Use a separate error parameter for lower bound.

bench/gen_bench_expectations.py

 #!/usr/bin/env python
 # Copyright (c) 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """ Generate bench_expectations file from a given set of bench data files. """
 
 import argparse
 import bench_util
 import os
 import re
 import sys
 
 # Parameters for calculating bench ranges.
 RANGE_RATIO_UPPER = 1.2  # Ratio of range for upper bounds.
 RANGE_RATIO_LOWER = 2.0  # Ratio of range for lower bounds.
 ERR_RATIO = 0.08  # Further widens the range by the ratio of average value.
-ERR_ABS = 0.5  # Adds an absolute error margin to cope with very small benches.
+ERR_UB = 0.5  # Adds an absolute upper error to cope with small benches.
+ERR_LB = 1.5
 
 # List of bench configs to monitor. Ignore all other configs.
 CONFIGS_TO_INCLUDE = ['simple_viewport_1000x1000',
                       'simple_viewport_1000x1000_gpu',
                       'simple_viewport_1000x1000_scalar_1.100000',
                       'simple_viewport_1000x1000_scalar_1.100000_gpu',
                      ]
 
 # List of flaky SKPs that should be excluded.
 SKPS_TO_EXCLUDE = [
                   ]
 
 
 def compute_ranges(benches):
   """Given a list of bench numbers, calculate the alert range.
 
   Args:
     benches: a list of float bench values.
 
   Returns:
     a list of float [lower_bound, upper_bound].
   """
   minimum = min(benches)
   maximum = max(benches)
   diff = maximum - minimum
   avg = sum(benches) / len(benches)
 
-  return [minimum - diff * RANGE_RATIO_LOWER - avg * ERR_RATIO - ERR_ABS,
-          maximum + diff * RANGE_RATIO_UPPER + avg * ERR_RATIO + 2 * ERR_ABS]
+  return [minimum - diff * RANGE_RATIO_LOWER - avg * ERR_RATIO - ERR_UB,
+          maximum + diff * RANGE_RATIO_UPPER + avg * ERR_RATIO + ERR_LB]
 
 
 def create_expectations_dict(revision_data_points):
   """Convert list of bench data points into a dictionary of expectations data.
 
   Args:
     revision_data_points: a list of BenchDataPoint objects.
 
   Returns:
     a dictionary of this form:
@@ skipping 56 matching lines @@
               'expected': expected,
               'lower_bound': lower_bound,
               'upper_bound': upper_bound})
 
     with open(args.output_file, 'w') as file_handle:
       file_handle.write('\n'.join(out_lines))
 
 
 if __name__ == "__main__":
     main()