Torch - RuntimeError: Directly access saved variables after they have already been freed. (2nd use of .backward)

autograd
1

Hey guys,
In my code i am trying to train a model so that it moves a given sample to a given target distribution. The next step is to introduce intermediate distributions and to use a loop so that the particles (the samples) are moved from one distribution to another iteratively. Unfortunately, at the second iteration I get the following Error-Message when running my code: “Trying to backward through the graph a second time (or directly access saved variables after they have already been freed). Saved intermediate values of the graph are freed when you call .backward() or autograd.grad(). Specify retain_graph=True if you need to backward through the graph a second time or if you need to access saved variables after calling backward”

I don’t think that retain_graph = True would fit my problem, since I would rather kind of clear the model after every iteraion than retain it. However, i gave it a shot, the result is the following error:

one of the variables needed for gradient computation has been modified by an inplace operation: [torch.FloatTensor [1, 2]] is at version 2251; expected version 2250 instead. Hint: enable anomaly detection to find the operation that failed to compute its gradient, with torch.autograd.set_detect_anomaly(True).

Here are the relevant parts of my code:

  for k in range(1, K_intermediate+1):

           flow = BasicFlow(dim=d, n_flows=n_flows, flow_layer=flow_layer)
           ldj = train_flow(
                 flow, x, W, f_intermediate(x,k-1), lambda x: 
                      f_intermediate(x,k), epochs=2500
    )
           x, xtransp = flow(x)
           x = xtransp.data

def train_flow(flow, sample, weights, f0, f1, epochs=1000):
    optim = torch.optim.Adam(flow.parameters(), lr=1e-2)

    for i in range(epochs):
        x0, xtransp = flow(sample)
        ldj = accumulate_kl_div(flow).reshape(sample.size(0))
    
        loss = det_loss(
            x_0 = x0, 
            x_transp = xtransp, 
            weights = weights, 
            ldj = ldj, 
            f0 = f0, 
            f1 = f1
        )
        loss.backward(retain_graph = True)
        optim.step()
        optim.zero_grad()
        reset_kl_div(flow)
        if i % 250 == 0:
            if i > 0 and previous_loss - loss.item() < 1e-06:
                break                
            print(loss.item())
            previous_loss = loss.item()
            if torch.isnan(loss) == True:
                break
    return ldj

Note that the problem only arises since I start capturing the ldj-value (log of the determinant jacobian, for those who wonder). Since this value is crucial for further computations i can not just delete this.

Happy for any kind of advices,
Christian

2

I don’t know what the methods are exactly doing, but it seems you might have introduced a dependency on the computation graph history from previous iterations.
Check, if any used tensor might have been created in a previous iteration (and would also still be attached to the graph). If this is indeed needed (and you want to backpropagate through multiple iterations), you should use retain_graph=True, if not, you should detach() this tensor before using it in the current iteration.

3

Hi, many thank for your answer. In fact the problem could be solved just by “transforming” the input W using W.data, since after the first iteration it was of the type “backwardMul” or something like that.
Greeting, Christian