[benchmark] Limit setup overhead detection (>20)

For really small runtimes < 20 μs this method of setup overhead detection doesn’t work. Even 1μs change in 20μs runtime is 5%. Just return no overhead.

Author: palimondo

benchmark/scripts/Benchmark_Driver

@@ -442,7 +442,7 @@ class BenchmarkDoctor(object):
                     [[result.samples.min for result in i_series]
                      for i_series in
                      [select(measurements, num_iters=i) for i in [1, 2]]]]
-        setup = int(round(2.0 * (ti1 - ti2)))
+        setup = int(round(2.0 * (ti1 - ti2))) if ti2 > 20 else 0
         ratio = (setup / ti1) if ti1 > 0 else 0
         return (setup, ratio)
 

benchmark/scripts/test_Benchmark_Driver.py

@@ -728,11 +728,18 @@ def test_benchmark_has_no_significant_setup_overhead(self):
                 'SO O i2a': _PTR(min=67), 'SO O i2b': _PTR(min=68)})
             doctor.analyze({'name': 'Zero', 'Zero O i1a': _PTR(min=0),
                             'Zero O i2a': _PTR(min=0)})
+            doctor.analyze({
+                'name': 'OOO',  # Out Of Order
+                # Impossible to detect overhead -- limits of precision:
+                # Even 1μs change in 20μs runtime is 5%.
+                'OOO O i1a': _PTR(min=21),
+                'OOO O i2a': _PTR(min=20)})
         output = out.getvalue()
 
         self.assertIn('runtime: ', output)
         self.assertNotIn('NoOverhead', output)
         self.assertNotIn('ZeroRuntime', output)
+        self.assertNotIn('OOO', output)
         self.assert_contains(
             ["'SO' has setup overhead of 4 μs (5.8%)."],
             self.logs['error'])