Update multiprocessing context to be Pool specific (pybamm-team#4001)

* convert global start context to individual pool context

* Update context creation, add tests for coverage

---------

Co-authored-by: Valentin Sulzer <valentinsulzer@hotmail.com>
Co-authored-by: Eric G. Kratz <kratman@users.noreply.github.com>

pybamm/solvers/base_solver.py

@@ -13,10 +13,6 @@
 import pybamm
 from pybamm.expression_tree.binary_operators import _Heaviside
 
-# Set context for parallel processing depending on the platform
-if platform.system() == "Darwin" or platform.system() == "Linux":
-    mp.set_start_method("fork")
-
 
 class BaseSolver:
     """Solve a discretised model.
@@ -70,6 +66,7 @@ def __init__(
         self.algebraic_solver = False
         self._on_extrapolation = "warn"
         self.computed_var_fcns = {}
+        self._mp_context = self.get_platform_context(platform.system())
 
     @property
     def root_method(self):
@@ -917,7 +914,7 @@ def solve(
                         model_inputs_list,
                     )
                 else:
-                    with mp.Pool(processes=nproc) as p:
+                    with mp.get_context(self._mp_context).Pool(processes=nproc) as p:
                         new_solutions = p.starmap(
                             self._integrate,
                             zip(
@@ -1395,6 +1392,12 @@ def check_extrapolation(self, solution, events):
                         "outside these bounds."
                     )
 
+    def get_platform_context(self, system_type: str):
+        # Set context for parallel processing depending on the platform
+        if system_type.lower() in ["linux", "darwin"]:
+            return "fork"
+        return "spawn"
+
     @staticmethod
     def _set_up_model_inputs(model, inputs):
         """Set up input parameters"""

tests/unit/test_solvers/test_base_solver.py

@@ -348,6 +348,12 @@ def test_multiple_models_error(self):
         with self.assertRaisesRegex(RuntimeError, "already been initialised"):
             solver.solve(model2, t_eval=[0, 1])
 
+    def test_multiprocess_context(self):
+        solver = pybamm.BaseSolver()
+        assert solver.get_platform_context("Win") == "spawn"
+        assert solver.get_platform_context("Linux") == "fork"
+        assert solver.get_platform_context("Darwin") == "fork"
+
     @unittest.skipIf(not pybamm.have_idaklu(), "idaklu solver is not installed")
     def test_sensitivities(self):
         def exact_diff_a(y, a, b):