[UKM] Adjust CC and Blink Update sampling to reduce counts

Adjust the exponential decay to decay more rapidly (about 3x higher
exponent) to reduce the number of samples we send.

`

Bug: 1035531
Change-Id: Ie6de6eaca5c6dc49cd2e674e4c9d82f30d59c294
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003291
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732400}

cc/metrics/latency_ukm_reporter.cc

@@ -42,11 +42,11 @@ unsigned LatencyUkmReporter::SampleFramesToNextEvent() {
   double float_sample = 0;
   do {
     float_sample = -(sample_rate_multiplier_ *
-                     std::exp(samples_so_far_ / sample_decay_rate_) *
+                     std::exp(samples_so_far_ * sample_decay_rate_) *
                      std::log(1.0 - base::RandDouble()));
   } while (float_sample == 0);
   // float_sample is positive, so we don't need to worry about underflow.
-  // After around 100 samples we will end up with a super high
+  // After around 30 samples we will end up with a super high
   // sample. That's OK because it just means we'll stop reporting metrics
   // for that session, but we do need to be careful about overflow and NaN.
   samples_so_far_++;

cc/metrics/latency_ukm_reporter.h

@@ -48,13 +48,14 @@ class CC_EXPORT LatencyUkmReporter {
   // multiplier. The goal is to get many randomly distributed samples early
   // during page load and initial interaction, then samples at an exponentially
   // decreasing rate to effectively cap the number of samples. The particular
-  // parameters chosen here give roughly 10-15 samples in the first 100 frames,
+  // parameters chosen here give roughly 5-10 samples in the first 100 frames,
   // decaying to several hours between samples by the 40th sample. The
   // multiplier value should be tuned to achieve a total sample count that
   // avoids throttling by the UKM system.
-  // The sample_rate_multiplier_ has been set to meet UKM goals for data volume.
-  double sample_decay_rate_ = 3;
-  double sample_rate_multiplier_ = 4;
+  // The sample_rate_multiplier_ and sample_decay_rate_ have been set to meet
+  // UKM goals for data volume.
+  double sample_decay_rate_ = 1;
+  double sample_rate_multiplier_ = 2;
   unsigned samples_so_far_ = 0;
   unsigned frames_to_next_event_ = 0;
 

third_party/blink/renderer/core/frame/local_frame_ukm_aggregator.cc

@@ -433,11 +433,11 @@ unsigned LocalFrameUkmAggregator::SampleFramesToNextEvent() {
   double float_sample = 0;
   do {
     float_sample = -(sample_rate_multiplier_ *
-                     std::exp(samples_so_far_ / sample_decay_rate_) *
+                     std::exp(samples_so_far_ * sample_decay_rate_) *
                      std::log(1.0 - base::RandDouble()));
   } while (float_sample == 0);
   // float_sample is positive, so we don't need to worry about underflow.
-  // After around 100 samples we will end up with a super high
+  // After around 30 samples we will end up with a super high
   // sample. That's OK because it just means we'll stop reporting metrics
   // for that session, but we do need to be careful about overflow and NaN.
   samples_so_far_++;

third_party/blink/renderer/core/frame/local_frame_ukm_aggregator.h

@@ -314,12 +314,12 @@ class CORE_EXPORT LocalFrameUkmAggregator
   // multiplier. The goal is to get many randomly distributed samples early
   // during page load and initial interaction, then samples at an exponentially
   // decreasing rate to effectively cap the number of samples. The particular
-  // parameters chosen here give roughly 10-15 samples in the first 100 frames,
+  // parameters chosen here give roughly 5-10 samples in the first 100 frames,
   // decaying to several hours between samples by the 40th sample. The
-  // multiplier value should be tuned to achieve a total sample count that
-  // avoids throttling by the UKM system.
-  double sample_decay_rate_ = 3;
-  double sample_rate_multiplier_ = 4;
+  // multiplier value and sample_decay_rate_ should be tuned to achieve a total
+  // sample count that avoids throttling by the UKM system.
+  double sample_decay_rate_ = 1;
+  double sample_rate_multiplier_ = 2;
   unsigned samples_so_far_ = 0;
   unsigned frames_to_next_event_ = 0;