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Update GPU-aware MPI docs #949

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a8ee57a
Update GPU-aware MPI docs
abussy Apr 30, 2026
97e10d0
Update docs/src/usage.md
abussy Apr 30, 2026
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14 changes: 14 additions & 0 deletions docs/src/usage.md
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Expand Up @@ -115,6 +115,20 @@ your ROCm-aware MPI implementation to use multiple AMD GPUs (one GPU per rank).
If using OpenMPI, the status of ROCm support can be checked via the
[`MPI.has_rocm()`](@ref) function.

### Safe MPI communication
When using GPU-aware MPI (CUDA or ROCm), it is required to synchronize the (task-local) stream, before initiating MPI communication. Because GPU operations are asynchronous with respect to the host, GPU buffers may not be fully updated when a MPI call is issued. This can lead to race conditions and incorrect results.

With CUDA:
```
CUDA.synchronize()
MPI.Isend(my_CuArray, mpi_comm, dest, tag)
```
And with AMDGPU:
```
AMDGPU.synchronize()
MPI.Allreduce!(my_ROCArray, +, mpi_comm)
```

### Multiple GPUs per node

In a configuration with multiple GPUs per node, mapping GPU ID to node local MPI rank can be achieved either (1) on the application side using node-local communicator (`MPI.COMM_TYPE_SHARED`) or (2) on the system side setting device visibility accordingly.
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