用linux perf命令來分析程序的cpu cache miss現象

 

#include <stdio.h>
#include <unistd.h>
 
int main(int argc, char **argv)
{
    int a[1000][1000];
    if(1 == argc)
    {
        for(int i = 0; i < 1000; ++i)
        {
                for(int j = 0; j < 1000; ++j)
                {
                        a[i][j] = 0;
                }
        }
    }
    else
    {
        for(int i = 0; i < 1000; ++i)
        {
                for(int j = 0; j < 1000; ++j)
                {
                        a[j][i] = 0;
                }
        }
    }
 
    return 0;
}
 

 

 上面有兩個小程序片段， 哪段效率高？ 顯然， 第一段效率高， 為什么呢？ 因為在C/C++中，數組是按行存儲的，程序的按行訪問可以充分利用程序的局部性原理（空間局部性）， 用time命令來看看結果：

 

[root@bogon c++]# g++ miss.c  -o miss
[root@bogon c++]# ./miss 
[root@bogon c++]# time ./miss 

real    0m0.009s
user    0m0.009s
sys     0m0.000s
[root@bogon c++]# time ./miss 1

real    0m0.013s
user    0m0.013s
sys     0m0.000s
[root@bogon c++]# time ./miss 

real    0m0.010s
user    0m0.010s
sys     0m0.000s
[root@bogon c++]# time ./miss 1

real    0m0.013s
user    0m0.013s
sys     0m0.000s

 

[root@bogon c++]# perf stat -e L1-dcache-load-misses ./miss

 Performance counter stats for './miss':

            88,780      L1-dcache-load-misses                                       

       0.009002291 seconds time elapsed

       0.009174000 seconds user
       0.000000000 seconds sys


[root@bogon c++]# perf stat -e L1-dcache-load-misses ./miss 1

 Performance counter stats for './miss 1':

         1,015,683      L1-dcache-load-misses                                       

       0.012000335 seconds time elapsed

       0.006059000 seconds user
       0.006059000 seconds sys


[root@bogon c++]# perf stat -e L1-dcache-load-misses ./miss 1

 Performance counter stats for './miss 1':

         1,015,363      L1-dcache-load-misses                                       

       0.012145156 seconds time elapsed

       0.006134000 seconds user
       0.006134000 seconds sys


[root@bogon c++]# perf stat -e L1-dcache-load-misses ./miss 0

 Performance counter stats for './miss 0':

         1,011,740      L1-dcache-load-misses                                       

       0.012363858 seconds time elapsed

       0.012484000 seconds user
       0.000000000 seconds sys


[root@bogon c++]# perf stat -e L1-dcache-load-misses ./miss 0

 Performance counter stats for './miss 0':

         1,015,347      L1-dcache-load-misses                                       

       0.012348778 seconds time elapsed

       0.006237000 seconds user
       0.006237000 seconds sys