我有以下程序，我在 stackoverflow 上的其他人的帮助下编写了该程序以了解缓存线和 CPU 缓存。我在下面发布了计算结果。
1 450.0 440.0
2 420.0 230.0
4 400.0 110.0
8 390.0 60.0
16 380.0 30.0
32 320.0 10.0
64 180.0 10.0
128 60.0 0.0
256 40.0 10.0
512 10.0 0.0
1024 10.0 0.0
我已经使用 gnuplot 绘制了一个图表，该图表发布在下面。
我有以下问题。
我以毫秒为单位的计时计算是否正确？440ms似乎很多时间？
从图中 cache_access_1（红线）我们可以得出结论，缓存线的大小是 32 位（而不是 64 位？）
在代码中的 for 循环之间清除缓存是个好主意吗？如果是，我该如何以编程方式做到这一点？
如您所见，我
0.0在上面的结果中有一些值。？这说明什么？测量的粒度是否太粗？
请回复。
#include <stdio.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#define MAX_SIZE (512*1024*1024)
int main()
{
clock_t start, end;
double cpu_time;
int i = 0;
int k = 0;
int count = 0;
/*
* MAX_SIZE array is too big for stack.This is an unfortunate rough edge of the way the stack works.
* It lives in a fixed-size buffer, set by the program executable's configuration according to the
* operating system, but its actual size is seldom checked against the available space.
*/
/*int arr[MAX_SIZE];*/
int *arr = (int*)malloc(MAX_SIZE * sizeof(int));
/*cpu clock ticks count start*/
for(k = 0; k < 3; k++)
{
start = clock();
count = 0;
for (i = 0; i < MAX_SIZE; i++)
{
arr[i] += 3;
/*count++;*/
}
/*cpu clock ticks count stop*/
end = clock();
cpu_time = ((double) (end - start)) / CLOCKS_PER_SEC;
printf("cpu time for loop 1 (k : %4d) %.1f ms.\n",k,(cpu_time*1000));
}
printf("\n");
for (k = 1 ; k <= 1024 ; k <<= 1)
{
/*cpu clock ticks count start*/
start = clock();
count = 0;
for (i = 0; i < MAX_SIZE; i += k)
{
/*count++;*/
arr[i] += 3;
}
/*cpu clock ticks count stop*/
end = clock();
cpu_time = ((double) (end - start)) / CLOCKS_PER_SEC;
printf("cpu time for loop 2 (k : %4d) %.1f ms.\n",k,(cpu_time*1000));
}
printf("\n");
/* Third loop, performing the same operations as loop 2,
but only touching 16KB of memory
*/
for (k = 1 ; k <= 1024 ; k <<= 1)
{
/*cpu clock ticks count start*/
start = clock();
count = 0;
for (i = 0; i < MAX_SIZE; i += k)
{
count++;
arr[i & 0xfff] += 3;
}
/*cpu clock ticks count stop*/
end = clock();
cpu_time = ((double) (end - start)) / CLOCKS_PER_SEC;
printf("cpu time for loop 3 (k : %4d) %.1f ms.\n",k,(cpu_time*1000));
}
return 0;
}