Skip test_issue_3108.py tests if run with more than one MPI rank on G…

…ithub Linux runners due to performance issues; the tests are run under macOS

.github/workflows/nestbuildmatrix.yml

@@ -707,6 +707,8 @@ jobs:
           echo "backend : svg" > $HOME/.matplotlib/matplotlibrc
 
       - name: "Run NEST testsuite"
+        env:
+          DO_TESTS_SKIP_TEST_REQUIRING_MANY_CORES: foo
         run: |
           pwd
           cd "$NEST_VPATH"

testsuite/do_tests.sh

@@ -520,8 +520,16 @@ if test "${PYTHON}"; then
             for numproc in $(cd ${PYNEST_TEST_DIR}/mpi/; ls -d */ | tr -d '/'); do
                 XUNIT_FILE="${REPORTDIR}/${XUNIT_NAME}_mpi_${numproc}.xml"
                 PYTEST_ARGS="--verbose --timeout $TIME_LIMIT --junit-xml=${XUNIT_FILE} ${PYNEST_TEST_DIR}/mpi/${numproc}"
+
+		if "${DO_TESTS_SKIP_TEST_REQUIRING_MANY_CORES:-false}"; then
+		    PYTEST_ARGS="${PYTEST_ARGS} -m 'not requires_many_cores'"
+		fi
+
 		set +e
-                $(sli -c "${numproc} (${PYTHON} -m pytest) (${PYTEST_ARGS}) mpirun =only") 2>&1 | tee -a "${TEST_LOGFILE}"
+
+		# We need to use eval here because $() splits run_command in weird ways
+                run_command="$(sli -c "${numproc} (${PYTHON} -m pytest) (${PYTEST_ARGS}) mpirun =only")"
+		eval "${run_command}" 2>&1 | tee -a "${TEST_LOGFILE}"
 		set -e
             done
         fi

testsuite/pytests/conftest.py

@@ -47,6 +47,18 @@ def test_gsl():
 import testutil  # noqa
 
 
+def pytest_configure(config):
+    """
+    Add NEST-specific markers.
+
+    See https://docs.pytest.org/en/8.0.x/how-to/mark.html.
+    """
+    config.addinivalue_line(
+        "markers",
+        "requires_many_cores: mark tests as needing many cores (deselect with '-m \"not requires_many_cores\"')",
+    )
+
+
 @pytest.fixture(scope="module", autouse=True)
 def safety_reset():
     """

testsuite/pytests/test_issue_3108.py

@@ -34,6 +34,10 @@
 The connect tests test that NodeCollection::thread_local_begin() works.
 """
 
+# Needs up to 16 cores when run on 4 MPI ranks.
+# Experiences severe slowdown on Github runners under Linux with MPI and OpenMP
+pytestmark = pytest.mark.requires_many_cores
+
 if nest.ll_api.sli_func("is_threaded"):
     num_threads = [1, 2, 3, 4]
 else: