Add a sampling scheme to ScrollUpdate.Touch UKM

content/browser/renderer_host/input/render_widget_host_latency_tracker.cc

Index: content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
diff --git a/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc b/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
index 62e42952dd6e9400f3de541511172139ca8b229d..77292aa43ab8427d3ae6be921db2dd8421744e2c 100644
--- a/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
+++ b/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
@@ -9,6 +9,7 @@
 #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"
@@ -29,6 +30,8 @@ using ui::LatencyInfo;
 namespace content {
 namespace {
 
+constexpr int kSamplingInterval = 10;
+
 std::string WebInputEventTypeToInputModalityString(WebInputEvent::Type type) {
   if (type == blink::WebInputEvent::kMouseWheel) {
     return "Wheel";
@@ -138,7 +141,8 @@ void RecordEQTAccuracy(base::TimeDelta queueing_time,
 
 }  // namespace
 
-RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker()
+RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker(
+    bool metric_sampling)
     : ukm_source_id_(-1),
       last_event_id_(0),
       latency_component_id_(0),
@@ -146,7 +150,12 @@ RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker()
       has_seen_first_gesture_scroll_update_(false),
       active_multi_finger_gesture_(false),
       touch_start_default_prevented_(false),
-      render_widget_host_delegate_(nullptr) {}
+      metric_sampling_(metric_sampling),
+      metric_sampling_countdown_(-1),
+      render_widget_host_delegate_(nullptr) {
+  if (metric_sampling)
+    metric_sampling_countdown_ = base::RandUint64() % kSamplingInterval;
+}
 
 RenderWidgetHostLatencyTracker::~RenderWidgetHostLatencyTracker() {}
 
@@ -414,6 +423,15 @@ void RenderWidgetHostLatencyTracker::ReportUkmScrollLatency(
     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") {
+    metric_sampling_countdown_++;

[tdresser, 2017/07/17 20:25:22]

"countdown" implies it counts down, but it counts up!

I'm not sure what the best name would be though.
metric_sampling_events_since_last_sample ?

Any better ideas?

[Navid Zolghadr, 2017/07/19 15:57:46]

Done.

+    metric_sampling_countdown_ %= kSamplingInterval;
+    if (metric_sampling_ && metric_sampling_countdown_)
+      return;
+    metric_sampling_countdown_ = 0;

[dtapuska, 2017/07/17 20:21:41]

This line is useless.

[Navid Zolghadr, 2017/07/19 15:57:46]

Done.

+  }
+
   ukm::SourceId ukm_source_id = GetUkmSourceId();
   ukm::UkmRecorder* ukm_recorder = ukm::UkmRecorder::Get();