NCCL 源码解析②：Bootstrap 网络连接的建立

2023-04-10
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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;}

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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;    }  }

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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}

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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;};

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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)

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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;};

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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)

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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;}

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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<nranks; ++r) {    int next = (r+1) % nranks;    void *tmpSendComm;    NCCLCHECKGOTO(bootstrapNetConnect(0, rankHandlesRoot+r, &tmpSendComm), res, out);    NCCLCHECKGOTO(bootstrapNetSend(tmpSendComm, rankHandles+next, sizeof(ncclNetHandle_t)), res, out);    NCCLCHECKGOTO(bootstrapNetCloseSend(tmpSendComm), res, out);  }  TRACE(NCCL_INIT, "SENT OUT ALL %d HANDLES", nranks); out:  bootstrapNetCloseListen(listenComm);  if (rankHandles) free(rankHandles);  if (rankHandlesRoot) free(rankHandlesRoot);   TRACE(NCCL_INIT, "DONE");  return NULL;}

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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)

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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;}

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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; i<nranks-1; i++) {    size_t rslice = (rank - i - 1 + nranks) % nranks;    size_t sslice = (rank - i + nranks) % nranks;     // Send slice to the right    NCCLCHECK(bootstrapNetSend(state->extBstrapRingSendComm, 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;}