Introduce Coefficient of variation aggregate (#1220)

* Introduce Coefficient of variation aggregate

I believe, it is much more useful / use to understand,
because it is already normalized by the mean,
so it is not affected by the duration of the benchmark,
unlike the standard deviation.

Example of real-world output:
```
raw.pixls.us-unique/GoPro/HERO6 Black$ ~/rawspeed/build-old/src/utilities/rsbench/rsbench GOPR9172.GPR --benchmark_repetitions=27 --benchmark_display_aggregates_only=true --benchmark_counters_tabular=true
2021-09-03T18:05:56+03:00
Running /home/lebedevri/rawspeed/build-old/src/utilities/rsbench/rsbench
Run on (32 X 3596.16 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x16)
  L1 Instruction 32 KiB (x16)
  L2 Unified 512 KiB (x16)
  L3 Unified 32768 KiB (x2)
Load Average: 7.00, 2.99, 1.85
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benchmark                                                      Time             CPU   Iterations  CPUTime,s CPUTime/WallTime     Pixels Pixels/CPUTime Pixels/WallTime Raws/CPUTime Raws/WallTime WallTime,s
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GOPR9172.GPR/threads:32/process_time/real_time_mean         11.1 ms          353 ms           27   0.353122          31.9473        12M       33.9879M        1085.84M      2.83232       90.4864  0.0110535
GOPR9172.GPR/threads:32/process_time/real_time_median       11.0 ms          352 ms           27   0.351696          31.9599        12M       34.1203M        1090.11M      2.84336       90.8425  0.0110081
GOPR9172.GPR/threads:32/process_time/real_time_stddev      0.159 ms         4.60 ms           27   4.59539m        0.0462064          0       426.371k        14.9631M    0.0355309       1.24692   158.944u
GOPR9172.GPR/threads:32/process_time/real_time_cv           1.44 %          1.30 %            27  0.0130136         1.44633m          0      0.0125448       0.0137802    0.0125448     0.0137802  0.0143795
```

Fixes https://github.com/google/benchmark/issues/1146

* Be consistent, it's CV, not 'rel std dev'

docs/user_guide.md

@@ -930,7 +930,7 @@ Note that `ClobberMemory()` is only available for GNU or MSVC based compilers.
 
 <a name="reporting-statistics" />
 
-## Statistics: Reporting the Mean, Median and Standard Deviation of Repeated Benchmarks
+## Statistics: Reporting the Mean, Median and Standard Deviation / Coefficient of variation of Repeated Benchmarks
 
 By default each benchmark is run once and that single result is reported.
 However benchmarks are often noisy and a single result may not be representative
@@ -940,16 +940,17 @@ benchmark.
 The number of runs of each benchmark is specified globally by the
 `--benchmark_repetitions` flag or on a per benchmark basis by calling
 `Repetitions` on the registered benchmark object. When a benchmark is run more
-than once the mean, median and standard deviation of the runs will be reported.
+than once the mean, median, standard deviation and coefficient of variation
+of the runs will be reported.
 
 Additionally the `--benchmark_report_aggregates_only={true|false}`,
 `--benchmark_display_aggregates_only={true|false}` flags or
 `ReportAggregatesOnly(bool)`, `DisplayAggregatesOnly(bool)` functions can be
 used to change how repeated tests are reported. By default the result of each
 repeated run is reported. When `report aggregates only` option is `true`,
-only the aggregates (i.e. mean, median and standard deviation, maybe complexity
+only the aggregates (i.e. mean, median, standard deviation and coefficient
-measurements if they were requested) of the runs is reported, to both the
+of variation, maybe complexity measurements if they were requested) of the runs
-reporters - standard output (console), and the file.
+is reported, to both the reporters - standard output (console), and the file.
 However when only the `display aggregates only` option is `true`,
 only the aggregates are displayed in the standard output, while the file
 output still contains everything.
@@ -961,11 +962,10 @@ benchmark.
 
 ## Custom Statistics
 
-While having mean, median and standard deviation is nice, this may not be
+While having these aggregates is nice, this may not be enough for everyone.
-enough for everyone. For example you may want to know what the largest
+For example you may want to know what the largest observation is, e.g. because
-observation is, e.g. because you have some real-time constraints. This is easy.
+you have some real-time constraints. This is easy. The following code will
-The following code will specify a custom statistic to be calculated, defined
+specify a custom statistic to be calculated, defined by a lambda function.
-by a lambda function.
 
 ```c++
 void BM_spin_empty(benchmark::State& state) {

src/benchmark_register.cc

@@ -215,6 +215,7 @@ Benchmark::Benchmark(const char* name)
   ComputeStatistics("mean", StatisticsMean);
   ComputeStatistics("median", StatisticsMedian);
   ComputeStatistics("stddev", StatisticsStdDev);
+  ComputeStatistics("cv", StatisticsCV, kPercentage);
 }
 
 Benchmark::~Benchmark() {}

src/statistics.cc

@@ -74,6 +74,15 @@ double StatisticsStdDev(const std::vector<double>& v) {
   return Sqrt(v.size() / (v.size() - 1.0) * (avg_squares - Sqr(mean)));
 }
 
+double StatisticsCV(const std::vector<double>& v) {
+  if (v.size() < 2) return 0.0;
+
+  const auto stddev = StatisticsStdDev(v);
+  const auto mean = StatisticsMean(v);
+
+  return stddev / mean;
+}
+
 std::vector<BenchmarkReporter::Run> ComputeStats(
     const std::vector<BenchmarkReporter::Run>& reports) {
   typedef BenchmarkReporter::Run Run;

src/statistics.h

@@ -31,6 +31,7 @@ std::vector<BenchmarkReporter::Run> ComputeStats(
 double StatisticsMean(const std::vector<double>& v);
 double StatisticsMedian(const std::vector<double>& v);
 double StatisticsStdDev(const std::vector<double>& v);
+double StatisticsCV(const std::vector<double>& v);
 
 }  // end namespace benchmark
 

test/display_aggregates_only_test.cc

@@ -19,21 +19,23 @@ BENCHMARK(BM_SummaryRepeat)->Repetitions(3)->DisplayAggregatesOnly();
 int main(int argc, char* argv[]) {
   const std::string output = GetFileReporterOutput(argc, argv);
 
-  if (SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3") != 6 ||
+  if (SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3") != 7 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3\"") != 3 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_mean\"") != 1 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_median\"") !=
           1 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_stddev\"") !=
-          1) {
+          1 ||
-    std::cout << "Precondition mismatch. Expected to only find 6 "
+      SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_cv\"") != 1) {
+    std::cout << "Precondition mismatch. Expected to only find 8 "
                  "occurrences of \"BM_SummaryRepeat/repeats:3\" substring:\n"
                  "\"name\": \"BM_SummaryRepeat/repeats:3\", "
                  "\"name\": \"BM_SummaryRepeat/repeats:3\", "
                  "\"name\": \"BM_SummaryRepeat/repeats:3\", "
                  "\"name\": \"BM_SummaryRepeat/repeats:3_mean\", "
                  "\"name\": \"BM_SummaryRepeat/repeats:3_median\", "
-                 "\"name\": \"BM_SummaryRepeat/repeats:3_stddev\"\nThe entire "
+                 "\"name\": \"BM_SummaryRepeat/repeats:3_stddev\", "
+                 "\"name\": \"BM_SummaryRepeat/repeats:3_cv\"\nThe entire "
                  "output:\n";
     std::cout << output;
     return 1;

test/output_test_helper.cc

@@ -49,6 +49,7 @@ SubMap& GetSubstitutions() {
       {" %s ", "[ ]+"},
       {"%time", "[ ]*" + time_re + "[ ]+ns"},
       {"%console_report", "[ ]*" + time_re + "[ ]+ns [ ]*" + time_re + "[ ]+ns [ ]*[0-9]+"},
+      {"%console_percentage_report", "[ ]*" + time_re + "[ ]+% [ ]*" + time_re + "[ ]+% [ ]*[0-9]+"},
       {"%console_us_report", "[ ]*" + time_re + "[ ]+us [ ]*" + time_re + "[ ]+us [ ]*[0-9]+"},
       {"%console_ms_report", "[ ]*" + time_re + "[ ]+ms [ ]*" + time_re + "[ ]+ms [ ]*[0-9]+"},
       {"%console_s_report", "[ ]*" + time_re + "[ ]+s [ ]*" + time_re + "[ ]+s [ ]*[0-9]+"},

test/report_aggregates_only_test.cc

@@ -19,17 +19,19 @@ BENCHMARK(BM_SummaryRepeat)->Repetitions(3)->ReportAggregatesOnly();
 int main(int argc, char* argv[]) {
   const std::string output = GetFileReporterOutput(argc, argv);
 
-  if (SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3") != 3 ||
+  if (SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3") != 4 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_mean\"") != 1 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_median\"") !=
           1 ||
       SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_stddev\"") !=
-          1) {
+          1 ||
-    std::cout << "Precondition mismatch. Expected to only find three "
+      SubstrCnt(output, "\"name\": \"BM_SummaryRepeat/repeats:3_cv\"") != 1) {
+    std::cout << "Precondition mismatch. Expected to only find four "
                  "occurrences of \"BM_SummaryRepeat/repeats:3\" substring:\n"
                  "\"name\": \"BM_SummaryRepeat/repeats:3_mean\", "
                  "\"name\": \"BM_SummaryRepeat/repeats:3_median\", "
-                 "\"name\": \"BM_SummaryRepeat/repeats:3_stddev\"\nThe entire "
+                 "\"name\": \"BM_SummaryRepeat/repeats:3_stddev\", "
+                 "\"name\": \"BM_SummaryRepeat/repeats:3_cv\"\nThe entire "
                  "output:\n";
     std::cout << output;
     return 1;

test/statistics_gtest.cc

@@ -25,4 +25,11 @@ TEST(StatisticsTest, StdDev) {
                    1.151086443322134);
 }
 
+TEST(StatisticsTest, CV) {
+  EXPECT_DOUBLE_EQ(benchmark::StatisticsCV({101, 101, 101, 101}), 0.0);
+  EXPECT_DOUBLE_EQ(benchmark::StatisticsCV({1, 2, 3}), 1. / 2.);
+  EXPECT_DOUBLE_EQ(benchmark::StatisticsCV({2.5, 2.4, 3.3, 4.2, 5.1}),
+                   0.32888184094918121);
+}
+
 }  // end namespace

test/user_counters_tabular_test.cc

@@ -19,11 +19,13 @@ ADD_CASES(TC_ConsoleOut,
       {"^BM_Counters_Tabular/repeats:2/threads:1_mean %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
       {"^BM_Counters_Tabular/repeats:2/threads:1_median %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
             {"^BM_Counters_Tabular/repeats:2/threads:1_stddev %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
+            {"^BM_Counters_Tabular/repeats:2/threads:1_cv %console_percentage_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
       {"^BM_Counters_Tabular/repeats:2/threads:2 %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
       {"^BM_Counters_Tabular/repeats:2/threads:2 %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
       {"^BM_Counters_Tabular/repeats:2/threads:2_mean %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
       {"^BM_Counters_Tabular/repeats:2/threads:2_median %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
             {"^BM_Counters_Tabular/repeats:2/threads:2_stddev %console_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
+            {"^BM_Counters_Tabular/repeats:2/threads:2_cv %console_percentage_report [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat [ ]*%hrfloat$", MR_Next},
     {"^BM_CounterRates_Tabular/threads:%int %console_report [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s$", MR_Next},
     {"^BM_CounterRates_Tabular/threads:%int %console_report [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s$", MR_Next},
     {"^BM_CounterRates_Tabular/threads:%int %console_report [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s [ ]*%hrfloat/s$", MR_Next},
@@ -181,6 +183,28 @@ ADD_CASES(TC_JSONOut,
            {"\"Frob\": %float,$", MR_Next},
            {"\"Lob\": %float$", MR_Next},
            {"}", MR_Next}});
+ADD_CASES(TC_JSONOut,
+          {{"\"name\": \"BM_Counters_Tabular/repeats:2/threads:1_cv\",$"},
+           {"\"family_index\": 0,$", MR_Next},
+           {"\"per_family_instance_index\": 0,$", MR_Next},
+           {"\"run_name\": \"BM_Counters_Tabular/repeats:2/threads:1\",$",
+            MR_Next},
+           {"\"run_type\": \"aggregate\",$", MR_Next},
+           {"\"repetitions\": 2,$", MR_Next},
+           {"\"threads\": 1,$", MR_Next},
+           {"\"aggregate_name\": \"cv\",$", MR_Next},
+           {"\"aggregate_unit\": \"percentage\",$", MR_Next},
+           {"\"iterations\": %int,$", MR_Next},
+           {"\"real_time\": %float,$", MR_Next},
+           {"\"cpu_time\": %float,$", MR_Next},
+           {"\"time_unit\": \"ns\",$", MR_Next},
+           {"\"Bar\": %float,$", MR_Next},
+           {"\"Bat\": %float,$", MR_Next},
+           {"\"Baz\": %float,$", MR_Next},
+           {"\"Foo\": %float,$", MR_Next},
+           {"\"Frob\": %float,$", MR_Next},
+           {"\"Lob\": %float$", MR_Next},
+           {"}", MR_Next}});
 
 ADD_CASES(TC_JSONOut,
           {{"\"name\": \"BM_Counters_Tabular/repeats:2/threads:2\",$"},
@@ -268,6 +292,28 @@ ADD_CASES(TC_JSONOut,
            {"\"Frob\": %float,$", MR_Next},
            {"\"Lob\": %float$", MR_Next},
            {"}", MR_Next}});
+ADD_CASES(TC_JSONOut,
+          {{"\"name\": \"BM_Counters_Tabular/repeats:2/threads:2_cv\",$"},
+           {"\"family_index\": 0,$", MR_Next},
+           {"\"per_family_instance_index\": 1,$", MR_Next},
+           {"\"run_name\": \"BM_Counters_Tabular/repeats:2/threads:2\",$",
+            MR_Next},
+           {"\"run_type\": \"aggregate\",$", MR_Next},
+           {"\"repetitions\": 2,$", MR_Next},
+           {"\"threads\": 2,$", MR_Next},
+           {"\"aggregate_name\": \"cv\",$", MR_Next},
+           {"\"aggregate_unit\": \"percentage\",$", MR_Next},
+           {"\"iterations\": %int,$", MR_Next},
+           {"\"real_time\": %float,$", MR_Next},
+           {"\"cpu_time\": %float,$", MR_Next},
+           {"\"time_unit\": \"ns\",$", MR_Next},
+           {"\"Bar\": %float,$", MR_Next},
+           {"\"Bat\": %float,$", MR_Next},
+           {"\"Baz\": %float,$", MR_Next},
+           {"\"Foo\": %float,$", MR_Next},
+           {"\"Frob\": %float,$", MR_Next},
+           {"\"Lob\": %float$", MR_Next},
+           {"}", MR_Next}});
 ADD_CASES(TC_CSVOut,
           {{"^\"BM_Counters_Tabular/repeats:2/threads:1\",%csv_report,"
             "%float,%float,%float,%float,%float,%float$"}});
@@ -283,6 +329,9 @@ ADD_CASES(TC_CSVOut,
 ADD_CASES(TC_CSVOut,
           {{"^\"BM_Counters_Tabular/repeats:2/threads:1_stddev\",%csv_report,"
             "%float,%float,%float,%float,%float,%float$"}});
+ADD_CASES(TC_CSVOut,
+          {{"^\"BM_Counters_Tabular/repeats:2/threads:1_cv\",%csv_report,"
+            "%float,%float,%float,%float,%float,%float$"}});
 ADD_CASES(TC_CSVOut,
           {{"^\"BM_Counters_Tabular/repeats:2/threads:2\",%csv_report,"
             "%float,%float,%float,%float,%float,%float$"}});
@@ -298,6 +347,9 @@ ADD_CASES(TC_CSVOut,
 ADD_CASES(TC_CSVOut,
           {{"^\"BM_Counters_Tabular/repeats:2/threads:2_stddev\",%csv_report,"
             "%float,%float,%float,%float,%float,%float$"}});
+ADD_CASES(TC_CSVOut,
+          {{"^\"BM_Counters_Tabular/repeats:2/threads:2_cv\",%csv_report,"
+            "%float,%float,%float,%float,%float,%float$"}});
 // VS2013 does not allow this function to be passed as a lambda argument
 // to CHECK_BENCHMARK_RESULTS()
 void CheckTabular(Results const& e) {