Hi, I’m getting this RuntimeError while running an optimization algorithm. I was able to run without problems 2 days ago, but now I occur in this error every time. The error doesn’t show up always on the same iteration, but sometimes the algorithm can run for 23 iterations or sometimes only one or less.

This is the error message:

```
RuntimeError Traceback (most recent call last)
in
44 tilts, outputs, rewards = Algorithm5.update_data(next_config)
45 #aggiorno il modello
---> 46 Algorithm5.fit_model(next_config)
47
48 end_k = time.time()
in fit_model(self, new_config)
54 optimizer.zero_grad()
55 output = self.model(self.model.train_inputs[0])
---> 56 loss = -mll(output, self.model.train_targets)
57 loss.backward()
58 optimizer.step()
~\anaconda3\lib\site-packages\gpytorch\module.py in __call__(self, *inputs, **kwargs)
26
27 def __call__(self, *inputs, **kwargs):
---> 28 outputs = self.forward(*inputs, **kwargs)
29 if isinstance(outputs, list):
30 return [_validate_module_outputs(output) for output in outputs]
~\anaconda3\lib\site-packages\gpytorch\mlls\exact_marginal_log_likelihood.py in forward(self, function_dist, target, *params)
49 # Get the log prob of the marginal distribution
50 output = self.likelihood(function_dist, *params)
---> 51 res = output.log_prob(target)
52
53 # Add additional terms (SGPR / learned inducing points, heteroskedastic likelihood models)
~\anaconda3\lib\site-packages\gpytorch\distributions\multivariate_normal.py in log_prob(self, value)
133
134 # Get log determininat and first part of quadratic form
--> 135 inv_quad, logdet = covar.inv_quad_logdet(inv_quad_rhs=diff.unsqueeze(-1), logdet=True)
136
137 res = -0.5 * sum([inv_quad, logdet, diff.size(-1) * math.log(2 * math.pi)])
~\anaconda3\lib\site-packages\gpytorch\lazy\lazy_tensor.py in inv_quad_logdet(self, inv_quad_rhs, logdet, reduce_inv_quad)
1000 from .chol_lazy_tensor import CholLazyTensor
1001
-> 1002 cholesky = CholLazyTensor(self.cholesky())
1003 return cholesky.inv_quad_logdet(inv_quad_rhs=inv_quad_rhs, logdet=logdet, reduce_inv_quad=reduce_inv_quad)
1004
~\anaconda3\lib\site-packages\gpytorch\lazy\lazy_tensor.py in cholesky(self, upper)
737 (LazyTensor) Cholesky factor (lower triangular)
738 """
--> 739 res = self._cholesky()
740 if upper:
741 res = res.transpose(-1, -2)
~\anaconda3\lib\site-packages\gpytorch\utils\memoize.py in g(self, *args, **kwargs)
32 cache_name = name if name is not None else method
33 if not is_in_cache(self, cache_name):
---> 34 add_to_cache(self, cache_name, method(self, *args, **kwargs))
35 return get_from_cache(self, cache_name)
36
~\anaconda3\lib\site-packages\gpytorch\lazy\lazy_tensor.py in _cholesky(self)
412
413 # contiguous call is necessary here
--> 414 cholesky = psd_safe_cholesky(evaluated_mat).contiguous()
415 return NonLazyTensor(cholesky)
416
~\anaconda3\lib\site-packages\gpytorch\utils\cholesky.py in psd_safe_cholesky(A, upper, out, jitter)
46 except RuntimeError:
47 continue
---> 48 raise e
~\anaconda3\lib\site-packages\gpytorch\utils\cholesky.py in psd_safe_cholesky(A, upper, out, jitter)
23 """
24 try:
---> 25 L = torch.cholesky(A, upper=upper, out=out)
26 return L
27 except RuntimeError as e:
RuntimeError: cholesky_cpu: U(135,135) is zero, singular U.
```

What can I do?

Thanks