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[英]Is using a for loop to iterate a fixed array slower than manually going through it?
[英]Why is this sequential array loop slower than a loop that uses a “lookup” array?
我最近一直在研究緩存局部性,我試圖了解CPU如何訪問內存。 我寫了一個實驗,看看在順序循環數組時是否存在性能差異,而使用某種查找表來索引數據數組。 我很驚訝地發現查找方法稍快一些。 我的代碼如下。 我在Windows上用GCC編譯(MinGW)。
#include <stdlib.h>
#include <stdio.h>
#include <windows.h>
int main()
{
DWORD dwElapsed, dwStartTime;
//random arrangement of keys to lookup
int lookup_arr[] = {0, 3, 8, 7, 2, 1, 4, 5, 6, 9};
//data for both loops
int data_arr1[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int data_arr2[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
//first loop, sequential access
dwStartTime = GetTickCount();
for (int n = 0; n < 9000000; n++) {
for (int i = 0; i < 10; i++)
data_arr1[i]++;
}
dwElapsed = GetTickCount() - dwStartTime;
printf("Normal loop completed: %d\n", dwElapsed);
//second loop, indexes into data_arr2 using the lookup array
dwStartTime = GetTickCount();
for (int n = 0; n < 9000000; n++) {
for (int i = 0; i < 10; i++)
data_arr2[lookup_arr[i]]++;
}
dwElapsed = GetTickCount() - dwStartTime;
printf("Lookup loop completed: %d\n", dwElapsed);
return 0;
}
運行這個,我得到:
Normal loop completed: 375
Lookup loop completed: 297
按照我之前的評論,這是你如何做這件事。
結果是速度與“隨機索引”有顯着差異。
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <math.h>
#define N 1000000
int main(void) {
int *rArr;
int *rInd; // randomized indices
int *lInd; // linear indices
int ii;
rArr = malloc(N * sizeof(int) );
rInd = malloc(N * sizeof(int) );
lInd = malloc(N * sizeof(int) );
for(ii = 0; ii < N; ii++) {
lInd[ii] = ii;
rArr[ii] = rand();
rInd[ii] = rand()%N;
}
int loopCount;
int sum;
time_t startT, stopT;
double dt, totalT=0, tt2=0;
startT = clock();
for(loopCount = 0; loopCount < 100; loopCount++) {
for(ii = 0; ii < N; ii++) {
sum += rArr[lInd[ii]];
}
stopT = clock();
dt = stopT - startT;
totalT += dt;
tt2 += dt * dt;
startT = stopT;
}
printf("sum is %d\n", sum);
printf("total time: %lf += %lf\n", totalT/(double)(CLOCKS_PER_SEC), (tt2 - totalT * totalT / 100.0)/100.0 / (double)(CLOCKS_PER_SEC));
totalT = 0; tt2 = 0;
startT = clock();
for(loopCount = 0; loopCount < 100; loopCount++) {
for(ii = 0; ii < N; ii++) {
sum += rArr[rInd[ii]];
}
stopT = clock();
dt = stopT - startT;
totalT += dt;
tt2 += dt * dt;
startT = stopT;
}
printf("sum is %d\n", sum);
printf("total time: %lf += %lf\n", totalT/(double)(CLOCKS_PER_SEC), sqrt((tt2 - totalT * totalT / 100.0)/100.0) / (double)(CLOCKS_PER_SEC));
}
結果 - 順序訪問速度提高了2倍(在我的機器上):
sum is -1444272372
total time: 0.396539 += 0.000219
sum is 546230204
total time: 0.756407 += 0.001165
通過-O3
優化,差異甚至更加明顯 - 快3倍:
sum is -318372465
total time: 0.142444 += 0.013230
sum is 1672130111
total time: 0.455804 += 0.000402
我相信你正在編譯而沒有打開優化。 使用-O2
g ++可以優化所有內容,因此運行時間為0,沒有標志我得到類似的結果。
在修改程序以便data_arr1
和data_arr2
中的值實際用於某些東西時,我得到78ms。
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