[README] Tips on using less than 8 MI300 GPUs (#1270)

Signed-off-by: nileshnegi <Nilesh.Negi@amd.com>

[ROCm/rccl commit: a2474846f5]

projects/rccl/README.md

@@ -148,6 +148,17 @@ pip3 install -r sphinx/requirements.txt
 python3 -m sphinx -T -E -b html -d _build/doctrees -D language=en . _build/html
 ```
 
+### Improving performance on MI300 when using less than 8 GPUs
+
+On a system with 8\*MI300X GPUs, each pair of GPUs are connected with dedicated XGMI links in a fully-connected topology. So, for collective operations, one can achieve good performance when all 8 GPUs (and all XGMI links) are used. When using less than 8 GPUs, one can only achieve a fraction of the potential bandwidth on the system.
+
+But, if your workload warrants using less than 8 MI300 GPUs on a system, you can set the run-time variable `NCCL_MIN_NCHANNELS` to increase the number of channels.\
+E.g.: `export NCCL_MIN_NCHANNELS=32`
+
+Increasing the number of channels can be beneficial to performance, but it also increases GPU utilization for collective operations.
+
+Additionally, we have pre-defined higher number of channels when using only 2 GPUs or 4 GPUs on a 8\*MI300 system. Here, RCCL will use **32 channels** for the 2 MI300 GPUs scenario and **24 channels** for the 4 MI300 GPUs scenario.
+
 ## Copyright
 
 All source code and accompanying documentation is copyright (c) 2015-2022, NVIDIA CORPORATION. All rights reserved.