zero3: SDMA allgather via mori (sdma_allgather) (#7999)
## Summary
RFC: https://github.com/deepspeedai/DeepSpeed/issues/7884
Wire `sdma_allgather` into ZeRO-3's parameter prefetch path
(`_dist_allgather_fn`). When enabled, ZeRO-3 allgather routes through
`mori_cpp.AllGatherIntoTensor` (intra-node SDMA copy on AMD MI300), with
a
transparent fallback to `dist.allgather_fn` (RCCL/NCCL) on init failure.
End-to-end demo + repro steps + verified numbers live in
[`examples/sdma_allgather/README.md`](examples/sdma_allgather/README.md).
Headline (8x MI300X, DeepSpeed default ZeRO-3 buckets, 100 steps):
| | GPT-7B-ish | Qwen3-32B |
|---|---|---|
| SDMA off | 697.7 ms / step | 1402.5 ms / step |
| SDMA on | 622.0 ms / step | 1263.2 ms / step |
| **gain** | **+10.85 %** | **+9.93 %** |
Loss curves match off ↔ on, peak memory unchanged.
Speedup is workload-dependent — gains shrink (or invert) when allgather
can't be overlapped with compute
Co-authored-by: wuyl1 <yangwu@amd.com>
---------
Signed-off-by: wuyl1 <yangwu@amd.com>
Signed-off-by: inkcherry <mingzhi.liu@amd.com>
Co-authored-by: wuyl1 <yangwu@amd.com>
