不同机器之间std时钟的显着性能差异

测试其他东西我偶然发现了一些我还没有想到的东西.

我们来看看这个片段:

#include 
#include 

int main () {
  int i = 0;
  using namespace std::chrono_literals;

  auto const end = std::chrono::system_clock::now() + 5s;
  while (std::chrono::system_clock::now() 

我注意到计数在很大程度上取决于我执行它的机器.

我用gcc 7.3,8.2和clang 6.0编译了std=c++17 -O3.

在i7-4790(4.17.14-arch1-1-ARCH内核):〜3e8

但在Xeon E5-2630 v4(3.10.0-514.el7.x86_64)上:〜8e6

现在这是我想要理解的差异,所以我已经检查过了 perf stat -d 

在i7上:   

       4999.419546      task-clock:u (msec)       #    0.999 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
               120      page-faults:u             #    0.024 K/sec                  
    19,605,598,394      cycles:u                  #    3.922 GHz                      (49.94%)
    33,601,884,120      instructions:u            #    1.71  insn per cycle           (62.48%)
     7,397,994,820      branches:u                # 1479.771 M/sec                    (62.53%)
            34,788      branch-misses:u           #    0.00% of all branches          (62.58%)
    10,809,601,166      L1-dcache-loads:u         # 2162.171 M/sec                    (62.41%)
            13,632      L1-dcache-load-misses:u   #    0.00% of all L1-dcache hits    (24.95%)
             3,944      LLC-loads:u               #    0.789 K/sec                    (24.95%)
             1,034      LLC-load-misses:u         #   26.22% of all LL-cache hits     (37.42%)

       5.003180401 seconds time elapsed

       4.969048000 seconds user
       0.016557000 seconds sys


至强:

       5001.000000      task-clock (msec)         #    0.999 CPUs utilized          
                42      context-switches          #    0.008 K/sec                  
                 2      cpu-migrations            #    0.000 K/sec                  
               412      page-faults               #    0.082 K/sec                  
    15,100,238,798      cycles                    #    3.019 GHz                      (50.01%)
       794,184,899      instructions              #    0.05  insn per cycle           (62.51%)
       188,083,219      branches                  #   37.609 M/sec                    (62.49%)
            85,924      branch-misses             #    0.05% of all branches          (62.51%)
       269,848,346      L1-dcache-loads           #   53.959 M/sec                    (62.49%)
           246,532      L1-dcache-load-misses     #    0.09% of all L1-dcache hits    (62.51%)
            13,327      LLC-loads                 #    0.003 M/sec                    (49.99%)
             7,417      LLC-load-misses           #   55.65% of all LL-cache hits     (50.02%)

       5.006139971 seconds time elapsed


弹出的是Xeon上每个周期的指令数量少以及我不理解的非零上下文切换.但是,我无法使用这些诊断程序来解释.

并且为了给问题添加一点怪异,在尝试调试时我也在一台机器上静态编译并在另一台机器上执行.

在Xeon上,静态编译的可执行文件输出降低了约10%,在xeon或i7上编译没有区别.

在i7上做同样的事情,计数器实际上都从3e8〜下降2e7

所以最后我还有两个问题:


为什么我在两台机器之间看到如此显着的差异.
为什么静态链接的exectuable表现更差,而我期望相反？


编辑:在将centos 7机器上的内核更新到4.18之后,我们实际上看到了从〜8e6到的额外下降5e6.

有趣地显示不同的数字:    

   5002.000000      task-clock:u (msec)       #    0.999 CPUs utilized          
             0      context-switches:u        #    0.000 K/sec                  
             0      cpu-migrations:u          #    0.000 K/sec                  
           119      page-faults:u             #    0.024 K/sec                  
   409,723,790      cycles:u                  #    0.082 GHz                      (50.00%)
   392,228,592      instructions:u            #    0.96  insn per cycle           (62.51%)
   115,475,503      branches:u                #   23.086 M/sec                    (62.51%)
        26,355      branch-misses:u           #    0.02% of all branches          (62.53%)
   115,799,571      L1-dcache-loads:u         #   23.151 M/sec                    (62.51%)
        42,327      L1-dcache-load-misses:u   #    0.04% of all L1-dcache hits    (62.50%)
            88      LLC-loads:u               #    0.018 K/sec                    (49.96%)
             2      LLC-load-misses:u         #    2.27% of all LL-cache hits     (49.98%)

   5.005940327 seconds time elapsed

   0.533000000 seconds user
   4.469000000 seconds sys


有趣的是,没有更多的上下文切换和每个周期的结构显着上升,但周期和colck是超低!