add a sanity check ensuring that Numba threading actually works

.github/workflows/pypi.yml

@@ -38,7 +38,7 @@ jobs:
           unset CI
           cd ${{ matrix.packages-dir }}
           python -m build 2>&1 | tee build.log
-          exit `fgrep -i warning build.log | grep -v impl_numba/warnings.py \
+          exit `fgrep -i warning build.log | grep -v warnings.py \
             | grep -v "no previously-included files matching" \
             | grep -v "version of {dist_name} already set" \
             | grep -v -E "UserWarning: version of PySDM(-examples)? already set" \

PySDM/backends/numba.py

@@ -7,6 +7,8 @@
 import warnings
 
 import numba
+from numba import prange
+import numpy as np
 
 from PySDM.backends.impl_numba import methods
 from PySDM.backends.impl_numba.random import Random as ImportedRandom
@@ -45,21 +47,43 @@ def __init__(
         self.formulae_flattened = self.formulae.flatten
 
         parallel_default = True
-        if platform.machine() == "arm64":
-            if "CI" not in os.environ:
-                warnings.warn(
-                    "Disabling Numba threading due to ARM64 CPU (atomics do not work yet)"
-                )
-            parallel_default = False  # TODO #1183 - atomics don't work on ARM64!
-
-        try:
-            numba.parfors.parfor.ensure_parallel_support()
-        except numba.core.errors.UnsupportedParforsError:
-            if "CI" not in os.environ:
-                warnings.warn(
-                    "Numba version used does not support parallel for (32 bits?)"
-                )
-            parallel_default = False
+
+        if override_jit_flags is not None and "parallel" in override_jit_flags:
+            parallel_default = override_jit_flags["parallel"]
+
+        if parallel_default:
+            if platform.machine() == "arm64":
+                if "CI" not in os.environ:
+                    warnings.warn(
+                        "Disabling Numba threading due to ARM64 CPU (atomics do not work yet)"
+                    )
+                parallel_default = False  # TODO #1183 - atomics don't work on ARM64!
+
+            try:
+                numba.parfors.parfor.ensure_parallel_support()
+            except numba.core.errors.UnsupportedParforsError:
+                if "CI" not in os.environ:
+                    warnings.warn(
+                        "Numba version used does not support parallel for (32 bits?)"
+                    )
+                parallel_default = False
+
+            if not numba.config.DISABLE_JIT:  # pylint: disable=no-member
+
+                @numba.jit(parallel=True, nopython=True)
+                def fill_array_with_thread_id(arr):
+                    """writes thread id to corresponding array element"""
+                    for i in prange(  # pylint: disable=not-an-iterable
+                        numba.get_num_threads()
+                    ):
+                        arr[i] = numba.get_thread_id()
+
+                fill_array_with_thread_id(arr := np.full(numba.get_num_threads(), -1))
+                if not max(arr) == arr[-1] == numba.get_num_threads() - 1:
+                    raise ValueError(
+                        "Numba threading enabled but does not work"
+                        " (try other setting of the NUMBA_THREADING_LAYER env var?)"
+                    )
 
         assert "fastmath" not in (override_jit_flags or {})
         self.default_jit_flags = {

tests/unit_tests/backends/test_ctor_defaults.py → tests/unit_tests/backends/test_ctor_defaults_and_warnings.py

@@ -1,10 +1,14 @@
 # pylint: disable=missing-module-docstring,missing-class-docstring,missing-function-docstring
+from unittest import mock
+import warnings
 import inspect
+import pytest
+import numba
 
 from PySDM.backends import Numba, ThrustRTC
 
 
-class TestCtorDefaults:
+class TestCtorDefaultsAndWarnings:
     @staticmethod
     def test_gpu_ctor_defaults():
         signature = inspect.signature(ThrustRTC.__init__)
@@ -17,3 +21,15 @@ def test_gpu_ctor_defaults():
     def test_cpu_ctor_defaults():
         signature = inspect.signature(Numba.__init__)
         assert signature.parameters["formulae"].default is None
+
+    @staticmethod
+    @mock.patch("PySDM.backends.numba.prange", new=range)
+    def test_check_numba_threading_warning():
+        if numba.config.DISABLE_JIT:  # pylint: disable=no-member
+            pytest.skip()
+
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore")
+            with pytest.raises(ValueError) as exc_info:
+                Numba()
+            assert exc_info.match(r"^Numba threading enabled but does not work")