I am trying to communicate between processes so that every processes are notified when all other processes are ready. The code snippet below does that. Is there a more elegant way to do this?
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)
If all the processes need to be aware which other processes are ready then you can use the comm.Allgather
routine:
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))
Output:
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]
As pointed out in the comments by @Gilles Gouaillardet:
if all processes only have to know if all processes are ready, then MPI_Allreduce() is an even better fit.
The idea is that in theory Allreduce should be faster then Allgather
because the former can use a tree communication pattern and because it will require to allocate and communicate less memory. More information can be found here .
In your case, you use MPI.LAND
(ie, logical and) as the Allreduce operation operator.
An example:
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))
The Output:
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]
In the first part of the output ( ie, before "Second RUN"), the result is FALSE
because the processes with even rank where not ready (ie, False
) and the processes with odd rank where ready. Hence, False & True => False
. In the second part, the result is True
because all processes were ready.
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