Add a sampling scheme to ScrollUpdate.Touch UKM

content/browser/renderer_host/input/render_widget_host_latency_tracker.cc

 // Copyright 2014 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.
 
 #include "content/browser/renderer_host/input/render_widget_host_latency_tracker.h"
 
 #include <stddef.h>
 
 #include "base/logging.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/rand_util.h"
 #include "build/build_config.h"
 #include "components/rappor/public/rappor_utils.h"
 #include "content/browser/renderer_host/render_widget_host_delegate.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/content_browser_client.h"
 #include "content/public/common/content_client.h"
 #include "services/metrics/public/cpp/ukm_entry_builder.h"
 #include "ui/events/blink/web_input_event_traits.h"
 #include "ui/latency/latency_histogram_macros.h"
 
@@ skipping 109 matching lines @@
   if (expected_queueing_time_ms > 450) {
     UMA_HISTOGRAM_TIMES(
         "RendererScheduler."
         "QueueingDurationWhenExpectedQueueingTime_GreaterThan.450ms",
         queueing_time);
   }
 }
 
 }  // namespace
 
-RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker()
+RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker(
+    bool metric_sampling)
     : ukm_source_id_(-1),
       last_event_id_(0),
       latency_component_id_(0),
       device_scale_factor_(1),
       has_seen_first_gesture_scroll_update_(false),
       active_multi_finger_gesture_(false),
       touch_start_default_prevented_(false),
+      metric_sampling_(metric_sampling),
       render_widget_host_delegate_(nullptr) {}
 
 RenderWidgetHostLatencyTracker::~RenderWidgetHostLatencyTracker() {}
 
 void RenderWidgetHostLatencyTracker::Initialize(int routing_id,
                                                 int process_id) {
   DCHECK_EQ(0, last_event_id_);
   DCHECK_EQ(0, latency_component_id_);
   last_event_id_ = static_cast<int64_t>(process_id) << 32;
   latency_component_id_ = routing_id | last_event_id_;
@@ skipping 248 matching lines @@
   return ukm_source_id_;
 }
 
 void RenderWidgetHostLatencyTracker::ReportUkmScrollLatency(
     const std::string& event_name,
     const std::string& metric_name,
     const LatencyInfo::LatencyComponent& start_component,
     const LatencyInfo::LatencyComponent& end_component) {
   CONFIRM_VALID_TIMING(start_component, end_component)
 
+  // Only report a subset of this metric as the volume is too high.
+  if (event_name == "Event.ScrollUpdate.Touch") {
+    if (metric_sampling_ && base::RandUint64() % 10)

[tdresser, 2017/07/17 16:26:40]

RandUint64 is thread safe and cryptographically secure. I think we could use
something a bit cheaper here. Maybe even just record every nth value?

[Navid Zolghadr, 2017/07/17 20:15:14]

I changed the implementation to only use random once at the beginning to also
avoid any bias towards any particular cycle.

+      return;
+  }
+
   ukm::SourceId ukm_source_id = GetUkmSourceId();
   ukm::UkmRecorder* ukm_recorder = ukm::UkmRecorder::Get();
 
   if (ukm_source_id == -1 || !ukm_recorder)
     return;
 
   std::unique_ptr<ukm::UkmEntryBuilder> builder =
       ukm_recorder->GetEntryBuilder(ukm_source_id, event_name.c_str());
   builder->AddMetric(metric_name.c_str(), (end_component.last_event_time -
                                            start_component.first_event_time)
                                               .InMicroseconds());
 }
 }  // namespace content