Softplus returning negative values in training loop

I’m using (torch.nn.functional.softplus(x) + 1e-6) to compute the variance that is trained in a policy gradient network. It occasionally throws a negative value which crashes the Normal(mean,variance) distribution creation for the error function. When I try and detect it and then print out the offending input, and run the same function outside of the loop, I get a different (valid) output for the same input.

I’ve been fighting this for 3 days - even using different softplus approximations like a manual squareplus function, and even clamping gradients to small values, and cannot figure out what’s going on. I’m using Pytorch-Lightning for the training module. Any ideas here? What could be going on in the training loop that might corrupt the final model output?


class GradientPolicy(nn.Module):
  def __init__(self, in_features, out_dims, hidden_size=128, xrange=[0.0,1.0],min_variance=1e-6):
    assert(xrange[0] < xrange[1])
    assert(min_variance >= 1e-6)
    self.min_var = nn.Parameter(torch.zeros(out_dims, requires_grad=False) + min_variance)
    self.range_min = nn.Parameter(torch.zeros(out_dims, requires_grad=False) + xrange[0])
    self.range_size = nn.Parameter(torch.zeros(out_dims, requires_grad=False) + (xrange[1] - xrange[0]))
    self.fc1 = nn.Linear(in_features, hidden_size)
    self.fc2 = nn.Linear(hidden_size, hidden_size)
    self.fc_mu = nn.Linear(hidden_size, out_dims)
    self.fc_var = nn.Linear(hidden_size, out_dims)

  def forward(self, x):
    x = torch.tensor(x).float().to(device)
    x = F.mish(self.fc1(x))
    x = F.mish(self.fc2(x))
    mu = F.sigmoid(self.fc_mu(x)) * self.range_size + self.range_min
    v = F.softplus(self.fc_var(x)) + self.min_var
    if (v <= 0.).any().item():
      print("softplus input",self.fc_var(x))
      print("gp variance: ",v)
    return mu, v

And here’s where it prints the detected bad value immediately before the crash:

softplus input tensor([[-3.7529]], device='cuda:0')
gp variance:  tensor([[-0.0329]], device='cuda:0')

Of course, when you manually run that input through softplus:


You get a positive output: tensor(0.0232)

WTF is going on here?

Is there a reason you make the min variance a param? I know it does not require a grad but I don’t see the full code base. In general I would put it as a buffer. Can you check that it’s value is unchanged?

It’s just a parameter for convenience during development.

I’m starting to think the problem is in the mu network since they share 2 layers and the output is a sigmoid, which could cause underflow in the gradients. Checking that next.

ARRGGHH!!! My bad. I figured it out.

This call:

self.min_var = nn.Parameter(torch.zeros(out_dims, requires_grad=False) + min_variance)

Passes a tensor into the nn.Parameter function without a gradient… but the nn.Parameter function defaults to using a gradient anyway. The function call should be:

self.min_var = nn.Parameter(torch.zeros(out_dims) + min_variance, requires_grad=False)

So that the parameter itself gets the no-grad parameter. Sheesh!!! I spent days on this stupid bug.

I would register a buffer instead :wink: