telemtry: better error message for not enought frames cases.

tools/perf/metrics/rendering_stats.py

 # Copyright 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.
 
 from operator import attrgetter
+from telemetry.page import page_measurement
 
 # These are LatencyInfo component names indicating the various components
 # that the input event has travelled through.
 # This is when the input event first reaches chrome.
 UI_COMP_NAME = 'INPUT_EVENT_LATENCY_UI_COMPONENT'
 # This is when the input event was originally created by OS.
 ORIGINAL_COMP_NAME = 'INPUT_EVENT_LATENCY_ORIGINAL_COMPONENT'
 # This is when the input event was sent from browser to renderer.
 BEGIN_COMP_NAME = 'INPUT_EVENT_LATENCY_BEGIN_RWH_COMPONENT'
 # This is when the input event has reached swap buffer.
 END_COMP_NAME = 'INPUT_EVENT_LATENCY_TERMINATED_FRAME_SWAP_COMPONENT'
 
 
+class NotEnoughFramesError(page_measurement.MeasurementFailure):
+  def __init__(self, frame_count):
+    super(NotEnoughFramesError, self).__init__(
+      'Only %i frame timestamps were collected ' % frame_count +
+      '(at least two are required).\n'
+      'Issues that have caused this in the past:\n' +
+      '- Browser bugs that prevents the page from redrawing\n' +
+      '- Bugs in the synthetic gesture code\n' +
+      '- Page and benchmark out of sync (e.g. clicked element was renamed)\n' +
+      '- Pages that render extremely slow\n' +
+      '- Pages that can\'t be scrolled')
+
+
 def GetScrollInputLatencyEvents(scroll_type, browser_process, timeline_range):
   """Get scroll events' LatencyInfo from the browser process's trace buffer
      that are within the timeline_range.
 
   Scroll events (MouseWheel, GestureScrollUpdate or JS scroll on TouchMove)
   dump their LatencyInfo into trace buffer as async trace event with name
   "InputLatency". The trace event has a memeber 'step' containing its event
   type and a memeber 'data' containing its latency history.
 
   """
@@ skipping 124 matching lines @@
       self.mouse_wheel_scroll_latency.append([])
       self.touch_scroll_latency.append([])
       self.js_touch_scroll_latency.append([])
 
       if timeline_range.is_empty:
         continue
       self.initMainThreadStatsFromTimeline(timeline_range)
       self.initImplThreadStatsFromTimeline(timeline_range)
       self.initScrollLatencyStatsFromTimeline(browser_process, timeline_range)
 
+    # Check if we have collected at least 2 frames in every range. Otherwise we
+    # can't compute any meaningful metrics.
+    for segment in self.frame_timestamps:
+      if len(segment) < 2:
+        raise NotEnoughFramesError(len(segment))
+
   def initScrollLatencyStatsFromTimeline(self, browser_process, timeline_range):
     mouse_wheel_events = GetScrollInputLatencyEvents(
         "MouseWheel", browser_process, timeline_range)
     self.mouse_wheel_scroll_latency = ComputeMouseWheelScrollLatency(
         mouse_wheel_events)
 
     touch_scroll_events = GetScrollInputLatencyEvents(
         "GestureScrollUpdate", browser_process, timeline_range)
     self.touch_scroll_latency = ComputeTouchScrollLatency(touch_scroll_events)
 
@@ skipping 52 matching lines @@
         self.frame_timestamps[-1].append(
             event.start)
         if not first_frame:
           self.frame_times[-1].append(round(self.frame_timestamps[-1][-1] -
                                             self.frame_timestamps[-1][-2], 2))
         first_frame = False
       self.rasterize_times[-1].append(1000.0 *
           event.args['data']['rasterize_time'])
       self.rasterized_pixel_counts[-1].append(
           event.args['data']['rasterized_pixel_count'])