MB-33735 Make HdrHistogram mean test more reliable

Make HdrHistogram mean test more reliable by testing the error
percentage of the mean, calculated by HdrHistogram. This error
percentage should be no more than +/-0.05%.

Change-Id: I10e3486b9e7498eb8c22fd98015260184fca8953
Reviewed-on: http://review.couchbase.org/107948
Reviewed-by: Dave Rigby <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: rdemellow

engines/ep/tests/module_tests/hdrhistogram_test.cc

@@ -22,6 +22,7 @@
 #include <hdr_histogram.h>
 
 #include <cmath>
+#include <iomanip>
 #include <memory>
 #include <random>
 #include <thread>
@@ -173,14 +174,13 @@ TEST(HdrHistogramTest, addValueAndCountTest) {
 #define LOG_NORMAL_STD 2.0
 #define LOG_NORMAL_SCALE_UP_MULT 35000
 #define LOG_NORMAL_MIN 50000
-
+static std::vector<uint64_t> valuesToAdd(10000);
+static bool initialised = false;
 // static function to return a log normal value scaled by
 // LOG_NORMAL_SCALE_UP_MULT. It creates an array of 10000 static values that
 // using std::lognormal_distribution and returners them in an incrementing
 // linear fashion so that they can be used for the meanTest
 static uint64_t GetNextLogNormalValue() {
-    static bool initialised = false;
-    static std::vector<uint64_t> valuesToAdd(10000);
     static unsigned int i = 0;
 
     if (!initialised) {
@@ -219,7 +219,7 @@ static uint64_t GetNextLogNormalValue() {
 TEST(HdrHistogramTest, meanTest) {
     HdrHistogram histogram{0, 60000000, 3};
     uint64_t sum = 0;
-    double total_count = 0;
+    uint64_t total_count = 0;
 
     for (uint64_t i = 0; i < 1000000; i++) {
         uint64_t count = GetNextLogNormalValue();
@@ -236,24 +236,13 @@ TEST(HdrHistogramTest, meanTest) {
     }
 
     // calculate the mean
-    double_t avg = (sum / total_count);
-    int normaliseBy = 1;
-
-    // the mean will only be accurate to getSigFigAccuracy() in this case
-    // 3 sig fig. Which means we only want to compare the 3 sig fig, thus
-    // we need to normalise our mean to the one returned by the histogram.
-    // To do this we work out the power of ten of our mean and then remove two
-    // from this to leave us with two sig fig
-    int tens = static_cast<int>(log10(avg)) + 1 - histogram.getSigFigAccuracy();
-
-    // no point in normalising if the mean is 3 sig fig or less.
-    if (tens <= histogram.getSigFigAccuracy()) {
-        normaliseBy = static_cast<int>(std::pow(10, tens));
-    }
+    double_t avg = (sum / static_cast<double_t>(total_count));
+
+    uint64_t meanDiff = std::abs(avg - histogram.getMean());
+    double_t errorPer = (meanDiff / avg) * 100.0;
 
-    EXPECT_EQ(static_cast<unsigned int>(round(avg) / normaliseBy),
-              static_cast<unsigned int>(round(histogram.getMean()) /
-                                        normaliseBy));
+    // check that the error percentage is less than 0.05%
+    EXPECT_GT(0.05, std::abs(errorPer));
 }
 
 void addValuesThread(HdrHistogram& histo,