并行程序效率太低，高手帮忙看看！

bingxingbingxing163 2009-12-09 04:19:05

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下面是一个搜索最大数的程序，但并行子函数运行时间大约是串行函数的两倍，大家帮忙看看原因是什么？

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#include <omp.h>

#define CACHE_LINE_LEN 64
#define data(x) data[x]
#define pnMax(x) pnMax[x]
int *data;

void randData(int count)
{
int i;

data = (int *)malloc(sizeof(int)*count);
srand((unsigned)time(NULL));
for(i=0; i<count; i++)
{
data(i) = (int)(rand()%1000);
}
}

int Parallel_SearchMaxData(int *data,int nLen)
{
int i,k;
int *pnMax;
int pos;
int nCore,nStep,begin,end;
int nMax;

nCore = omp_get_num_procs();
pnMax = (int *)malloc(nCore*(sizeof(int)+CACHE_LINE_LEN));

if(pnMax == NULL)
{
return -1;
}

nStep = nLen/nCore;

#pragma omp parallel for
for(k=0; k<nCore; k++)
{
begin = k*nStep;
end = (k+1)*nStep-1;
if(k == nCore-1)
{
end = nLen-1;
}

nMax = data(begin);
for(i=begin+1; i<=end; i++)
{
if(data(i) > nMax)
{
nMax = data(i);
}
}
pos = k*(sizeof(int)+CACHE_LINE_LEN)/sizeof(int);
pnMax[pos] = nMax;
}

nMax = pnMax(0);
for(i=1; i<nCore; i++)
{
pos = i*(sizeof(int)+CACHE_LINE_LEN)/sizeof(int);
if(pnMax(pos) > nMax)
{
nMax = pnMax(pos);
}
}

free(pnMax);
return nMax;
}

int SearchMaxData(int *data, int nLen)
{
int i;
int nMax;

nMax = 0;
for(i=1; i<nLen; i++)
{
if(data(i) > data(nMax))
{
nMax = i;
}
}

nMax = data(nMax);

return nMax;
}

void main(int argc,char **argv)
{
int count;
int rank1,rank2;
clock_t t1,t2,t3,t4;

printf("Input datasize:\n");
scanf("%d",&count);
randData(count);

t1 = clock();
rank1 = Parallel_SearchMaxData(data,count);
t2 = clock();
printf("Result of parallel searching:\n");
printf("The max data is:%d\n",rank1);
printf("Searching time is:%d\n",t2-t1);

printf("\n");
t3 = clock();
rank2 = SearchMaxData(data,count);
t4 = clock();
printf("Result of serial searching:\n");
printf("The max data is:%d\n",rank2);
printf("Searching time is:%d\n",t4-t3);
}

Input datasize:
100000000
Result of parallel searching:
The max data is:999
Searching time is:1016

Result of serial searching:
The max data is:999
Searching time is:719
请按任意键继续. . .

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