MPI I / O，单进程和多进程输出的混合

问题描述

我需要一个MPI C代码，通过MPI I / O将数据写入二进制文件。我需要进程0来编写一个短标题，然后我需要整个进程范围来编写标题所指示的自己的数组。然后我需要进程0来编写另一个头，然后是所有进程编写下一个数组的部分，等等。我想出了以下测试代码，它实际上做了我想要的。没有人比我更惊讶。

I need an MPI C code to write data to a binary file via MPI I/O. I need process 0 to write a short header, then I need the whole range of processes to write their own pieces of the array indicated by the header. Then I need process 0 to write another header, followed by all processes writing their pieces of the next array, etc. I came up with the following test code which actually does what I want. No one will be more surprised about that than me.

我的问题是，我是MPI I / O的新手。所以我得到它？我这样做是正确的方式还是有更高效或更紧凑的方式来做到这一点？

My question is, I am new at MPI I/O. So am I "getting it"? Am I doing this the "right way" or is there some more efficient or compact way to do it?

代码是:(顺便说一句，如果你想测试一下，只试试4次触发。）

Code is: (BTW, if you think of testing this, try it with 4 procs only.)

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

#include "mpi.h"

#define ROWS 9
#define COLS 10

int main(int argc, char *argv[]) {

   int size_mpi, rank_mpi, row_mpi, col_mpi;
   int i,j,p,ttlcols;
   int sizes[]= {2*ROWS,2*COLS};
   int subsizes[]= {ROWS,COLS};
   int starts[] = {0,0};
   int vals[ROWS][COLS];
   char hdr[] = "This is just a header.\n";
   MPI_Status stat_mpi;
   MPI_Datatype subarray;
   MPI_File fh;
   MPI_Offset offset, end_of_hdr;
   MPI_Info info_mpi;

   MPI_Init(&argc, &argv);
   MPI_Comm_size(MPI_COMM_WORLD,&size_mpi);
   MPI_Comm_rank(MPI_COMM_WORLD,&rank_mpi);

   ttlcols = 2*COLS;
   /* Where are we in the array of processes? */
   col_mpi = rank_mpi%2;
   row_mpi = rank_mpi/2;
   /* Populate the array */
   for (j=0; j<ROWS; j++){
      for (i=0; i<COLS; i++){
         vals[j][i] = ttlcols*(ROWS*row_mpi + j) +
                      COLS*col_mpi + i;
      }
   }
   /* MPI derived datatype for setting a file view */
   starts[0] = row_mpi*ROWS;
   starts[1] = col_mpi*COLS;
   MPI_Type_create_subarray(2, sizes, subsizes, starts,
                            MPI_ORDER_C, MPI_INT,
                            &subarray);
   MPI_Type_commit(&subarray);
   /* open the file */
   printf("opening file\n");
   MPI_File_open(MPI_COMM_WORLD, "arrdata.dat",
                 MPI_MODE_WRONLY | MPI_MODE_CREATE,
                 MPI_INFO_NULL, &fh);
   printf("opened file\n");
   /* set the initial file view */
   MPI_File_set_view(fh, 0, MPI_CHAR, MPI_CHAR, "native", MPI_INFO_NULL);
   /* proc 0 writes first header */
   if (rank_mpi == 0) {
      MPI_File_write(fh, (void*)hdr, strlen(hdr), MPI_CHAR, &stat_mpi);
      MPI_File_get_position(fh, &offset);
      MPI_File_get_byte_offset(fh, offset, &end_of_hdr);
   }
   /* everybody has to know where proc 0 stopped writing */
   MPI_Bcast((void*)&end_of_hdr, 1, MPI_INT, 0, MPI_COMM_WORLD);
   /* re-set file view for writing first array */
   MPI_File_set_view(fh, end_of_hdr, MPI_INT,
                     subarray, "native",
                     MPI_INFO_NULL);
   /* and write the array */
   MPI_File_write(fh, (void*)vals, ROWS*COLS, MPI_INT,
                  &stat_mpi);

   /* now go through the whole thing again to test */
   MPI_File_get_position(fh, &offset);
   MPI_File_get_byte_offset(fh, offset, &end_of_hdr);
   MPI_File_set_view(fh, end_of_hdr, MPI_CHAR, MPI_CHAR, "native", MPI_INFO_NULL);
   if (rank_mpi == 0) {
      MPI_File_write(fh, (void*)hdr, strlen(hdr), MPI_CHAR, &stat_mpi);
      MPI_File_get_position(fh, &offset);
      MPI_File_get_byte_offset(fh, offset, &end_of_hdr);
   }

   MPI_Bcast((void*)&end_of_hdr, 1, MPI_INT, 0, MPI_COMM_WORLD);

   MPI_File_set_view(fh, end_of_hdr, MPI_INT,
                     subarray, "native",
                     MPI_INFO_NULL);
   MPI_File_write(fh, (void*)vals, ROWS*COLS, MPI_INT,
                  &stat_mpi);
   MPI_File_close(&fh);

   MPI_Finalize();

   return 0;

}

推荐答案

您的方法很好，如果你现在需要一些东西将比特放在一个文件中，请继续自称完成。

Your approach is fine and if you need something right now to put bits in a file, go ahead and call yourself done.

以下是一些提高效率的建议：

Here are some suggestions for more efficiency:

• 您可以查询状态对象，了解写入的字节数，而不是获取位置并转换为字节。

• You can consult the status object for how many bytes were written, instead of getting the position and translating into bytes.

如果你有内存在写入之前保存所有数据，你可以用MPI数据类型来描述你的I / O（诚然，这可能最终会让你很难创建） ）。然后所有进程都会发出一个集体调用。

If you have the memory to hold all the data before you write, you could describe your I/O with an MPI datatype (admittedly, one that might end up being a pain to create). Then all processes would issue a single collective call.

您应该使用集体I / O而不是独立的I / O. 质量库应该能够为您提供相同甚至更好的性能（如果没有，您可以提出MPI实现的问题）。

You should use collective I/O instead of independent I/O. A "quality library" should be able to give you equal if not better performance (and if not, you could raise the issue with your MPI implementation).

如果进程有不同的数据量要写，MPI_EXSCAN是收集谁拥有哪些数据的好方法。然后你可以调用MPI_FILE_WRITE_AT_ALL到文件中的正确偏移量。

If the processes have different amounts of data to write, MPI_EXSCAN is a good way to collect who has what data. Then you can call MPI_FILE_WRITE_AT_ALL to the correct offset in the file.

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