < 1e-6:
            break
    return alpha_k<ECODE>
OCY
Could you please submit a PR with the changes?
apaszke
I made pull request. It is showing some error message Travis-CI though.
OCY
I’ve been trying to use this code. The _backtracking linesearch method doesn’t take bounds. Both _weak_wolfe and _goldstein take bounds, but don’t obey them. Further, the _max_alpha method breaks unless you are working with both upper and lower bounds. I’m not convinced that it calculates the max_alpha parameter correctly either. I’m trying to work on a fix on my own, but I’m hoping that the author has already fixed these issues and could offer a new version to play around with.
rapidsnow
Any news related to this topic?
magnus_w
palimboa