benchmark - 技术栈

好久没写了

简单写一篇吧

最近在用benchmark，看了下wiki

In all cases, the number of iterations for which the benchmark is run is

governed by the amount of time the benchmark takes. Concretely, the number of

iterations is at least one, not more than 1e9, until CPU time is greater than

the minimum time, or the wallclock time is 5x minimum time. The minimum time is

set per benchmark by calling `MinTime` on the registered benchmark object.

这段是关于统计时间的，我简单思考了下，猜测了基本实现

然后又看了下源码，发现和想的大差不差吧

时间的统计

先获取real time

再获取cputime

获取方法如下：

cpp 复制代码

 inline double ChronoClockNow() {
    typedef ChooseClockType::type ClockType;
    using FpSeconds = std::chrono::duration<double, std::chrono::seconds::period>;
    return FpSeconds(ClockType::now().time_since_epoch()).count();
  }

 double ThreadCPUUsage() {
   struct timespec ts;
   if (clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts) == 0) return MakeTime(ts);
   DiagnoseAndExit("clock_gettime(CLOCK_THREAD_CPUTIME_ID, ...) failed");

所以这个统计时间，要注意，会包含获取cpu时间的overhead

循环

cpp 复制代码

for (;;) {
     b.Setup();
     i = DoNIterations();
     b.Teardown();

通过这个应该可以更清楚理解，使用方法介绍的 Setup/Teardown了

这里会一直循环下去，每一次操作都会统计时间，直到触发下面条件

cpp 复制代码

bool BenchmarkRunner::ShouldReportIterationResults(
     const IterationResults& i) const {
   // Determine if this run should be reported;
   // Either it has run for a sufficient amount of time
   // or because an error was reported.
   bool b_flag = i.results.skipped_ ||
          i.iters >= kMaxIterations ||  // Too many iterations already.
          i.seconds >=
              GetMinTimeToApply() ||  // The elapsed time is large enough.
          // CPU time is specified but the elapsed real time greatly exceeds
          // the minimum time.
          // Note that user provided timers are except from this test.
          ((i.results.real_time_used >= 5 * GetMinTimeToApply()) &&
           !b.use_manual_time());
   return b_flag;
 }