Raise error if resource limitation and parallelization is requested (#…

…1023)

* Add warnings for parallelism doc

* Check for parallelization + pynisher

Does not work together, throw error.
Does not work bc Dask pickles jobs and
pynisher runs the target function in a process.

* Fix whitespaces

---------

Co-authored-by: Carolin Benjamins <benjamins@tnt.uni-hannover.de>

docs/advanced_usage/9_parallelism.rst

@@ -21,6 +21,39 @@ SMAC supports multiple workers natively via Dask. Just specify ``n_workers`` in
  When using multiple workers, SMAC is not reproducible anymore.
 
 
+.. warning ::
+
+ You cannot use resource limitation (pynisher, via the `scenario` arguments `trail_walltime_limit` and `trial_memory_limit`).
+ This is because pynisher works by running your function inside of a subprocess.
+ Once in the subprocess, the resources will be limited for that process before running your function. 
+ This does not work together with pickling - which is required by dask to schedule jobs on the cluster, even on a local one.
+
+
+.. warning ::
+
+ Start/run SMAC inside ``if __name__ == "__main__"`` in your script otherwise Dask is not able to correctly
+ spawn jobs and probably this runtime error will be raised:
+
+ .. code-block ::
+
+ RuntimeError: 
+ An attempt has been made to start a new process before the
+ current process has finished its bootstrapping phase.
+
+ This probably means that you are not using fork to start your
+ child processes and you have forgotten to use the proper idiom
+ in the main module:
+
+ if __name__ == '__main__':
+ freeze_support()
+ ...
+
+ The "freeze_support()" line can be omitted if the program
+ is not going to be frozen to produce an executable.
+
+
+
+
 Running on a Cluster
 --------------------
 You can also pass a custom dask client, e.g. to run on a slurm cluster.

smac/facade/abstract_facade.py

@@ -455,6 +455,18 @@ def _validate(self) -> None:
  # Make sure the same acquisition function is used
  assert self._acquisition_function == self._acquisition_maximizer._acquisition_function
 
+ if isinstance(self._runner, DaskParallelRunner) and (
+ self.scenario.trial_walltime_limit is not None or self.scenario.trial_memory_limit is not None
+ ):
+ # This is probably due to pickling dask jobs
+ raise ValueError(
+ "Parallelization via Dask cannot be used in combination with limiting "
+ "the resources "
+ "of the target function via `scenario.trial_walltime_limit` or "
+ "`scenario.trial_memory_limit`. Set those to `None` if you want "
+ "parallelization. "
+ )
+
  def _get_signature_arguments(self) -> list[str]:
  """Returns signature arguments, which are required by the intensifier."""
  arguments = []