MPI_Send 不适用于更高的缓冲区大小? [英] MPI_Send does not work with higher buffer size?

问题描述

当 MPI_Send 缓冲区大小为 100 时程序可以运行，但是当它为 1000 或更大时它会卡住.为什么?

When MPI_Send buffer size is 100 program works, but it stucks when it is 1000 or greater. Why?

if(id == 0){
    rgb_image = stbi_load(argv[1], &width, &height, &bpp, CHANNEL_NUM);

    for(int i = 0; i < size -1; i++)
        MPI_Send(rgb_image,1000,MPI_UINT8_T,i,0,MPI_COMM_WORLD);
        
}
uint8_t *part = (uint8_t*) malloc(sizeof(uint8_t)*(1000));

if(id != size-1 && size > 1)
    MPI_Recv(part,1000,MPI_UINT8_T,0,0,MPI_COMM_WORLD,MPI_STATUS_IGNORE); 

推荐答案

此程序无效 w.r.t.MPI 标准，因为没有匹配的接收(排名 0)

This program is not valid w.r.t. MPI Standard since there is no matching receive (on rank 0) for

MPI_Send(..., dest=0, ...)

MPI_Send() 允许阻塞，直到发布匹配的接收(并且通常发生在消息大"时)......并且所需的匹配接收永远不会被发布.

MPI_Send() is allowed to block until a matching receive is posted (and that generally happens when the message is "large") ... and the required matching receive never gets posted.

一个典型的解决方法是在 MPI_Send()(和 MPI_Send() 之前在 rank 0 上发出一个 MPI_Irecv(...,src = 0,...)code>MPI_Wait() 之后)，或者处理 0 ->0 与 MPI_Sendrecv() 通信.

A typical fix would be to issue a MPI_Irecv(...,src = 0,...) on rank 0 before the MPI_Send() (and MPI_Wait() after), or to handle 0 -> 0 communication with MPI_Sendrecv().

话虽如此，创建一个通信器可能会更有效，将所有等级减去最后一个，以及此通信器中的 MPI_Bcast().

That being said, it would likely more efficient to create a communicator will all the ranks minus the last one, and MPI_Bcast() in this communicator.

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