[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 2015 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
+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'
+
+
+def _MaxDetachedContextAge(histogram_data):
+  if not 'buckets' in histogram_data:
+    return 0
+  buckets = json.loads(histogram_data)['buckets']
+  return max(x['high'] for x in buckets) if buckets else 0

[rmcilroy, 2015/02/18 11:20:31]

is '0' a good value for when we don't find buckets?  If the benchmark starts
failing this might get ignored since it would appear as a perf improvement of
the results to '0'.  Should we just fail here explicitly or can there
legitimately be no data for the V8.DetachedContextAgeInGC measurement?

[ulan, 2015/02/18 13:33:16]

Good point. There can be no detached contexts. If, for example, the page set
consists of a single page that is not reloaded. I replaced 0 with SkipValue (I
hope it works as the name suggests).

+
+
+class V8DetachedContextAgeInGC(page_test.PageTest):
+  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):

[rmcilroy, 2015/02/18 11:20:31]

nit - pull '8' out into a constant.

[ulan, 2015/02/18 13:33:16]

Done.

+      tab.CollectGarbage()
+    data = histogram_util.GetHistogram(_TYPE, _NAME, tab)
+    delta = histogram_util.SubtractHistogram(data, self._data_start)

[rmcilroy, 2015/02/18 11:20:31]

nit - I would do the getHistogram / SubtractHistogram in the
_MaxDetachedContextAge (maybe renamed _GetMaxDetachedContextAge) to keep the
code related to V8.DetachedContextAgeInGC together.

[ulan, 2015/02/18 13:33:16]

Done.

+    value = _MaxDetachedContextAge(delta)
+    results.AddValue(scalar.ScalarValue(
+        results.current_page, _DISPLAY_NAME, _UNITS, value,
+        description=_DESCRIPTION))