JUST DO IT ~

测试代码的locality。
数组的读区方式不同，按照行读，被cache也是按行连续加载的。
如果按照列读区，那么效率很低，除非cache足够大，而且也要遍历所有的数据，并且cache hash算法也好，实现的硬件还是多路组相联的cache硬件实现。

** valgrind --tool=cachegrind ./test2**

code1:

#include <stdio.h>
#define MAXROW 8000
#define MAXCOL 8000
int main () {
int i,j;
 static int x[MAXROW][MAXCOL];
 printf ("Starting!\n");
       for (i=0;i<MAXROW;i++)
       for (j=0;j<MAXCOL;j++)
              x[i][j] = i*j;
             printf("Completed!\n");
return 0;                                                    
 }

code2:

#include <stdio.h>                                                         
 #define MAXROW 8000
 #define MAXCOL 8000
 int main () {
 int i,j;
 static int x[MAXROW][MAXCOL];
 printf ("Starting!\n");
          for (j=0;j<MAXCOL;j++)
                         for (i=0;i<MAXROW;i++)
                 x[i][j] = i*j;
 printf("Completed!\n");
 return 0;
 }
 ```

##结果

Command: ./test1
Starting!
Completed!

I refs: 905,721,688
I1 misses: 4,177
LLi misses: 2,808
I1 miss rate: 0.00%
LLi miss rate: 0.00%

D refs: 514,830,867 (386,118,735 rd + 128,712,132 wr)
D1 misses: 4,025,828 ( 23,565 rd + 4,002,263 wr)
LLd misses: 4,008,456 ( 6,997 rd + 4,001,459 wr)

D1 miss rate: 0.8% ( 0.0% + 3.1% )
LLd miss rate: 0.8% ( 0.0% + 3.1% )

LL refs: 4,030,005 ( 27,742 rd + 4,002,263 wr)
LL misses: 4,011,264 ( 9,805 rd + 4,001,459 wr)
LL miss rate: 0.3% ( 0.0% + 3.1% )

gcc -o test2 test2.c
** valgrind --tool=cachegrind ./test2**

I refs: 905,720,801
I1 misses: 4,113
LLi misses: 2,811
I1 miss rate: 0.00%
LLi miss rate: 0.00%

D refs: 514,830,348 (386,118,427 rd + 128,711,921 wr)
D1 misses: 64,025,705 ( 23,462 rd + 64,002,243 wr)
LLd misses: 4,016,427 ( 6,977 rd + 4,009,450 wr)
D1 miss rate: 12.4% ( 0.0% + 49.7% )
LLd miss rate: 0.8% ( 0.0% + 3.1% )

LL refs: 64,029,818 ( 27,575 rd + 64,002,243 wr)
LL misses: 4,019,238 ( 9,788 rd + 4,009,450 wr)
LL miss rate: 0.3% ( 0.0% + 3.1% )

Starting!
Completed!
```

参考：

valgrind调试CPU缓存命中率和内存泄漏
http://laoxu.blog.51cto.com/4120547/1395236