通过恒定长度遍历数组的访问时间
我想测量缓存的速度（我问了一个问题 ），所以我写了这段代码：
#include #include #include const int mod = 100000007; volatile int f[100000007]; volatile int *p; int main() { FILE *f1; f1=fopen("CacheTest.txt","w"); for(int l=1;l<=100000;) { clock_t st,ed; st=clock(); int now=0; for(int i=0;i=mod) now-=mod; } ed=clock(); double extime=(double)(ed-st)/CLOCKS_PER_SEC; st=clock(); now=0; for(int i=0;i=mod) now-=mod; } ed=clock(); double cost=(double)(ed-st)/CLOCKS_PER_SEC; fprintf(f1,"length %d costs %f seconds access time:%f\n",l,cost,cost-extime); printf("length %d costs %f seconds access time:%f\n",l,cost,cost-extime); if (l<50) l++; else if (l<100) l+=2; else if (l<1000) l+=20; else if (l<10000) l+=200; else if (l<100000) l+=2000; else l+=20000; } } 结果如下：
length 1 costs 0.994000 seconds access time:0.153000 length 2 costs 0.651000 seconds access time:-0.048000 length 3 costs 1.034000 seconds access time:0.414000 length 4 costs 1.151000 seconds access time:0.018000 length 5 costs 1.221000 seconds access time:0.086000 length 6 costs 1.313000 seconds access time:0.179000 length 7 costs 1.475000 seconds access time:0.442000 length 8 costs 1.584000 seconds access time:0.437000 length 9 costs 1.700000 seconds access time:0.567000 length 10 costs 1.672000 seconds access time:0.665000 length 11 costs 1.578000 seconds access time:0.573000 length 12 costs 2.166000 seconds access time:1.280000 length 13 costs 2.070000 seconds access time:0.988000 length 14 costs 2.430000 seconds access time:1.402000 length 15 costs 2.564000 seconds access time:1.428000 length 16 costs 2.651000 seconds access time:1.519000 length 17 costs 2.721000 seconds access time:1.654000 length 18 costs 2.648000 seconds access time:1.518000 length 19 costs 2.775000 seconds access time:1.638000 length 20 costs 2.757000 seconds access time:1.636000 length 21 costs 2.816000 seconds access time:1.684000 length 22 costs 2.842000 seconds access time:1.706000 length 23 costs 2.856000 seconds access time:1.751000 length 24 costs 2.702000 seconds access time:1.568000 length 25 costs 2.624000 seconds access time:1.618000 length 26 costs 2.912000 seconds access time:1.777000 length 27 costs 2.617000 seconds access time:1.468000 length 28 costs 2.910000 seconds access time:1.778000 length 29 costs 2.779000 seconds access time:1.649000 length 30 costs 2.803000 seconds access time:1.727000 length 31 costs 2.596000 seconds access time:1.465000 length 32 costs 2.068000 seconds access time:1.188000 length 33 costs 2.012000 seconds access time:1.294000 length 34 costs 3.258000 seconds access time:2.381000 length 35 costs 3.538000 seconds access time:2.406000 length 36 costs 3.619000 seconds access time:2.556000 length 37 costs 3.847000 seconds access time:2.717000 length 38 costs 3.958000 seconds access time:2.827000 length 39 costs 3.910000 seconds access time:2.841000 length 40 costs 3.722000 seconds access time:2.592000 length 41 costs 4.095000 seconds access time:2.960000 length 42 costs 3.440000 seconds access time:2.420000 length 43 costs 4.217000 seconds access time:3.085000 length 44 costs 4.413000 seconds access time:3.457000 length 45 costs 4.637000 seconds access time:3.507000 length 46 costs 4.248000 seconds access time:3.115000 length 47 costs 4.924000 seconds access time:3.856000 length 48 costs 5.072000 seconds access time:3.942000 length 49 costs 4.619000 seconds access time:3.615000 length 50 costs 5.025000 seconds access time:4.136000 length 52 costs 5.196000 seconds access time:4.059000 length 54 costs 4.937000 seconds access time:3.806000 length 56 costs 5.264000 seconds access time:4.132000 length 58 costs 5.137000 seconds access time:4.004000 length 60 costs 5.113000 seconds access time:4.107000 length 62 costs 4.665000 seconds access time:3.537000 length 64 costs 5.030000 seconds access time:3.900000 length 66 costs 4.969000 seconds access time:4.016000 length 68 costs 5.201000 seconds access time:4.096000 length 70 costs 5.066000 seconds access time:3.931000 ... for the length 70~10000 , the access time is around 4s.
前几行的访问时间非常短，我不知道为什么。
然后程序稳定，访问时间大约为0.4秒，我认为原因是大多数访问都达到了L1级缓存。
随着长度的增加，时间变成大约1.5秒。 也许L1缓存不起作用，但L2缓存不起作用。
然后时间到4s，可能根本没有缓存命中？
上面的结果似乎总体上符合我的期望。
但！ 出乎意料的事情发生了，我感到很惊讶。
当长度超过20000时，访问时间变为：
length 20000 costs 1.828000 seconds access time:1.453000 length 22000 costs 1.867000 seconds access time:1.492000 length 24000 costs 1.959000 seconds access time:1.584000 length 26000 costs 1.977000 seconds access time:1.603000 length 28000 costs 1.987000 seconds access time:1.613000 length 30000 costs 2.045000 seconds access time:1.669000 length 32000 costs 1.951000 seconds access time:1.575000 length 34000 costs 1.980000 seconds access time:1.611000 length 36000 costs 1.988000 seconds access time:1.613000 length 38000 costs 1.993000 seconds access time:1.619000 length 40000 costs 1.850000 seconds access time:1.473000 length 42000 costs 2.010000 seconds access time:1.634000 length 44000 costs 1.991000 seconds access time:1.615000 length 46000 costs 1.977000 seconds access time:1.601000 length 48000 costs 1.977000 seconds access time:1.601000 length 50000 costs 1.927000 seconds access time:1.552000 length 52000 costs 1.907000 seconds access time:1.531000 length 54000 costs 2.013000 seconds access time:1.637000 length 56000 costs 2.014000 seconds access time:1.639000 length 58000 costs 2.011000 seconds access time:1.640000 length 60000 costs 1.978000 seconds access time:1.606000 length 62000 costs 2.046000 seconds access time:1.672000 length 64000 costs 2.076000 seconds access time:1.700000 length 66000 costs 2.068000 seconds access time:1.693000 length 68000 costs 2.014000 seconds access time:1.639000 length 70000 costs 2.123000 seconds access time:1.748000 length 72000 costs 2.032000 seconds access time:1.646000 length 74000 costs 2.015000 seconds access time:1.638000 length 76000 costs 2.023000 seconds access time:1.646000 length 78000 costs 1.873000 seconds access time:1.497000 length 80000 costs 1.655000 seconds access time:1.278000
谁能解释为什么这些奇怪的事情发生了？ 我现在被困住了，对此一无所知。
提前致谢。
当你测试这样的东西时，你应该让你的系统处于空闲状态。
我在电脑上测试了三次，长度> 5000时保持3秒。