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

tools/perf/measurements/v8_detached_context_age_in_gc_unittest.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.
+
+from measurements import v8_detached_context_age_in_gc
+from telemetry.core import wpr_modes
+from telemetry.page import page_test
+from telemetry.results import page_test_results
+from telemetry.unittest_util import options_for_unittests
+from telemetry.unittest_util import page_test_test_case
+from telemetry.value import skip
+
+
+class FakePage(object):
+  def __init__(self, url):
+    self.url = url
+    self.is_file = url.startswith('file://')
+
+
+class FakeTab(object):
+  def __init__(self, histograms):
+    self.histograms = histograms
+    self.current_histogram_index = 0
+
+  def EvaluateJavaScript(self, script):
+    if 'V8.DetachedContextAgeInGC' in script:
+      self.current_histogram_index += 1
+      return self.histograms[self.current_histogram_index - 1]
+    return "{}"

[Sami, 2015/02/18 15:30:26]

nit: single quotes

[ulan, 2015/02/18 15:58:41]

Done.

+
+  def CollectGarbage(self):
+    pass
+
+
+def _MeasureFakePage(histograms):
+    results = page_test_results.PageTestResults()
+    page = FakePage("file://blank.html")

[Sami, 2015/02/18 15:30:26]

ditto

[ulan, 2015/02/18 15:58:41]

Done.

+    tab = FakeTab(histograms)
+    metric = v8_detached_context_age_in_gc.V8DetachedContextAgeInGC()
+    results.WillRunPage(page)
+    metric.DidNavigateToPage(page, tab)
+    metric.ValidateAndMeasurePage(page, tab, results)
+    results.DidRunPage(page)
+    return results
+
+
+def _ActualValues(results):
+  return dict(list((v.name, v)) for v in results.all_page_specific_values)

[Sami, 2015/02/18 15:30:27]

nit: "list()" is redundant

[ulan, 2015/02/18 15:58:41]

Done.

+
+
+class V8DetachedContextAgeInGCTests(page_test_test_case.PageTestTestCase):
+
+  def setUp(self):
+    self._options = options_for_unittests.GetCopy()
+    self._options.browser_options.wpr_mode = wpr_modes.WPR_OFF
+
+  def testWithNoData(self):
+    histograms = [
+        """{"count": 0, "buckets": []}""",
+        """{"count": 0, "buckets": []}"""
+    ]
+    results = _MeasureFakePage(histograms)
+    actual = _ActualValues(results)
+    self.assertTrue('skip' in actual)
+    self.assertFalse('V8_DetachedContextAgeInGC' in actual)
+
+  def testWithData(self):

[Sami, 2015/02/18 15:30:26]

Could we write this test in a way that uses the real browser? That way we can
catch breakage where the browser stops sending us the necessary data for some
reason.

[ulan, 2015/02/18 15:58:41]

Do you mean: create a real html page, let the browser run it, do GC and collect
data?
Dependency on GC would make this test non-deterministic and might require us to
adjust expectation if GC implementation changes. If browser breaks we would see
that in perf graphs anyway, right? 

[Sami, 2015/02/18 16:20:56]

Breaking a unit test instead triggering a perf regression is better because the
bad patch never gets to land in the first scenario. Maybe we can do what Oilpan
did and verify that we get at least some data from the browser instead of
expecting exact values:
https://code.google.com/p/chromium/codesearch#chromium/src/tools/perf/measure...

+    histograms = [
+        """{"count": 3, "buckets": [{"low": 1, "high": 2, "count": 1},
+                                    {"low": 2, "high": 3, "count": 2}]}""",
+        """{"count": 4, "buckets": [{"low": 1, "high": 2, "count": 2},
+                                    {"low": 2, "high": 3, "count": 2}]}""",
+    ]
+    results = _MeasureFakePage(histograms)
+    actual = _ActualValues(results)['V8_DetachedContextAgeInGC']
+    self.assertEqual(2, actual.value)
+    self.assertEqual('garbage_collections', actual.units)