Telemetry: add missing functionality to aggregate histograms.

This way we'll get saner and less noisy histogram-based results.

Rationale for moving histogram_utils.py: HistogramValue is part of
telemetry.value, so it makes sense that all utils it needs are there too;
doesn't make sense to import the utils from metrics.

BUG=

Review URL: https://codereview.chromium.org/578593004

Cr-Commit-Position: refs/heads/master@{#295254}

tools/perf/measurements/memory_pressure.py

@@ -10,7 +10,7 @@ the entire test rather than each single page.
 
 import logging
 
-from metrics import histogram_util
+from telemetry.value import histogram_util
 from telemetry.page import page_test
 
 

tools/perf/measurements/session_restore.py

@@ -6,9 +6,9 @@ import collections
 
 from measurements import startup
 from metrics import cpu
-from metrics import histogram_util
 from metrics import startup_metric
 from telemetry.core import util
+from telemetry.value import histogram_util
 
 
 class SessionRestore(startup.Startup):

tools/perf/measurements/tab_switching.py

@@ -12,11 +12,11 @@ Power usage is also measured.
 
 import time
 
-from metrics import histogram_util
 from metrics import power
 from telemetry.core import util
 from telemetry.page import page_test
 from telemetry.value import histogram
+from telemetry.value import histogram_util
 
 # TODO: Revisit this test once multitab support is finalized.
 

tools/perf/metrics/memory.py

@@ -4,9 +4,9 @@
 
 import sys
 
-from metrics import histogram_util
 from metrics import Metric
 from telemetry.value import histogram
+from telemetry.value import histogram_util
 from telemetry.value import scalar
 
 

tools/perf/metrics/startup_metric.py

@@ -6,9 +6,9 @@ import json
 import logging
 
 from metrics import Metric
-from metrics import histogram_util
 
 from telemetry.core import util
+from telemetry.value import histogram_util
 from telemetry.value import scalar
 
 

tools/telemetry/telemetry/value/histogram.py

@@ -5,7 +5,7 @@ import json
 
 from telemetry import value as value_module
 from telemetry import perf_tests_helper
-
+from telemetry.value import histogram_util
 
 class HistogramValueBucket(object):
   def __init__(self, low, high, count=0):
@@ -113,7 +113,8 @@ class HistogramValue(value_module.Value):
     v0 = values[0]
     return HistogramValue(
         v0.page, v0.name, v0.units,
-        raw_value_json=v0.ToJSONString(),
+        raw_value_json=histogram_util.AddHistograms(
+            [v.ToJSONString() for v in values]),
         important=v0.important)
 
   @classmethod

tools/perf/metrics/histogram_util.py → tools/telemetry/telemetry/value/histogram_util.py

@@ -8,6 +8,7 @@ The histogram data is the same data as is visible from "chrome://histograms".
 More information can be found at: chromium/src/base/metrics/histogram.h
 """
 
+import collections
 import json
 import logging
 
@@ -59,6 +60,32 @@ def SubtractHistogram(histogram_json, start_histogram_json):
   return json.dumps(histogram)
 
 
+def AddHistograms(histogram_jsons):
+  """Adds histograms together. Used for aggregating data.
+
+  The parameter is a list of json serializations and the returned result is a
+  json serialization too.
+
+  Note that the histograms to be added together are typically from different
+  processes.
+  """
+
+  buckets = collections.defaultdict(int)
+  for histogram_json in histogram_jsons:
+    h = json.loads(histogram_json)
+    for b in h['buckets']:
+      key = (b['low'], b['high'])
+      buckets[key] += b['count']
+
+  buckets = [{'low': key[0], 'high': key[1], 'count': value}
+      for key, value in buckets.iteritems()]
+  buckets.sort(key = lambda h : h['low'])
+
+  result_histogram = {}
+  result_histogram['buckets'] = buckets
+  return json.dumps(result_histogram)
+
+
 def GetHistogram(histogram_type, histogram_name, tab):
   """Get a json serialization of a histogram."""
   assert histogram_type in [BROWSER_HISTOGRAM, RENDERER_HISTOGRAM]

tools/perf/metrics/histogram_util_unittest.py → tools/telemetry/telemetry/value/histogram_util_unittest.py

@@ -5,7 +5,7 @@
 import json
 import unittest
 
-from metrics import histogram_util
+from telemetry.value import histogram_util
 
 class TestHistogram(unittest.TestCase):
   def testSubtractHistogram(self):
@@ -26,3 +26,27 @@ class TestHistogram(unittest.TestCase):
     self.assertFalse(1 in new_buckets)
     self.assertEquals(1, new_buckets[2])
     self.assertEquals(10, new_buckets[3])
+
+
+  def testAddHistograms(self):
+    histograms = []
+    histograms.append("""{"count": 3, "buckets": [
+        {"low": 1, "high": 2, "count": 1},
+        {"low": 2, "high": 3, "count": 2}]}""")
+
+    histograms.append("""{"count": 20, "buckets": [
+        {"low": 2, "high": 3, "count": 10},
+        {"low": 3, "high": 4, "count": 10}]}""")
+
+    histograms.append("""{"count": 15, "buckets": [
+        {"low": 1, "high": 2, "count": 4},
+        {"low": 3, "high": 4, "count": 11}]}""")
+
+    new_histogram = json.loads(
+        histogram_util.AddHistograms(histograms))
+    new_buckets = dict()
+    for b in new_histogram['buckets']:
+      new_buckets[b['low']] = b['count']
+    self.assertEquals(5, new_buckets[1])
+    self.assertEquals(12, new_buckets[2])
+    self.assertEquals(21, new_buckets[3])