malloc in openmp for parallel loop

I am bit confused what is a better way to use malloc()/free() in openmp parallel for loop. Here are two ways I thought of but I am not aware of which method is better. I learned from previous answers that malloc/free in loop can fragment the memory.

Suppose I have a loop which runs over million times

for (size_t i = 0 ; i< 1000000; ++i){
    double * p = malloc(sizeof(double)*FIXED_SIZE); 

    /* FIXED_SIZE is some size constant 
    for the entire loop but is only determined dynamically */

    ....... /* Do some stuff using p array */

    free(p);
}

Now I want to parallelize the above loop with openmp

Method -1. simply adding a pragma on top of for loop

#pragma omp parallel for
for (size_t i = 0 ; i< 1000000; ++i){

    #pragma omp atomic
    double * p = malloc(sizeof(double)*FIXED_SIZE); 
    
    ....... /* Do some stuff using p array */

    #pragma omp atomic
    free(p);
}

Method -2. allocate a common array outside loop for each thread

int num_threads = omp_get_num_threads();
double * p = malloc(sizeof(double)*FIXED_SIZE * num_threads); 

#pragma omp parallel for
for (size_t i = 0 ; i< 1000000; ++i){

    int thread_num = omp_get_thread_num();

    double * p1 = p + FIXED_SIZE*thread_num ;
    
    ....... /* Do some stuff using p1 array */
}
free(p);

</details>


# 答案1
**得分**: 2

首先创建一个并行块，为每个线程分配资源，然后将线程拆分以执行并行循环。

```c
#pragma omp parallel
{
  double * p = malloc(sizeof(double) * FIXED_SIZE);

  #pragma omp for
  for (size_t i = 0; i < 1000000; ++i) { ... }

  free(p);
}

#pragma omp parallel
{
  double * p = malloc(sizeof(double)*FIXED_SIZE);

  #pragma omp for
  for (size_t i = 0 ; i< 1000000; ++i) { ... }

  free(p);
}