From 1869db04d9e174a663e2422bc1d20812beb3c25e Mon Sep 17 00:00:00 2001
From: Laurent Demailly <ldemailly@google.com>
Date: Tue, 4 Jul 2017 10:30:15 -0700
Subject: [PATCH] Improve stats accuracy at low percentile And corresponding
 test

---
 test/Stats_test.cpp |  6 ++++++
 util/Stats.cpp      | 19 +++++++++++++------
 util/Stats.h        |  8 +++++++-
 3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/test/Stats_test.cpp b/test/Stats_test.cpp
index 5d7e3334..cde09c31 100644
--- a/test/Stats_test.cpp
+++ b/test/Stats_test.cpp
@@ -94,7 +94,13 @@ TEST(StatsTest, HistogramPercentile) {
     std::cout << i << "% at " << h.calcPercentile(i) << std::endl;
   }
   EXPECT_EQ(501, h.getAverage());
+  EXPECT_EQ(1, h.calcPercentile(-1));
+  EXPECT_EQ(1, h.calcPercentile(0));
+  EXPECT_EQ(1.045, h.calcPercentile(0.1));
+  EXPECT_EQ(1.45, h.calcPercentile(1));
+  EXPECT_EQ(2.8, h.calcPercentile(4));
   EXPECT_EQ(10, h.calcPercentile(20));
+  EXPECT_EQ(250.25, h.calcPercentile(20.1));
   EXPECT_EQ(550, h.calcPercentile(50));
   EXPECT_EQ(775, h.calcPercentile(75));
   EXPECT_EQ(1000.5, h.calcPercentile(90));
diff --git a/util/Stats.cpp b/util/Stats.cpp
index 1c88e573..9a8b5e32 100644
--- a/util/Stats.cpp
+++ b/util/Stats.cpp
@@ -49,18 +49,20 @@ int32_t* initHistLookup() {
 static const int32_t* kVal2Bucket = initHistLookup();
 
 void Counter::record(int64_t v) {
-  int64_t newCount = ++count_;  // atomic_add(count_, 1);
-  // atomic_add(sum_, v);
+  int64_t newCount = ++count_;
   sum_ += v;
   if (newCount == 1) {
-    min_ = max_ = v;
+    realMin_ = min_ = max_ = v;
   }
-  // this can actually miss/revert the min/max (race condition)
-  // don't count 0 in the min - makes for a slightly more interesting min
+  // don't count 0s in the min - makes for a slightly more interesting min
   // in most cases
   if (v && ((min_ == 0) || (v < min_))) {
     min_ = v;
   }
+  // real min (can be 0):
+  if (v < realMin_ ) {
+    realMin_ = v;
+  }
   if (v > max_) {
     max_ = v;
   }
@@ -176,7 +178,7 @@ double Histogram::calcPercentile(double percentile) const {
     return getMax();
   }
   if (percentile <= 0) {
-    return 0;
+    return getRealMin();
   }
   // Initial value of prev should in theory be offset_
   // but if the data is wrong (smaller than offset - eg 'negative') that
@@ -185,6 +187,7 @@ double Histogram::calcPercentile(double percentile) const {
   int64_t total = 0;
   const int64_t ctrTotal = getCount();
   const int64_t ctrMax = getMax();
+  const int64_t ctrMin = getRealMin();
   double prevPerc = 0;
   double perc = 0;
   bool found = false;
@@ -213,6 +216,10 @@ double Histogram::calcPercentile(double percentile) const {
     cur = ctrMax;
     perc = 100.;  // can't be removed
   }
+  // Fix up the min too to never return < min and increase low p accuracy
+  if (prev < ctrMin) {
+    prev = ctrMin;
+  }
   return (prev + (percentile - prevPerc) * (cur - prev) / (perc - prevPerc));
 }
 
diff --git a/util/Stats.h b/util/Stats.h
index df0770f0..b752e801 100644
--- a/util/Stats.h
+++ b/util/Stats.h
@@ -145,6 +145,7 @@ class Counter : crashifcopied {
  public:
   explicit Counter(double printScale = 1.0)
       : count_(0),
+        realMin_(0),
         min_(0),
         max_(0),
         sum_(0),
@@ -161,6 +162,11 @@ class Counter : crashifcopied {
     return count_;
   }
 
+  // Includes 0s
+  inline int64_t getRealMin() const {
+    return realMin_;
+  }
+  // Excludes 0
   inline int64_t getMin() const {
     return min_;
   }
@@ -209,7 +215,7 @@ class Counter : crashifcopied {
   void merge(const Counter& c);
 
  private:
-  int64_t count_, min_, max_, sum_, sumOfSquares_;
+  int64_t count_, realMin_, min_, max_, sum_, sumOfSquares_;
   double printScale_;
 };