PyTorch 1.7 does not free memory as PyTorch 1.6

J_Z: <p>Hi, I encountered a weird situation for the same code, in PyTorch 1.1 to 1.6, the code takes almost constant memory, but in PyTorch 1.7 the memory consumptions keep increasing. Could you help out check what’s the reason? Is the mechanism to free GPU memory experiencing a big change? Thanks a lot.</p>
<p>I defined an autograd function, where the backward function creates local variables, performs local backward, then deletes intermediate tensors.</p>
<p>The link to the repo is <a href="https://github.com/juntang-zhuang/torch_ACA" class="inline-onebox" rel="noopener nofollow ugc">GitHub - juntang-zhuang/torch_ACA: repo for paper: Adaptive Checkpoint Adjoint (ACA) method for gradient estimation in neural ODE</a>, and can be reproduced by running “python cifar_classification/train_mem.py”</p>
<p>Here’s the autograd function</p>
<pre><code class="lang-auto">import torch
import torch.nn as nn
from .ode_solver_endtime import odesolve_endtime
from torch.autograd import Variable
import copy
__all__ = ['odesolve_adjoint']

def flatten_params(params):
    flat_params = [p.contiguous().view(-1) for p in params]
    return torch.cat(flat_params) if len(flat_params) &gt; 0 else torch.tensor([])

def flatten_params_grad(params, params_ref):
    _params = [p for p in params]
    _params_ref = [p for p in params_ref]
    flat_params = [p.contiguous().view(-1) if p is not None else torch.zeros_like(q).view(-1)
        for p, q in zip(_params, _params_ref)]

    return torch.cat(flat_params) if len(flat_params) &gt; 0 else torch.tensor([])

class Checkpointing_Adjoint(torch.autograd.Function):

    @staticmethod
    def forward(ctx, *args):
        z0, func, flat_params, options= args[:-3], args[-3], args[-2], args[-1]

        if isinstance(z0,tuple):
            if len(z0) == 1:
                z0 = z0[0]

        ctx.func = func
        state0 = func.state_dict()
        ctx.state0 = state0
        if isinstance(z0, tuple):
            ctx.z0 = tuple([_z0.data for _z0 in z0])
        else:
            ctx.z0 = z0.data

        ctx.options = options

        with torch.no_grad():
            solver = odesolve_endtime(func, z0, options, return_solver=True, regenerate_graph = False)
            #solver.func.load_state_dict(state0)
            ans, steps = solver.integrate(z0, return_steps=True)

        ctx.steps = steps
        #ctx.ans = ans

        return ans

    @staticmethod
    def backward(ctx, *grad_output):

        if isinstance(ctx.z0, tuple):
            z0 = tuple([Variable(_z0, requires_grad=True) for _z0 in ctx.z0])
        else:
            z0 = Variable(ctx.z0, requires_grad=True)

        options = ctx.options
        func = ctx.func
        f_params = func.parameters()

        steps, state0 = ctx.steps, ctx.state0

        func.load_state_dict(state0)

        if isinstance(z0, tuple) or isinstance(z0, list):
            use_tuple = True
        else:
            use_tuple = False

        z = z0

        solver = odesolve_endtime(func, z, options, return_solver=True)

        # record inputs to each step
        inputs = []
        inputs.append(z)

        #t0 = solver.t0
        t_current = solver.t0
        y_current = z
        for point in steps:
            solver.neval += 1
            # print(y_current.shape)
            with torch.no_grad():
                y_current, error, variables = solver.step(solver.func, t_current, point - t_current, y_current, return_variables=True)
                t_current = point

            if not use_tuple:
                inputs.append(Variable(y_current.data, requires_grad = True))
            else:
                inputs.append([Variable(_y.data, requires_grad=True) for _y in y_current])

            if use_tuple:
                solver.delete_local_computation_graph(list(error) + list(variables))
            else:
                solver.delete_local_computation_graph([error] + list(variables))

        # delete the gradient directly applied to the original input
        # if use tuple, input is directly concatenated with output
        grad_output = list(grad_output)
        if use_tuple:
            input_direct_grad = grad_output[0][0,...]
            grad_output[0] = grad_output[0][1,...]
        grad_output = tuple(grad_output)

        ###################################
        #print(steps)
        # note that steps does not include the start point, need to include it
        steps = [options['t0']] + steps
        # now two list corresponds, steps = [t0, teval1, teval2, ... tevaln, t1]
        #                           inputs = [z0, z1, z2, ... , z_out]
        ###################################

        inputs.pop(-1)
        steps2 = copy.deepcopy(steps)
        steps2.pop(0)
        steps.pop(-1)

        # steps = [t0, eval1, eval2, ... evaln, t1], after pop is [t0, eval1, ... evaln]
        # steps2 = [t0, eval1, eval2, ... evaln, t1], after pop is [eval1, ... evaln, t1]

        # after reverse, they are
        # steps = [evaln, evaln-1, ... eval2, eval1, t0]
        # steps2 = [t1, evaln, ... eval2, eval1s]

        param_grads = []
        inputs.reverse()
        steps.reverse()
        steps2.reverse()

        assert len(inputs) == len(steps) == len(steps2), print('len inputs {}, len steps {}, len steps2 {}'.format(len(inputs), len(steps), len(steps2)))

        for input, point, point2 in zip(inputs, steps, steps2):
            if not use_tuple:
                input = Variable(input, requires_grad = True)
            else:
                input = [Variable(_, requires_grad = True) for _ in input]
                input = tuple(input)

            with torch.enable_grad():
                #print(type(z))
                y, error, variables = solver.step(solver.func, point, point2 - point, input, return_variables=True)

                param_grad = torch.autograd.grad(
                    y, f_params,
                    grad_output, retain_graph=True)

                grad_output = torch.autograd.grad(
                 y,  input,
                 grad_output)

                param_grads.append(param_grad)

                if use_tuple:
                    solver.delete_local_computation_graph(list(y) + list(error) + list(variables))
                else:
                    solver.delete_local_computation_graph([y, error] + list(variables))

        # sum up gradients w.r.t parameters at each step, stored in out2
        out2 = param_grads[0]
        for i in range(1, len(param_grads)):
            for _1, _2 in zip([*out2], [*param_grads[i]]):
                _1 += _2

        # attach direct gradient w.r.t input
        if use_tuple:
            grad_output = list(grad_output)
            # add grad output to direct gradient
            if input_direct_grad is not None:
                grad_output[0] = input_direct_grad + grad_output[0]#torch.stack((input_direct_grad, grad_output[0]), dim=0)

            grad_output = tuple(grad_output)
        out = tuple([*grad_output] + [None, flatten_params_grad(out2, func.parameters()), None])

        return out
        #return  out1[0], out1[1], None, flatten_params_grad(out2, func.parameters()), None


def odesolve_adjoint(func, z0, options = None):

    flat_params = flatten_params(func.parameters())
    if isinstance(z0, tuple) or isinstance(z0, list):
        zs = Checkpointing_Adjoint.apply(*z0, func, flat_params, options)
    else:
        zs = Checkpointing_Adjoint.apply(z0, func, flat_params, options)
    return zs
</code></pre>
<p>The definition of function to delete local variable is</p>
<pre><code class="lang-auto">        for i in inputs:
            i.set_()
            del i
        torch.cuda.empty_cache()
        return```</code></pre>