[問題] OpenMP
遇到的問題： (題意請描述清楚)
在Linux上練習OpenMP撰寫比較有用openMP與沒有用OpenMP
發現到有使用OpenMP部份並將其指定為多個執行緒會比沒有用OpenMP還慢
但是若在OpenMP區塊將其執行序指定數量改為1時
速度反而比沒有使用OpenMP時還要快
應理這說
當這兩個部份在單執行緒時(一個有OpenMP一個沒有)
速度應該要一樣快
而在Windows上時
OpenMP區塊在多執行緒與單執行緒時
都會比沒有使用OpenMP之區塊要來的快
有請高人解答...
謝謝
開發平台： (例: VC++ or gcc/g++ or Dev-C++, Windows or Linux)
gcc 4.2.1, CentOS5.4; Windows, Visual C++ 2008
有問題的code： (請善用置底文標色功能)
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#define MAX_THREAD 2 //define the thread number
int element[100000000];
int main( int argc, char *argv )
{
int counter;
double stageStartTime;
srand(( unsigned ) time ( NULL ));
omp_set_num_threads( MAX_THREAD );
printf( "Entering non-OpenMP Stage...\n" );
stageStartTime=omp_get_wtime();
for( counter=omp_get_thread_num(); counter<100000000;
counter+=omp_get_num_threads())
element[counter]=rand()%100;
printf( "Exiting non-OpenMP Stage...:%.3fs\n\n",
omp_get_wtime()-stageStartTime );
printf( "Entering OpenMP Stage...\n" );
stageStartTime=omp_get_wtime();
#pragma omp parallel for private (counter)
for( counter=omp_get_thread_num(); counter<100000000;
counter+=omp_get_num_threads())
element[counter]=rand()%100;
printf( "Exiting OpenMP Stage...:%.3fs\n", omp_get_wtime()-stageStartTime );
return 0;
}
--
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◆ From: 140.136.144.13
※ 編輯: cywec 來自: 140.136.144.13 (04/28 18:45)
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