diff --git a/_unittests/ut_pycode/test_profiling.py b/_unittests/ut_pycode/test_profiling.py
index 211c1663..fa87aa9a 100644
--- a/_unittests/ut_pycode/test_profiling.py
+++ b/_unittests/ut_pycode/test_profiling.py
@@ -6,7 +6,11 @@
 import unittest
 import warnings
 import time
-from pstats import SortKey
+try:
+    from pstats import SortKey
+except ImportError:
+    # python < 3.7
+    from pyquickhelper.pycode.profiling import SortKey
 import pandas
 from pyquickhelper.pycode import ExtTestCase
 from pyquickhelper.pandashelper import df2rst
@@ -119,6 +123,8 @@ def simple():
         self.assertIn('"start_time"', res)
         self.assertNotEmpty(ps)
 
+    @unittest.skipIf(sys.version_info[:2] < (3, 7),
+                     reason="not supported")
     def test_profile_graph(self):
         calls = [0]
 
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 135719a1..376f53ac 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -6,6 +6,8 @@ jobs:
     matrix:
       Python39:
         python.version: '3.9'
+      Python36:
+        python.version: '3.6'
     maxParallel: 3
 
   steps:
diff --git a/requirements.txt b/requirements.txt
index 12d901a8..5efdfd36 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -51,9 +51,8 @@ sphinx-gallery
 sphinxcontrib-imagesvg
 sphinx_rtd_theme
 tabulate
-thebe
 tqdm
-traitlets>=5.0
+traitlets
 unify
 virtualenv
 wheel
diff --git a/src/pyquickhelper/pycode/profiling.py b/src/pyquickhelper/pycode/profiling.py
index 786e73fb..07e40b30 100644
--- a/src/pyquickhelper/pycode/profiling.py
+++ b/src/pyquickhelper/pycode/profiling.py
@@ -9,7 +9,17 @@
 import os
 import site
 import cProfile
-from pstats import SortKey, Stats
+from pstats import Stats
+
+try:
+    from pstarts import SortKey
+except ImportError:  # pragma: no cover
+    # Python < 3.7
+
+    class SortKey:
+        LINE = 'line'
+        CUMULATIVE = 'cumulative'
+        TIME = 'time'
 
 
 class ProfileNode:
@@ -53,9 +63,7 @@ def add_calls_to(self, pnode, time_elements):
 
     @staticmethod
     def _key(filename, line, fct):
-        key = "%s:%d" % (filename, line)
-        if key == "~:0":
-            key += ":%s" % fct
+        key = "%s:%d:%s" % (filename, line, fct)
         return key
 
     @property
@@ -68,11 +76,13 @@ def get_root(self):
         "Returns the root of the graph."
         done = set()
 
-        def _get_root(node):
+        def _get_root(node, stor=None):
+            if stor is not None:
+                stor.append(node)
             if len(node.called_by) == 0:
                 return node
             if len(node.called_by) == 1:
-                return _get_root(node.called_by[0])
+                return _get_root(node.called_by[0], stor=stor)
             res = None
             for ct in node.called_by:
                 k = id(node), id(ct)
@@ -81,12 +91,20 @@ def _get_root(node):
                 res = ct
                 break
             if res is None:
-                raise RuntimeError(  # pragma: no cover
-                    "All paths have been explored and no entry point was found.")
+                # All paths have been explored and no entry point was found.
+                # Choosing the most consuming function.
+                return None
             done.add((id(node), id(res)))
-            return _get_root(res)
+            return _get_root(res, stor=stor)
 
-        return _get_root(self)
+        root = _get_root(self)
+        if root is None:
+            candidates = []
+            _get_root(self, stor=candidates)
+            tall = [(n.tall, n) for n in candidates]
+            tall.sort()
+            root = tall[-1][-1]
+        return root
 
     def __repr__(self):
         "usual"
@@ -537,7 +555,7 @@ def better_name(row):
             return ps, res
     if as_df:
         raise ValueError(  # pragma: no cover
-            "as_df is not a compatible option with pyinst_format")
+            "as_df is not a compatible option with pyinst_format.")
 
     try:
         from pyinstrument import Profiler