Horovod core principles are based on the MPI concepts size, rank, local rank, allreduce, allgather, and broadcast. These are best explained by example. Say we launched a training script on 4 servers, each having 4 GPUs. If we launched one copy of the script per GPU:
Size would be the number of processes, in this case, 16.
Rank would be the unique process ID from 0 to 15 (size - 1).
Local rank would be the unique process ID within the server from 0 to 3.
Allreduce is an operation that aggregates data among multiple processes and distributes results back to them. Allreduce is used to average dense tensors. Here’s an illustration from the MPI Tutorial:
Allgather is an operation that gathers data from all processes on every process. Allgather is used to collect values of sparse tensors. Here’s an illustration from the MPI Tutorial:
Broadcast is an operation that broadcasts data from one process, identified by root rank, onto every other process. Here’s an illustration from the MPI Tutorial: