我正在一个执行任务的循环中创建MPI组，但是当我想要释放组时，计算中止。我应该何时释放组？
我得到的错误是：

[KLArch:13617] *** An error occurred in MPI_Comm_free
[KLArch:13617] *** reported by process [1712324609,2]
[KLArch:13617] *** on communicator MPI_COMM_WORLD
[KLArch:13617] *** MPI_ERR_COMM: invalid communicator
[KLArch:13617] *** MPI_ERRORS_ARE_FATAL (processes in this communicator will now abort,
[KLArch:13617] ***    and potentially your MPI job)
[KLArch:13611] 2 more processes have sent help message help-mpi-errors.txt / mpi_errors_are_fatal
[KLArch:13611] Set MCA parameter "orte_base_help_aggregate" to 0 to see all help / error messages

我打算做的是并行执行一个进程数量不断增加的计算，作为MPI的基准，即只用1个进程执行整个计算，花点时间，用2个进程运行相同的计算，花点时间，用4个进程运行相同的计算，花点时间......然后比较问题如何随进程数量而扩展。
MWE：

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>

int main(int argc, char *argv[])
{
    int j = 0;
    int size = 0;
    int rank = 0;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    // Get the group of processes in MPI_COMM_WORLD
    MPI_Group world_group;
    MPI_Comm_group(MPI_COMM_WORLD, &world_group);

    // Construct a group containing all of the ranks smaller than i in MPI_COMM_WORLD
    for (i = 1; i <= size; i++)
    {
        int group_ranks[i];
        for (int j = 0; j < i; j++)
        {
            group_ranks[j] = j;
        }

        // Construct a group with all the ranks smaller than i
        MPI_Group sub_group;
        MPI_Group_incl(world_group, i, group_ranks, &sub_group);

        // Create a communicator based on the group
        MPI_Comm sub_comm;
        MPI_Comm_create(MPI_COMM_WORLD, sub_group, &sub_comm);

        int sub_rank = -1;
        int sub_size = -1;
        // If this rank isn't in the new communicator, it will be
        // MPI_COMM_NULL. Using MPI_COMM_NULL for MPI_Comm_rank or
        // MPI_Comm_size is erroneous
        if (MPI_COMM_NULL != sub_comm)
        {
            MPI_Comm_rank(sub_comm, &sub_rank);
            MPI_Comm_size(sub_comm, &sub_size);
        }

        // Do some work
        printf("WORLD RANK/SIZE: %d/%d \t Group RANK/SIZE: %d/%d\n",
               rank, size, sub_rank, sub_size);

        // Free the communicator and group
        MPI_Barrier(MPI_COMM_WORLD);
        //MPI_Comm_free(&sub_comm);
        //MPI_Group_free(&sub_group);
        j = 0;
    }
    MPI_Group_free(&world_group);

    MPI_Finalize();
    return 0;
}

如果我在循环结束时取消注解MPI_Comm_free(&sub_comm);和MPI_Group_free(&sub_group);，代码将抛出错误。