TMS320C6678用OpenMP多核开发时候的额外开销有哪些？为什么会出现多核用时比单核多的问题？

#include <ti/omp/omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <c6x.h>
void test()
{
int a = 0;
for (int j = 0; j<100000; j++)
a++;
}
void main()
{
int i;
printf("starting running\n");
unsigned long long t1,t2;
TSCL = 0;
TSCH = 0;
t1 = _itoll(TSCH,TSCL);

#pragma omp parallel for private(i)
for ( i = 0; i<100; i++){
test();
}
t2 = _itoll(TSCH,TSCL);
printf("time is: %ld\n",(t2-t1)/10000);
}

问题一：当我跑这个Demo的时候，单核用时0.21s，四核用时0.17s，八核用时0.14s，从时间效率上看，按理说是按倍数减少，请问下有什么额外的开销使得出现这样的结果。

问题二：当我跑我自己的程序的时候，单核时间时0.8s，四核是0.9s，八核是0.3s，请问下在这里为什么四核的时间效率反而还比单核还长？？？是因为调用多核产生了其他额外的开销还是什么原因？？

yuanchao gan：

麻烦TI的员工帮忙解答一下，谢谢！

Wesley He：

回复 yuanchao gan:

你好，

问题解决了吗？该问题已经反馈给相关专家，有什么更新的话，我们会及时通知您。

谢谢。

Wesley He：

回复 yuanchao gan:

你好，

The execution time of a OpenMP program depends on the "parallel" to "serial" percentage as well as the overhead from the inter processor communication for running multi-thread. Accordingly, users need to balance the parallelism percentage and the overhead from multi-threads to achieve optimal performance.TI OpenMP Runtime 2.2 implements support for the OpenMP 3.0 API specification, hence the resources in openmp.org should be helpful to customer's proprietary algorithm – www.openmp.org/…/ .

yuanchao gan：

回复 Wesley He:

问题还没解决