Question about SingleFlow.prev in ring all-reduce

[GoodFrost]

In the first round of Ring All-Reduce, why is the prev field of SingleFlow set to the previous rank?
Isn't the first-round flow supposed to be initiated independently by each rank?
Why does it need a prev rank?

Code is in MockNcclGroup.cc: line 1168.

if(rank_it->second[0]!=-1){
  prevranks={rank_it->second[0]};
}

[tianhao909]

Hello, regarding the two scenarios—whether the first-round flow should be initiated independently by each rank, and whether the prev field of SingleFlow should be set to the previous rank—it might be helpful to compare the implementation in the NCCL source code to see which approach NCCL adopts.

Then, maybe can attempt to conduct a comparative verification to determine which of the two schemes results in smaller errors in terms of precision in real-world scenarios.