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 detected : rank %d and rank %d both on CUDA device %x", rank, i, myInfo->busId);
      return ncclInvalidUsage;
    }
  }

看下bootstrapInit：

ncclResult_t bootstrapInit(ncclUniqueId * id, int rank, int nranks, void** commState) {
  ncclNetHandle_t* netHandle = (ncclNetHandle_t*) id;
  bool idFromEnv = getenv("NCCL_COMM_ID") != NULL;
  struct extState* state;
  NCCLCHECK(ncclCalloc(&state, 1));
  state->rank = rank;
  state->nranks = nranks;
  *commState = state;
 
  TRACE(NCCL_INIT, "rank %d nranks %d", rank, nranks);
 
  struct extInfo info = { 0 };
  info.rank = rank;
  info.nranks = nranks;
  void *tmpSendComm, *tmpRecvComm;
  // Pass the remote address to listen via info
  if (idFromEnv) {
    memcpy(&info.extHandleListen, netHandle, sizeof(ncclNetHandle_t));
    memcpy(&info.extHandleListenRoot, netHandle, sizeof(ncclNetHandle_t));
  }
  // listen will return the local address via info (specify interface type 'findSubnetIf')
  state->dev = idFromEnv ? findSubnetIf : 0;
  void* extBstrapListenCommRoot;
  NCCLCHECK(bootstrapNetListen(state->dev, &info.extHandleListen, &state->extBstrapListenComm));
  NCCLCHECK(bootstrapNetListen(state->dev, &info.extHandleListenRoot, &extBstrapListenCommRoot));
 
  // stagger connection times to avoid an overload of the root at very high rank counts
  if (nranks > 128) {
    long msec = rank;
    struct timespec tv;
    tv.tv_sec = msec / 1000;
    tv.tv_nsec = 1000000 * (msec % 1000);
    TRACE(NCCL_INIT, "rank %d delaying connection to root by %ld msec", rank, msec);
    (void) nanosleep(&tv, NULL);
  }
 
  // send info on my listening socket to root
  NCCLCHECK(bootstrapNetConnect(state->dev, netHandle, &tmpSendComm));
  NCCLCHECK(bootstrapNetSend(tmpSendComm, &info, sizeof(info)));
  NCCLCHECK(bootstrapNetCloseSend(tmpSendComm));
 
  // get info on my "next" rank in the bootstrap ring from root
}

首先看下commState，即ncclComm的bootstrap，类型为extState。

struct extState {
  void* extBstrapListenComm;
  void* extBstrapRingRecvComm;
  void* extBstrapRingSendComm;
  ncclNetHandle_t* peerBstrapHandles;
  struct unexConn* unexpectedConnections;
  int rank;
  int nranks;
  int dev;
};

其中extBstrapRingSendComm是当前节点连接next的socket连接，extBstrapRingRecvComm是当前节点和prev节点的socket连接，extBstrapListenComm是当前节点的监听socket，peerBstrapHandles是所有rank的ip port（对应extBstrapListenComm），dev默认为0，表示用第几个ip地址。

然后通过bootstrapNetListen创建extHandleListen和extHandleListenRoot两个bootstrap comm，如前文所述，bootstrap comm其实就是保存了fd，这里创建两个comm的原因是extHandleListen是rank之间实际使用的bootstrap连接，extHandleListenRoot是rank0节点和其他所有rank进行通信使用的连接。

static ncclResult_t bootstrapNetListen(int dev, ncclNetHandle_t* netHandle, void** listenComm)

bootstrapNetListen函数上节有介绍过，会获取到第dev个当前机器的ip，然后listen获取监听fd，将ip port写到nethandle，获取到的bootstrap comm写到listencomm。

然后将rank，nrank，extHandleListen和extHandleListenRoot写到extInfo里。

struct extInfo {
  int rank;
  int nranks;
  ncclNetHandle_t extHandleListenRoot;
  ncclNetHandle_t extHandleListen;
};

netHandle为ncclUniqueId，即rank0的ip port，然后通过bootstrapNetConnect创建bootstrap send comm，类比bootstrapNetListen，bootstrapNetConnect就是建立到netHandle的socket连接，将socket写到sendComm里，这里dev并没有用到。

static ncclResult_t bootstrapNetConnect(int dev, ncclNetHandle_t* netHandle, void** sendComm)

然后通过bootstrapNetSend将extInfo发送出去，即发给rank0：

static ncclResult_t bootstrapNetSend(void* sendComm, void* data, int size) {
  struct bootstrapNetComm* comm = (struct bootstrapNetComm*)sendComm;
  NCCLCHECK(socketSend(comm->fd, &size, sizeof(int)));
  NCCLCHECK(socketSend(comm->fd, data, size));
  return ncclSuccess;
}

其中socketSend就是执行send接口发送数据。

然后通过bootstrapNetCloseSend关闭fd。

rank0收到数据后会做什么工作呢，回顾一下，rank0的节执行ncclGetUniqueId生成ncclUniqueId，其中在执行bootstrapCreateRoot的最后会启动一个线程执行bootstrapRoot。

static void *bootstrapRoot(void* listenComm) {
  struct extInfo info;
  ncclNetHandle_t *rankHandles = NULL;
  ncclNetHandle_t *rankHandlesRoot = NULL; // for initial rank <-> root information exchange
  ncclNetHandle_t zero = { 0 }; // for sanity checking
  void* tmpComm;
  ncclResult_t res;
  setFilesLimit();
 
  TRACE(NCCL_INIT, "BEGIN");
  /* Receive addresses from all ranks */
  int nranks = 0, c = 0;
  do {
    NCCLCHECKGOTO(bootstrapNetAccept(listenComm, &tmpComm), res, out);
    NCCLCHECKGOTO(bootstrapNetRecv(tmpComm, &info, sizeof(info)), res, out);
    NCCLCHECKGOTO(bootstrapNetCloseRecv(tmpComm), res, out);
 
    if (c == 0) {
      nranks = info.nranks;
      NCCLCHECKGOTO(ncclCalloc(&rankHandles, nranks), res, out);
      NCCLCHECKGOTO(ncclCalloc(&rankHandlesRoot, nranks), res, out);
    }
 
    if (nranks != info.nranks) {
      WARN("Bootstrap Root : mismatch in rank count from procs %d : %d", nranks, info.nranks);
      goto out;
    }
 
    if (memcmp(&zero, &rankHandlesRoot[info.rank], sizeof(ncclNetHandle_t)) != 0) {
      WARN("Bootstrap Root : rank %d of %d ranks has already checked in", info.rank, nranks);
      goto out;
    }
 
    // Save the connection handle for that rank
    memcpy(rankHandlesRoot+info.rank, info.extHandleListenRoot, sizeof(ncclNetHandle_t));
    memcpy(rankHandles+info.rank, info.extHandleListen, sizeof(ncclNetHandle_t));
 
    ++c;
    TRACE(NCCL_INIT, "Received connect from rank %d total %d/%d",  info.rank, c, nranks);
  } while (c < nranks);
  TRACE(NCCL_INIT, "COLLECTED ALL %d HANDLES", nranks);
 
  // Send the connect handle for the next rank in the AllGather ring
  for (int r=0; r

listenComm是上一个博文中rank0创建的监听fd，bootstrapNetAccept是从listenComm中获取一个新连接，使用新连接的fd创建recvcomm。

static ncclResult_t bootstrapNetAccept(void* listenComm, void** recvComm)

然后通过bootstrapNetRecv读取tmpComm的数据，即其他rank发送来的extInfo，然后保存其他rank的extHandleListen和extHandleListenRoot，这个时候rank0就获取到其他所有rank的ip和port了。

获取完所有rank的info之后开始建环，将节点(r+1) % nranks的extHandleListen发送给节点r，就是说将节点r的next节点的nethandle发送给节点r。这里可以看出，每个节点创建了两个listen comm，其中rank0使用extHandleListenRoot进行通信，其他节点之间通过extHandleListen进行通信。

然后再回去接着看bootstrapInit。

ncclResult_t bootstrapInit(ncclUniqueId * id, int rank, int nranks, void** commState) {
  // get info on my "next" rank in the bootstrap ring from root
  ncclNetHandle_t extHandleNext;
  NCCLCHECK(bootstrapNetAccept(extBstrapListenCommRoot, &tmpRecvComm));
  NCCLCHECK(bootstrapNetRecv(tmpRecvComm, &extHandleNext, sizeof(extHandleNext)));
  NCCLCHECK(bootstrapNetCloseRecv(tmpRecvComm));
  NCCLCHECK(bootstrapNetCloseListen(extBstrapListenCommRoot));
 
  NCCLCHECK(bootstrapNetConnect(state->dev, &extHandleNext, &state->extBstrapRingSendComm));
  // Accept the connect request from the previous rank in the AllGather ring
  NCCLCHECK(bootstrapNetAccept(state->extBstrapListenComm, &state->extBstrapRingRecvComm));
 
  // AllGather all listen handlers
  NCCLCHECK(ncclCalloc(&state->peerBstrapHandles, nranks));
  memcpy(state->peerBstrapHandles+rank, info.extHandleListen, sizeof(ncclNetHandle_t));
  NCCLCHECK(bootstrapAllGather(state, state->peerBstrapHandles, sizeof(ncclNetHandle_t)));
 
  TRACE(NCCL_INIT, "rank %d nranks %d - DONE", rank, nranks);
 
  return ncclSuccess;
}

接着所有rank都会在extHandleListenRoot上接收新连接创建tmpRecvComm，然后接收到当前rank的next的ip，port；然后连接next创建bscomm到state->extBstrapRingSendComm，接收prev的连接创建bscomm到state->extBstrapRingRecvComm，到现在bootstrap网络连接就完全建立起来了，如下图：

最后gather所有rank的ip port，首先将自己的nethandle放到peerBstrapHandles的对应位置，如下所示。

然后执行bootstrapAllGather：

ncclResult_t bootstrapAllGather(void* commState, void* allData, int size) {
  struct extState* state = (struct extState*)commState;
  char* data = (char*)allData;
  int rank = state->rank;
  int nranks = state->nranks;
 
  TRACE(NCCL_INIT, "rank %d nranks %d size %d", rank, nranks, size);
 
  /* Simple ring based AllGather
   * At each step i receive data from (rank-i-1) from left
   * and send previous step's data from (rank-i) to right
   */
  for (int i=0; iextBstrapRingSendComm, data+sslice*size, size));
    // Recv slice from the left
    NCCLCHECK(bootstrapNetRecv(state->extBstrapRingRecvComm, data+rslice*size, size));
  }
 
  TRACE(NCCL_INIT, "rank %d nranks %d size %d - DONE", rank, nranks, size);
  return ncclSuccess;
}

每一次将自己的data发送给对应的rank，然后接收其他rank发送过来的data，如下图。

第一步：

第二步：

到这里每个rank就都有了全局所有rank的ip port。

最后总结一下，本节主要创建了bootstrap环形网络连接，并保存到ncclComm里。

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