我编写了以下代码,该代码在数组中执行一定数量的二进制搜索。我将其与OpenMP并行化,似乎我添加线程的次数越多,完成时间就越多。该程序将在其中应用Bsearch的数组的长度和在第一个数组中要搜索的值初始化的search
数组的长度作为参数。并行化在所有三个for循环中应用。
我在具有20个核心的单个节点上的HPC群集上使用以下脚本运行此程序:
for threads in 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ; do
export OMP_NUM_THREADS=${threads}
./binary_search_parallel.x 1000000000 100000000
done
我的问题是程序根本无法扩展:我添加线程越多,花费的时间就越多。串行版本的性能更好。有人知道问题出在哪里吗?也许事实是针对并行开销的性能吞吐量不足?
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <string.h>
#include <omp.h>
#define CPU_TIME (clock_gettime( CLOCK_PROCESS_CPUTIME_ID, &ts ), (double)ts.tv_sec + \
(double)ts.tv_nsec * 1e-9)
int mybsearch(int *data, int low, int high, int Key)
{
int register mid;
mid = (low + high) / 2;
while(low <= high) {
if(data[mid] < Key)
low = mid + 1;
else if(data[mid] > Key)
high = mid - 1;
else
return mid;
mid = (low + high) / 2;
}
/* if ( Key == data[low] ) */
/* return 0; */
/* else */
return -1;
}
#define N_DEFAULT (1024*1024*128)
#define N_search_DEFAULT (N_DEFAULT / 10)
int main(int argc, char **argv)
{
int N, Nsearch, i, n_threads = 1;
int *data, *search;
#ifndef _OPENMP
printf("serial binary search\n");
#else
#pragma omp parallel
{
#pragma omp master
{
n_threads = omp_get_num_threads();
printf("omp binary search with %d threads\n", n_threads );
}
}
#endif
if(argc > 1)
N = atoi( *(argv+1) );
else
N = N_DEFAULT;
if(argc > 2)
Nsearch = atoi ( *(argv + 2) );
else
Nsearch = N_search_DEFAULT;
printf("performing %d lookups on %d data..\n", Nsearch, N);
printf("set-up data.."); fflush(stdout);
data = (int*)malloc(N * sizeof(int));
#if defined(_OPENMP)
#pragma omp parallel for
for (i = 0; i < N; i++)
data[i] = i;
#else
for(i = 0; i < N; i++)
data[i] = i;
#endif
printf(" set-up lookups.. "); fflush(stdout);
search = (int*)malloc(Nsearch * sizeof(int));
srand(time(NULL));
#if defined(_OPENMP)
#pragma omp parallel for
for (i = 0; i < Nsearch; i++)
search[i] = rand() % N;
#else
for (i = 0; i < N; i++)
search[i] = rand() % N;
#endif
int found = 0;
double tstart, tstop;
struct timespec ts;
printf("\nstart cycle.. "); fflush(stdout);
tstart = CPU_TIME;
#if defined(_OPENMP)
#pragma omp parallel for
for (i = 0; i < Nsearch; i++)
if( mybsearch(data, N, search[i]) >= 0)
found++;
#else
for ( i = 0; i < Nsearch; i++)
if(mybsearch(data, N, search[i]) >= 0)
found++;
#endif
tstop = CPU_TIME;
printf("time elapsed: %g\n", tstop - tstart);
//free(data);
//free(search);
return 0;
}
您的程序在没有大量IO的情况下执行计算,因此添加更多线程(超出内核数量)意味着您只是增加了上下文切换的开销,而没有任何其他好处。这就是为什么它不扩展,因为每个线程都占用CPU。
添加更多线程仅在它们正在等待某些东西时才有用,而CPU可以执行其他任务。