当我使用“#pragma omp parallel num_threads(4)”时,为什么我没有得到不同的线程ID。在这种情况下,所有线程ID都为0。 但是当我评论该行并使用默认的线程数时,我得到了不同的线程ID。 注意: - 变量我使用变量tid来获取线程ID。
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char *argv[])
{
int nthreads, tid;
int x = 0;
#pragma omp parallel num_threads(4)
#pragma omp parallel private(nthreads, tid)
{
/* Obtain thread number */
tid = omp_get_thread_num();
printf("Hello World from thread = %d\n", tid);
// /* Only master thread does this */
if (tid == 0)
{
nthreads = omp_get_num_threads();
printf("Number of threads = %d\n", nthreads);
}
}
}
上述代码的输出: -
Hello World from thread = 0
Hello World from thread = 0
Number of threads = 1
Hello World from thread = 0
Number of threads = 1
Hello World from thread = 0
Number of threads = 1
Number of threads = 1
当我评论上述行时的输出: -
Hello World from thread = 3
Hello World from thread = 0
Number of threads = 4
Hello World from thread = 1
Hello World from thread = 2
答案 0 :(得分:14)
您正在创建两个嵌套的并行区域。这与此相同:
#pragma omp parallel num_threads(4)
{
#pragma omp parallel private(nthreads, tid)
{
/* Obtain thread number */
tid = omp_get_thread_num();
printf("Hello World from thread = %d\n", tid);
// /* Only master thread does this */
if (tid == 0)
{
nthreads = omp_get_num_threads();
printf("Number of threads = %d\n", nthreads);
}
}
}
omp_get_num_threads()
返回最里面区域中的线程数。所以你正在执行四个线程,每个线程都在执行一个线程。
内部并行区域仅执行一个线程,因为您尚未启用嵌套并行性。您可以致电omp_set_nested(1)
启用它。
http://docs.oracle.com/cd/E19205-01/819-5270/aewbi/index.html
如果不想创建两个嵌套的并行区域,而是想要创建一个并行区域并指定两个属性,则可以执行以下操作:
#pragma omp parallel num_threads(4) private(nthreads,tid)
{
.
.
.
}
答案 1 :(得分:0)
也可以通过将环境变量OMP_NESTED设置为true来启用嵌套