在不使用 mpi4py 收集和分散的情况下检查所有等级是否为真

Question

我正在尝试在进程之间进行通信，以便在所有其他进程就绪时通知每个进程。下面的代码片段就是这样做的。有没有更优雅的方法来做到这一点？

def get_all_ready_status(ready_batch):
    all_ready= all(ready_batch)
    return [all_ready for _ in ready_batch]

ready_batch= comm.gather(ready_agent, root=0)
if rank == 0:
    all_ready_batch = get_all_ready_status(ready_batch)
all_ready_flag = comm.scatter(all_ready_batch , root=0)

Answer 1

如果所有进程都需要知道哪些其他进程已准备就绪，那么您可以使用 comm.Allgather 例程：

from mpi4py import MPI
import numpy


comm = MPI.COMM_WORLD
size = comm.Get_size()
rank = comm.Get_rank()

sendBuffer = numpy.ones(1, dtype=bool)
recvBuffer = numpy.zeros(size, dtype=bool)

print("Before Allgather => Process %s | sendBuffer %s | recvBuffer %s" % (rank, sendBuffer, recvBuffer))
comm.Allgather([sendBuffer,  MPI.BOOL],[recvBuffer, MPI.BOOL])
print("After Allgather  => Process %s | sendBuffer %s | recvBuffer %s" % (rank, sendBuffer, recvBuffer))

输出：

Before Allgather => Process 0 | sendBuffer [ True] | recvBuffer [False False]
Before Allgather => Process 1 | sendBuffer [ True] | recvBuffer [False False]
After Allgather  => Process 0 | sendBuffer [ True] | recvBuffer [ True  True]
After Allgather  => Process 1 | sendBuffer [ True] | recvBuffer [ True  True]

正如@Gilles Gouaillardet 在评论中指出的那样：

if all processes only have to know if all processes are ready, then MPI_Allreduce() is an even better fit.

理论上 Allreduce 应该比 Allgather 快因为前者可以使用树通信模式，并且因为它需要分配和通信更少的内存。可以找到更多信息 .

在您的情况下，您使用 MPI.LAND（即逻辑与）作为 Allreduce 运算符。

一个例子：

from mpi4py import MPI
import numpy


comm = MPI.COMM_WORLD
size = comm.Get_size()
rank = comm.Get_rank()

sendBuffer =  numpy.ones(1, dtype=bool) if rank % 2 ==  0 else numpy.zeros(1, dtype=bool)
recvBuffer = numpy.zeros(1, dtype=bool)

print("Before Allreduce => Process %s | sendBuffer %s | recvBuffer %s" % (rank, sendBuffer, recvBuffer))
comm.Allreduce([sendBuffer,  MPI.BOOL],[recvBuffer, MPI.BOOL], MPI.LAND)
print("After Allreduce  => Process %s | sendBuffer %s | recvBuffer %s" % (rank, sendBuffer, recvBuffer))

comm.Barrier()
if rank == 0:
   print("Second RUN")
comm.Barrier()

sendBuffer =  numpy.ones(1, dtype=bool)
recvBuffer = numpy.zeros(1, dtype=bool)

print("Before Allreduce => Process %s | sendBuffer %s | recvBuffer %s" % (rank, sendBuffer, recvBuffer))
comm.Allreduce([sendBuffer,  MPI.BOOL],[recvBuffer, MPI.BOOL], MPI.LAND)
print("After Allreduce  => Process %s | sendBuffer %s | recvBuffer %s" % (rank, sendBuffer, recvBuffer))

输出：

Before Allreduce => Process 1 | sendBuffer [False] | recvBuffer [False]
Before Allreduce => Process 0 | sendBuffer [ True] | recvBuffer [False]
After Allreduce  => Process 1 | sendBuffer [False] | recvBuffer [False]
After Allreduce  => Process 0 | sendBuffer [ True] | recvBuffer [False]
Second RUN
Before Allreduce => Process 0 | sendBuffer [ True] | recvBuffer [False]
Before Allreduce => Process 1 | sendBuffer [ True] | recvBuffer [False]
After Allreduce  => Process 0 | sendBuffer [ True] | recvBuffer [ True]
After Allreduce  => Process 1 | sendBuffer [ True] | recvBuffer [ True]

在输出的第一部分（即“Second 运行”之前的），结果为FALSE，因为排名为偶数的进程未就绪的地方（即 False）和奇数等级的进程就绪。因此，False & True => False。在第二部分，结果是 True 因为所有进程都准备好了。

在不使用 mpi4py 收集和分散的情况下检查所有等级是否为真

Checking all rank is true without using mpi4py gather and scatter

python

performance

parallel-processing

mpi

mpi4py