Why using this following simple model:

```
with pm.Model() as linear_model:
data_x = pm.Data('data_x', matrix_train)
data_y = pm.Data('data_y', matrix_positive.flatten())
beta = pm.Normal('beta', mu=0, sd=20, shape=nfeatures)
sigma = pm.HalfNormal('sigma', sd=20)
mu = pm.Deterministic("mu", pm.math.dot(data_x, beta))
y_obs = pm.Normal('y', mu=mu, sd=sigma,
observed=data_y)
```

works with the following sampling:

```
trace = pm.sample(10000, tune=2000, cores=1)
```

But gets stuck with the following sampling:

```
trace = pm.sample(10000, tune=2000, cores=2)
```

Is this a known issue ? Seems to be related with multiprocessing. Iâ€™m using the master branch and MacOS.