集体通讯：使用gather通讯

gather 函数基本上是反向的 scatter ，即手机所有进程发送向root进程的数据。 mpi4py 实现的 gather 函数如下： :

recvbuf = comm.gather(sendbuf, rank_of_root_process)

这里， sendbuf 是要发送的数据， rank_of_root_process 代表要接收数据进程。

如何做

from mpi4py import MPI
comm = MPI.COMM_WORLD
size = comm.Get_size()
rank = comm.Get_rank()
data = (rank+1)**2
data = comm.gather(data, root=0)
if rank == 0:
    print ("rank = %s " %rank + "...receiving data to other process")
    for i in range(1, size):
        data[i] = (i+1)**2
        value = data[i]
        print(" process %s receiving %s from process %s" % (rank , value , i))

最后，我们用5个进程来演示： :

C:\>mpiexec -n 5 python gather.py
rank = 0 ...receiving data to other process
process 0 receiving 4 from process 1
process 0 receiving 9 from process 2
process 0 receiving 16 from process 3
process 0 receiving 25 from process 4

结果正如图中一样，root进程收到了其他四个进程的数据。

讨论

首先，我们有n个进程发送各自的数据： :

data = (rank+1)**2

如果rank是0，就在array中收集数据： :

if rank == 0:
    print ("rank = %s " %rank + "...receiving data to other process")
    for i in range(1, size):
        data[i] = (i+1)**2
        value = data[i]
        print(" process %s receiving %s from process %s" % (rank , value , i)) 

数据由下面的函数产生: :

data = (rank+1)**2

了解更多

mpi4py 提供了下面的函数收集数据：

• gathering to one task: comm.Gather, comm.Gatherv, 和 comm.gather
• gathering to all tasks: comm.Allgather, comm.Allgatherv, 和 comm.allgather