[telemetry] Add new measurement that counts number of GCs needed to free V8 context.

tools/perf/measurements/v8_detached_context_age_in_gc.py

+# Copyright 2012 The Chromium Authors. All rights reserved.

[Sami, 2015/02/17 17:14:06]

year += 3 :)

[ulan, 2015/02/18 10:35:56]

Done.

+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import json
+from telemetry.page import page_test
+from telemetry.value import histogram
+from telemetry.value import histogram_util
+from telemetry.value import scalar
+
+_NAME = 'V8.DetachedContextAgeInGC'
+_UNITS = 'garbage_collections'
+_DISPLAY_NAME = 'V8_DetachedContextAgeInGC'
+_TYPE = histogram_util.RENDERER_HISTOGRAM
+_DESCRIPTION = 'Number of GCs needed to collect detached context'
+

[Sami, 2015/02/17 17:14:06]

nit: add an extra blank line here.

[ulan, 2015/02/18 10:35:56]

Done.

+def _MaxDetachedContextAge(histogram_data):
+  if not 'buckets' in histogram_data:
+    return 0
+  buckets = json.loads(histogram_data)['buckets']
+  return max(map(lambda x: x['high'], buckets)) if buckets else 0

[Sami, 2015/02/17 17:14:06]

nit: max(x['high'] for x in buckets) works too.

[ulan, 2015/02/18 10:35:55]

Done.

+
+
+class V8DetachedContextAgeInGC(page_test.PageTest):

[Sami, 2015/02/17 17:14:06]

Could you please add a unit test for this too?

[ulan, 2015/02/18 10:35:56]

Done.

+  def __init__(self):
+    super(V8DetachedContextAgeInGC, self).__init__('RunPageInteractions')
+    self._data_start = None
+
+  def CustomizeBrowserOptions(self, options):
+    options.AppendExtraBrowserArgs(['--enable-stats-collection-bindings'])
+
+  def DidNavigateToPage(self, page, tab):
+    self._data_start = histogram_util.GetHistogram(_TYPE, _NAME, tab)
+
+  def ValidateAndMeasurePage(self, page, tab, results):
+    # Trigger GC to get histogram data.
+    # Seven GCs should be enough to collect any detached context.
+    # If a detached context survives more GCs then there is a leak.
+    for _ in xrange(8):
+      tab.CollectGarbage()
+    data = histogram_util.GetHistogram(_TYPE, _NAME, tab)
+    delta = histogram_util.SubtractHistogram(data, self._data_start)
+    value = _MaxDetachedContextAge(delta)
+    results.AddValue(scalar.ScalarValue(
+        results.current_page, _DISPLAY_NAME, _UNITS, value,
+        description=_DESCRIPTION))