c++ - 谷歌/基准测试结果不一致

Question

我是使用 Google Benchmark 的新手，在本地运行代码与在 Quick-Bench.com 上运行代码时，我收到了运行相同基准测试(下方)的不同结果，该基准测试使用 C++ 检索本地时间.两次我都使用 GCC 8.2 和 -O3。

为什么在本地运行与在 quick-bench.com 上运行的结果差异很大？哪个是对的？

#include <benchmark/benchmark.h>
#include <ctime>      
#include <sys/time.h> 
#include <chrono>     


static void BM_ctime(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    std::time_t sec = std::time(0);  

    benchmark::DoNotOptimize(count += sec);
  }
}

BENCHMARK(BM_ctime);


static void BM_sysTime(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    unsigned long sec = time(NULL);

    benchmark::DoNotOptimize(count += sec);
  }
}

BENCHMARK(BM_sysTime);


static void BM_chronoMilliseconds(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    unsigned long long ms = std::chrono::duration_cast<std::chrono::milliseconds>(
      std::chrono::system_clock::now().time_since_epoch()
    ).count();

    benchmark::DoNotOptimize(count += ms);
  }
}

BENCHMARK(BM_chronoMilliseconds);

static void BM_chronoSececonds(benchmark::State& state) {
  unsigned long long count = 0;

  for (auto _ : state) {
    unsigned long long sec = std::chrono::duration_cast<std::chrono::seconds>(
      std::chrono::system_clock::now().time_since_epoch()
    ).count();

    benchmark::DoNotOptimize(count += sec);
  }
}

BENCHMARK(BM_chronoSececonds);

本地产生以下结果:

-------------------------------------------------------------
Benchmark                      Time           CPU Iterations
-------------------------------------------------------------
BM_ctime                     183 ns        175 ns    4082013
BM_sysTime                   197 ns        179 ns    4004829
BM_chronoMilliseconds         37 ns         36 ns   19092506
BM_chronoSececonds            37 ns         36 ns   19057991

QuickBench 结果:

Answer 1

基准测试结果取决于平台/架构/机器。当您在同一台机器上运行基准测试时，假设它们始终相同是不切实际的，温度、性能扩展选项、磨损等都会影响性能。