Collective communication: many to one
Next step is to look into how to use collective communication to collect data from all tasks to a single task, i.e. how to move data from many to one.
Gather collects an equal amount of data from all the processes in a communicator to one process. One can think of it as an inverse scatter operation that allows one e.g. to collect partial results from the tasks.
Segments A, B, etc. may contain multiple elements just like in scatter.
An example of gathering a list of single values (
rank) from each process as
well as a numpy array of multiple elements (
data) that are then stored in a
larger receive array (
from mpi4py import MPI from numpy import arange, zeros comm = MPI.COMM_WORLD rank = comm.Get_rank() size = comm.Get_size() data = arange(10, dtype=float) * (rank + 1) buffer = zeros(size * 10, float) n = comm.gather(rank, root=0) # returns the value comm.Gather(data, buffer, root=0) # in-place modification
Reduce gathers data from all the processes in a communicator and applies an operation on the data before storing the result in a single process. Essentially, it is just like gather, but with an additional operation applied to the gathered data.
The operation that is applied on the data can be anything from a sum to a logical XOR and anything in between. MPI supports a wide variety of operations that can be used, including e.g. maximum value (MPI.MAX), minimum value (MPI.MIN), summation (MPI.SUM), and product (MPI.PROD).
An example of reduction using MPI.SUM to calculate the total sum of all received values:
from mpi4py import MPI from numpy import arange, empty comm = MPI.COMM_WORLD rank = comm.Get_rank() size = comm.Get_size() data = arange(10 * size, dtype=float) * (rank + 1) buffer = zeros(size * 10, float) n = comm.reduce(rank, op=MPI.SUM, root=0) # returns the value comm.Reduce(data, buffer, op=MPI.SUM, root=0) # in-place modification
Other common collective operations
- Scatterv (each process receives different amount of data)
- Gatherv (each process sends different amount of data)
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