diff --git a/parsl/tests/conftest.py b/parsl/tests/conftest.py
index a9c6efbe8c..c645d0af1e 100644
--- a/parsl/tests/conftest.py
+++ b/parsl/tests/conftest.py
@@ -20,6 +20,7 @@
 from itertools import chain
 
 import _pytest.runner as runner
+import psutil
 import pytest
 
 import parsl
@@ -191,6 +192,17 @@ def load_dfk_session(request, pytestconfig, tmpd_cwd_session):
     config = pytestconfig.getoption('config')[0]
 
     if config != 'local':
+        this_process = psutil.Process()
+        start_fds = this_process.num_fds()
+        logger.error(f"BENC: start open fds: {start_fds}")
+
+        # GC executor starts a thread at import(!)
+        # so we can't make this assertion...
+        # assert threading.active_count() == 1, "precondition: only one thread can be running before this test: " + repr(threading.enumerate())
+        pre_ac = threading.active_count()
+        if pre_ac > 1:
+            logger.warning("precondition: only one thread should be running before this test: " + repr(threading.enumerate()))
+
         spec = importlib.util.spec_from_file_location('', config)
         module = importlib.util.module_from_spec(spec)
         spec.loader.exec_module(module)
@@ -218,6 +230,17 @@ def load_dfk_session(request, pytestconfig, tmpd_cwd_session):
             raise RuntimeError("DFK changed unexpectedly during test")
         dfk.cleanup()
         assert DataFlowKernelLoader._dfk is None
+        end_fds = this_process.num_fds()
+        logger.error(f"BENC: end open fds: {end_fds}")
+
+        post_ac = threading.active_count()
+        assert pre_ac == post_ac, "test left threads running: " + repr(threading.enumerate())
+        # assert threading.active_count() == 1, "test left threads running: " + repr(threading.enumerate())
+
+        end_fds = this_process.num_fds()
+        logger.error(f"BENC: end open fds: {end_fds} (vs {start_fds} at start)")
+        assert start_fds == end_fds, "number of open fds changed across test run"
+
     else:
         yield
 
@@ -239,6 +262,16 @@ def load_dfk_local_module(request, pytestconfig, tmpd_cwd_session):
     config = pytestconfig.getoption('config')[0]
 
     if config == 'local':
+        this_process = psutil.Process()
+        start_fds = this_process.num_fds()
+        logger.error(f"BENC: start open fds: {start_fds}")
+        logger.error(f"BENC: start threads: {threading.active_count()}")
+
+        pre_ac = threading.active_count()
+        if pre_ac > 1:
+            logger.warning("precondition: only one thread should be running before this test: " + repr(threading.enumerate()))
+        # assert threading.active_count() == 1, "precondition: only one thread can be running before this test: " + repr(threading.enumerate())
+
         local_setup = getattr(request.module, "local_setup", None)
         local_teardown = getattr(request.module, "local_teardown", None)
         local_config = getattr(request.module, "local_config", None)
@@ -270,6 +303,21 @@ def load_dfk_local_module(request, pytestconfig, tmpd_cwd_session):
                 raise RuntimeError("DFK changed unexpectedly during test")
             dfk.cleanup()
             assert DataFlowKernelLoader._dfk is None
+        end_fds = this_process.num_fds()
+        logger.error(f"BENC: end open fds: {end_fds} (vs start {start_fds}")
+        logger.error(f"BENC: end threads: {threading.active_count()}")
+
+        post_ac = threading.active_count()
+        assert pre_ac == post_ac, "test left threads running: " + repr(threading.enumerate())
+
+        # assert threading.active_count() == 1, "test left threads running: " + repr(threading.enumerate())
+
+        end_fds = this_process.num_fds()
+        logger.error(f"BENC: open fds END: {end_fds}")
+        if end_fds > start_fds:
+            logger.error(f"Open files (not all fds, though?): {this_process.open_files()!r}")
+            os.system(f"ls -l /proc/{os.getpid()}/fd")
+            pytest.fail("BENC: number of open fds increased across test")
 
     else:
         yield