当我尝试发送2D整数数组时，为什么MPI_Send会阻塞？

我正在尝试使用mpi执行分形图片并行计算。我将程序分为4部分：

1. 平衡每个等级的行数
2. 对行的每一行属性进行计算
3. 发送行数和行数到等级0
4. 处理等级为0的数据（对于测试，只需打印int）

第1步和第2步正在工作，但是当我尝试将行发送到第0级时，程序正在停止并阻塞。我知道MPI_Send可以阻止bu，在这里没有理由。

这是第2步：

步骤1 ：

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

/* Include the MPI library for function calls */
#include <mpi.h>

/* Define tags for each MPI_Send()/MPI_Recv() pair so distinct messages can be
 * sent */
#define OTHER_N_ROWS_TAG 0
#define OTHER_PIXELS_TAG 1

int main(int argc, char **argv) {
  const int nRows = 513;
  const int nCols = 513;
  const int middleRow = 0.5 * (nRows - 1);
  const int middleCol = 0.5 * (nCols - 1);
  const double step = 0.00625;
  const int depth = 100;
  int pixels[nRows][nCols];
  int row;
  int col;
  double xCoord;
  double yCoord;
  int i;
  double x;
  double y;
  double tmp;
  int myRank;
  int nRanks;
  int evenSplit;
  int nRanksWith1Extra;
  int myRow0;
  int myNRows;
  int rank;
  int otherNRows;
  int otherPixels[nRows][nCols];

  /* Each rank sets up MPI */
  MPI_Init(&argc, &argv);

  /* Each rank determines its ID and the total number of ranks */
  MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
  MPI_Comm_size(MPI_COMM_WORLD, &nRanks);
  printf("My rank is %d \n",myRank);
  evenSplit = nRows / nRanks;
  nRanksWith1Extra = nRows % nRanks;

/*Each rank determine the number of rows that he will have to perform (well balanced)*/
  if (myRank < nRanksWith1Extra) {

    myNRows = evenSplit + 1;
    myRow0 = myRank * (evenSplit + 1);
  }
  else {
    myNRows = evenSplit;
    myRow0 = (nRanksWith1Extra * (evenSplit + 1)) +
      ((myRank - nRanksWith1Extra) * evenSplit);
  }
/*__________________________________________________________________________________*/

第2步 ：

/*_____________________PERFORM CALCUL ON EACH PIXEL________________________________ */
  for (row = myRow0; row < myRow0 + myNRows; row++) {

    /* Each rank loops over the columns in the given row */
    for (col = 0; col < nCols; col++) {

      /* Each rank sets the (x,y) coordinate for the pixel in the given row and 
       * column */
      xCoord = (col - middleCol) * step;
      yCoord = (row - middleRow) * step;

      /* Each rank calculates the number of iterations for the pixel in the
       * given row and column */
      i = 0;
      x = 0;
      y = 0;
      while ((x*x + y*y < 4) && (i < depth)) {
        tmp = x*x - y*y + xCoord;
        y = 2*x*y + yCoord;
        x = tmp;
        i++;
      }

      /* Each rank stores the number of iterations for the pixel in the given
       * row and column. The initial row is subtracted from the current row
       * so the array starts at 0 */
      pixels[row - myRow0][col] = i;
    }
      //printf("one row performed by %d \n",myRank);

  }
      printf("work done by %d \n",myRank);
/*_________________________________________________________________________________*/

第三步：

/*__________________________SEND DATA TO RANK 0____________________________________*/

  /* Each rank (including Rank 0) sends its number of rows to Rank 0 so Rank 0
   * can tell how many pixels to receive */
  MPI_Send(&myNRows, 1, MPI_INT, 0, OTHER_N_ROWS_TAG, MPI_COMM_WORLD);
  printf("test \n");
  /* Each rank (including Rank 0) sends its pixels array to Rank 0 so Rank 0
   * can print it */
  MPI_Send(&pixels, sizeof(int)*myNRows * nCols, MPI_BYTE, 0, OTHER_PIXELS_TAG,
      MPI_COMM_WORLD);
  printf("enter ranking 0 \n");
/*_________________________________________________________________________________*/

第四步：

/*________________________TREAT EACH ROW IN RANK 0_________________________________*/
  /* Only Rank 0 prints so the output is in order */
  if (myRank == 0) {

    /* Rank 0 loops over each rank so it can receive that rank's messages */
    for (rank = 0; rank < nRanks; rank++){

      /* Rank 0 receives the number of rows from the given rank so it knows how
       * many pixels to receive in the next message */
      MPI_Recv(&otherNRows, 1, MPI_INT, rank, OTHER_N_ROWS_TAG,
      MPI_COMM_WORLD, MPI_STATUS_IGNORE);

      /* Rank 0 receives the pixels array from each of the other ranks
       * (including itself) so it can print the number of iterations for each
       * pixel */
      MPI_Recv(&otherPixels, otherNRows * nCols, MPI_INT, rank,
          OTHER_PIXELS_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

      /* Rank 0 loops over the rows for the given rank */
      for (row = 0; row < otherNRows; row++) {

        /* Rank 0 loops over the columns within the given row */
        for (col = 0; col < nCols; col++) {

          /* Rank 0 prints the value of the pixel at the given row and column
           * followed by a comma */
          printf("%d,", otherPixels[row][col]);
        }

        /* In between rows, Rank 0 prints a newline character */
        printf("\n");
      }
    }
  }

  /* All processes clean up the MPI environment */
  MPI_Finalize();

  return 0;
}

我想了解它为什么会阻塞，您能解释一下吗？我是MPI的新用户，我不仅想拥有一个正在运行的程序，还想了解它。

先感谢您。