我正在尝试理解来自这里的一个OpenMP代码。下面是代码:
为了测量加速比,也就是串行版本与OpenMP版本之间的差异,我使用了time.h。你认为这个方法正确吗?
该程序在4核机器上运行。我指定
export OMP_NUM_THREADS="4"
,但没有看到明显的加速效果,通常只有1.2-1.7倍。我在并行化中遇到了哪些问题?我可以使用哪些调试/性能工具来查看性能损失?
代码(编译时使用xlc_r -qsmp=omp omp_workshare1.c -o omp_workshare1.exe
)
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#define CHUNKSIZE 1000000
#define N 100000000
int main (int argc, char *argv[])
{
int nthreads, tid, i, chunk;
float a[N], b[N], c[N];
unsigned long elapsed;
unsigned long elapsed_serial;
unsigned long elapsed_omp;
struct timeval start;
struct timeval stop;
chunk = CHUNKSIZE;
// ================= SERIAL start =======================
/* Some initializations */
for (i=0; i < N; i++)
a[i] = b[i] = i * 1.0;
gettimeofday(&start,NULL);
for (i=0; i<N; i++)
{
c[i] = a[i] + b[i];
//printf("Thread %d: c[%d]= %f\n",tid,i,c[i]);
}
gettimeofday(&stop,NULL);
elapsed = 1000000 * (stop.tv_sec - start.tv_sec);
elapsed += stop.tv_usec - start.tv_usec;
elapsed_serial = elapsed ;
printf (" \n Time SEQ= %lu microsecs\n", elapsed_serial);
// ================= SERIAL end =======================
// ================= OMP start =======================
/* Some initializations */
for (i=0; i < N; i++)
a[i] = b[i] = i * 1.0;
gettimeofday(&start,NULL);
#pragma omp parallel shared(a,b,c,nthreads,chunk) private(i,tid)
{
tid = omp_get_thread_num();
if (tid == 0)
{
nthreads = omp_get_num_threads();
printf("Number of threads = %d\n", nthreads);
}
//printf("Thread %d starting...\n",tid);
#pragma omp for schedule(static,chunk)
for (i=0; i<N; i++)
{
c[i] = a[i] + b[i];
//printf("Thread %d: c[%d]= %f\n",tid,i,c[i]);
}
} /* end of parallel section */
gettimeofday(&stop,NULL);
elapsed = 1000000 * (stop.tv_sec - start.tv_sec);
elapsed += stop.tv_usec - start.tv_usec;
elapsed_omp = elapsed ;
printf (" \n Time OMP= %lu microsecs\n", elapsed_omp);
// ================= OMP end =======================
printf (" \n speedup= %f \n\n", ((float) elapsed_serial) / ((float) elapsed_omp)) ;
}