[throughput] Ignore the first sample when reporting UKM

Currently we report throughput to UKM at:
  1. 5 seconds after a sequence starts.
  2. Every 500 seconds after that as long as the sequence lasts.
The problem is that in many cases users stays on a page less than 500
seconds, and then we would only get the report for the first 5 seconds,
which can be heavily biased by the page loading time.

This CL is a temp solution to ignore the first 5 seconds, so instead
we will report throughput at:
  1. The second 5 seconds after a sequence starts.
  2. Every 500 seconds after that as long as the sequence lasts.

Bug: 1040634
Change-Id: I808b3ff56acce12c3a81e8fca5b7d472f4fcf4a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129014
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755463}

cc/metrics/throughput_ukm_reporter.cc

@@ -16,19 +16,26 @@ constexpr unsigned kNumberOfSamplesToReport = 100u;
 ThroughputUkmReporter::ThroughputUkmReporter(UkmManager* ukm_manager)
     : ukm_manager_(ukm_manager) {
   DCHECK(ukm_manager_);
+  // TODO(crbug.com/1040634): Setting it to 1 such that the first sample is
+  // ignored. TThis is because the universal tracker is active during the page
+  // load and the first sample is heavily biased by loading as a result.
+  samples_to_next_event_[static_cast<int>(
+      FrameSequenceTrackerType::kUniversal)] = 1;
 }
 
+ThroughputUkmReporter::~ThroughputUkmReporter() = default;
+
 void ThroughputUkmReporter::ReportThroughputUkm(
     const base::Optional<int>& slower_throughput_percent,
     const base::Optional<int>& impl_throughput_percent,
     const base::Optional<int>& main_throughput_percent,
     FrameSequenceTrackerType type) {
-  if (samples_to_next_event_ == 0) {
-    // Sample every 2000 events. Using the Universal tracker as an example
+  if (samples_to_next_event_[static_cast<int>(type)] == 0) {
+    // Sample every 100 events. Using the Universal tracker as an example
     // which reports UMA every 5s, then the system collects UKM once per
-    // 2000*5 = 10000 seconds, which is about 3 hours. This number may need to
-    // be tuned to not throttle the UKM system.
-    samples_to_next_event_ = kNumberOfSamplesToReport;
+    // 100*5 = 500 seconds. This number may need to be tuned to not throttle
+    // the UKM system.
+    samples_to_next_event_[static_cast<int>(type)] = kNumberOfSamplesToReport;
     if (impl_throughput_percent) {
       ukm_manager_->RecordThroughputUKM(
           type, FrameSequenceMetrics::ThreadType::kCompositor,
@@ -43,8 +50,8 @@ void ThroughputUkmReporter::ReportThroughputUkm(
                                       FrameSequenceMetrics::ThreadType::kSlower,
                                       slower_throughput_percent.value());
   }
-  DCHECK_GT(samples_to_next_event_, 0u);
-  samples_to_next_event_--;
+  DCHECK_GT(samples_to_next_event_[static_cast<int>(type)], 0u);
+  samples_to_next_event_[static_cast<int>(type)]--;
 }
 
 void ThroughputUkmReporter::ReportAggregateThroughput(

cc/metrics/throughput_ukm_reporter.h

@@ -23,7 +23,7 @@ enum class AggregationType {
 class CC_EXPORT ThroughputUkmReporter {
  public:
   explicit ThroughputUkmReporter(UkmManager* ukm_manager);
-  ~ThroughputUkmReporter() = default;
+  ~ThroughputUkmReporter();
 
   ThroughputUkmReporter(const ThroughputUkmReporter&) = delete;
   ThroughputUkmReporter& operator=(const ThroughputUkmReporter&) = delete;
@@ -40,7 +40,8 @@ class CC_EXPORT ThroughputUkmReporter {
   // Sampling control. We sample the event here to not throttle the UKM system.
   // Currently, the same sampling rate is applied to all existing trackers. We
   // might want to iterate on this based on the collected data.
-  uint32_t samples_to_next_event_ = 0;
+  uint32_t samples_to_next_event_[static_cast<int>(
+      FrameSequenceTrackerType::kMaxType)] = {0};
   uint32_t samples_for_aggregated_report_ = 0;
 
   // This is pointing to the LayerTreeHostImpl::ukm_manager_, which is