NCCL源码解析②：Bootstrap网络连接的建立

作者｜KIDGINBROOK
更新｜潘丽晨

上次介绍到rank0的机器生成了ncclUniqueId，并完成了机器的bootstrap网络和通信网络的初始化，这节接着看下所有节点间bootstrap的连接是如何建立的。

rank0节点执行ncclGetUniqueId生成ncclUniqueId，通过mpi将Id广播到所有节点，然后所有节点都会执行ncclCommInitRank，这里其他节点也会进行初始化bootstrap网络和通信网络的操作，然后会执行到ncclCommInitRankSync。

ncclResult_t ncclCommInitRankSync(ncclComm_t* newcomm, int nranks, ncclUniqueId commId, int myrank, int cudaDev) {
  ncclResult_t res;
 
  CUDACHECK(cudaSetDevice(cudaDev));
  NCCLCHECKGOTO(commAlloc(newcomm, nranks, myrank), res, cleanup);
  NCCLCHECKGOTO(initTransportsRank(*newcomm, &commId), res, cleanup);
  NCCLCHECKGOTO(devCommSetup(*newcomm), res, cleanup);
 
  INFO(NCCL_INIT,"comm %p rank %d nranks %d cudaDev %d busId %x - Init COMPLETE", *newcomm, myrank, nranks, (*newcomm)->cudaDev, (*newcomm)->busId);
 
  return ncclSuccess;
cleanup:
  if ((*newcomm) && (*newcomm)->bootstrap) bootstrapAbort((*newcomm)->bootstrap);
  *newcomm = NULL;
  return res;
}

ncclComm_t是指向ncclComm的指针，ncclComm是一个大杂烩，包含了通信用到的所有上下文信息，里面的字段等用到的时候再介绍，然后通过commAlloc分配newcom，并且完成初始化，比如当前是哪个卡，对应的pcie busid是什么，然后执行initTransportsRank。

static ncclResult_t initTransportsRank(struct ncclComm* comm, ncclUniqueId* commId) {
  // We use 3 AllGathers
  // 1. { peerInfo, comm }
  // 2. ConnectTransport[nranks], ConnectValue[nranks]
  // 3. { nThreads, nrings, compCap, prev[MAXCHANNELS], next[MAXCHANNELS] }
 
  int rank = comm->rank;
  int nranks = comm->nRanks;
  uint64_t commHash = getHash(commId->internal, NCCL_UNIQUE_ID_BYTES);
  TRACE(NCCL_INIT, "comm %p, commHash %lx, rank %d nranks %d - BEGIN", comm, commHash, rank, nranks);
  NCCLCHECK(bootstrapInit(commId, rank, nranks, &comm->bootstrap));
 
  // AllGather1 - begin
  struct {
    struct ncclPeerInfo peerInfo;
    struct ncclComm* comm;
  } *allGather1Data;
 
  NCCLCHECK(ncclCalloc(&allGather1Data, nranks));
  allGather1Data[rank].comm = comm;
  struct ncclPeerInfo* myInfo = &allGather1Data[rank].peerInfo;
  NCCLCHECK(fillInfo(comm, myInfo, commHash));
  NCCLCHECK(bootstrapAllGather(comm->bootstrap, allGather1Data, sizeof(*allGather1Data)));
 
  NCCLCHECK(ncclCalloc(&comm->peerInfo, nranks+1)); // Extra rank to represent CollNet root
  for (int i = 0; i < nranks; i++) {
    memcpy(comm->peerInfo+i, &allGather1Data[i].peerInfo, sizeof(struct ncclPeerInfo));
    if ((i != rank) && (comm->peerInfo[i].hostHash == myInfo->hostHash) && (comm->peerInfo[i].busId == myInfo->busId)) {
      WARN("Duplicate GPU d