Telemetry / Memory benchmark fix: Separate histograms for different tests.

tools/perf/perf_tools/histogram.py

+# Copyright (c) 2013 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.
+import json
+import logging
+
+def SubtractHistogram(histogram_json, start_histogram_json):
+  """Subtracts a previous histogram from a histogram. Both parameters are json
+  serializations of histograms."""
+  start_histogram = json.loads(start_histogram_json)
+  # It's ok if the start histogram is empty (we had no data, maybe even no
+  # histogram at all, at the start of the test).
+  if 'buckets' not in start_histogram:
+    return histogram_json
+
+  start_histogram_buckets = dict()
+  for b in start_histogram['buckets']:
+    start_histogram_buckets[b['low']] = b['count']
+
+  new_buckets = []
+  histogram = json.loads(histogram_json)
+  for b in histogram['buckets']:
+    new_bucket = b
+    low = b['low']
+    if low in start_histogram_buckets:
+      new_bucket['count'] = b['count'] - start_histogram_buckets[low]
+      if new_bucket['count'] < 0:
+        logging.error('Histogram subtraction error, starting histogram most '
+                      'probably invalid.')
+    if new_bucket['count']:
+      new_buckets.append(new_bucket)
+  histogram['buckets'] = new_buckets
+  histogram['count'] -= start_histogram['count']
+
+  return json.dumps(histogram)