Include system load average in console and JSON reports

High system load can skew benchmark results. By including system load averages
in the library's output, we help users identify a potential issue in the
quality of their measurements, and thus assist them in producing better (more
reproducible) results.

I got the idea for this from Brendan Gregg's checklist for benchmark accuracy
(http://www.brendangregg.com/blog/2018-06-30/benchmarking-checklist.html).

include/benchmark/benchmark.h

@@ -1242,6 +1242,7 @@ struct CPUInfo {
   double cycles_per_second;
   std::vector<CacheInfo> caches;
   bool scaling_enabled;
+  std::vector<double> load_avg;
 
   static const CPUInfo& Get();
 

src/json_reporter.cc

@@ -111,6 +111,12 @@ bool JSONReporter::ReportContext(const Context& context) {
   }
   indent = std::string(4, ' ');
   out << indent << "],\n";
+  out << indent << "\"load_avg\": [";
+  for (auto it = info.load_avg.begin(); it != info.load_avg.end();) {
+    out << *it++;
+    if (it != info.load_avg.end()) out << ",";
+  }
+  out << "],\n";
 
 #if defined(NDEBUG)
   const char build_type[] = "release";

src/reporter.cc

@@ -22,6 +22,7 @@
 #include <vector>
 
 #include "check.h"
+#include "string_util.h"
 
 namespace benchmark {
 
@@ -54,6 +55,14 @@ void BenchmarkReporter::PrintBasicContext(std::ostream *out,
       Out << "\n";
     }
   }
+  if (!info.load_avg.empty()) {
+    Out << "Load Average: ";
+    for (auto It = info.load_avg.begin(); It != info.load_avg.end();) {
+      Out << StrFormat("%.2f", *It++);
+      if (It != info.load_avg.end()) Out << ", ";
+    }
+    Out << "\n";
+  }
 
   if (info.scaling_enabled) {
     Out << "***WARNING*** CPU scaling is enabled, the benchmark "

src/sysinfo.cc

@@ -571,6 +571,24 @@ double GetCPUCyclesPerSecond() {
   return static_cast<double>(cycleclock::Now() - start_ticks);
 }
 
+std::vector<double> GetLoadAvg() {
+#if defined BENCHMARK_OS_FREEBSD || defined(BENCHMARK_OS_LINUX) || \
+    defined BENCHMARK_OS_MACOSX || defined BENCHMARK_OS_NETBSD ||  \
+    defined BENCHMARK_OS_OPENBSD
+  constexpr int kMaxSamples = 3;
+  std::vector<double> res(kMaxSamples, 0.0);
+  const int nelem = getloadavg(res.data(), kMaxSamples);
+  if (nelem < 1) {
+    res.clear();
+  } else {
+    res.resize(nelem);
+  }
+  return res;
+#else
+  return {};
+#endif
+}
+
 }  // end namespace
 
 const CPUInfo& CPUInfo::Get() {
@@ -582,6 +600,7 @@ CPUInfo::CPUInfo()
     : num_cpus(GetNumCPUs()),
       cycles_per_second(GetCPUCyclesPerSecond()),
       caches(GetCacheSizes()),
-      scaling_enabled(CpuScalingEnabled(num_cpus)) {}
+      scaling_enabled(CpuScalingEnabled(num_cpus)),
+      load_avg(GetLoadAvg()) {}
 
 }  // end namespace benchmark

test/reporter_output_test.cc

@@ -28,7 +28,8 @@ static int AddContextCases() {
             {"\"mhz_per_cpu\": %float,$", MR_Next},
             {"\"cpu_scaling_enabled\": ", MR_Next},
             {"\"caches\": \\[$", MR_Next}});
-  auto const& Caches = benchmark::CPUInfo::Get().caches;
+  auto const& Info = benchmark::CPUInfo::Get();
+  auto const& Caches = Info.caches;
   if (!Caches.empty()) {
     AddCases(TC_ConsoleErr, {{"CPU Caches:$", MR_Next}});
   }
@@ -45,8 +46,13 @@ static int AddContextCases() {
                           {"\"num_sharing\": %int$", MR_Next},
                           {"}[,]{0,1}$", MR_Next}});
   }
-
   AddCases(TC_JSONOut, {{"],$"}});
+  auto const& LoadAvg = Info.load_avg;
+  if (!LoadAvg.empty()) {
+    AddCases(TC_ConsoleErr,
+             {{"Load Average: (%float, ){0,2}%float$", MR_Next}});
+  }
+  AddCases(TC_JSONOut, {{"\"load_avg\": \\[(%float,?){0,3}],$", MR_Next}});
   return 0;
 }
 int dummy_register = AddContextCases();