关于深度学习:NCCL源码解析②Bootstrap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,并实现了机器的bootstrap网络和通信网络的初始化,这节接着看下所有节点间bootstrap的连贯是如何建设的。

rank0节点执行ncclGetUniqueId生成ncclUniqueId,通过mpi将Id播送到所有节点,而后所有节点都会执行ncclCommInitRank,这里其余节点也会进行初始化bootstrap网络和通信网络的操作,而后会执行到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,蕴含了通信用到的所有上下文信息,外面的字段等用到的时候再介绍,而后通过commAlloc调配newcom,并且实现初始化,比方以后是哪个卡,对应的pcie busid是什么,而后执行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是以后节点连贯next的socket连贯,extBstrapRingRecvComm是以后节点和prev节点的socket连贯,extBstrapListenComm是以后节点的监听socket,peerBstrapHandles是所有rank的ip port(对应extBstrapListenComm),dev默认为0,示意用第几个ip地址。

而后通过bootstrapNetListen创立extHandleListen和extHandleListenRoot两个bootstrap comm,如前文所述,bootstrap comm其实就是保留了fd,这里创立两个comm的起因是extHandleListen是rank之间理论应用的bootstrap连贯,extHandleListenRoot是rank0节点和其余所有rank进行通信应用的连贯。

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个以后机器的ip,而后listen获取监听fd,将ip port写到nethandle,获取到的bootstrap comm写到listencomm。

而后将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,而后通过bootstrapNetConnect创立bootstrap send comm,类比bootstrapNetListen,bootstrapNetConnect就是建设到netHandle的socket连贯,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)

而后通过bootstrapNetSend将extInfo发送进来,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。

而后通过bootstrapNetCloseSend敞开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的最初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;
}

listenComm是上一个博文中rank0创立的监听fd,bootstrapNetAccept是从listenComm中获取一个新连贯,应用新连贯的fd创立recvcomm。

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

而后通过bootstrapNetRecv读取tmpComm的数据,即其余rank发送来的extInfo,而后保留其余rank的extHandleListen和extHandleListenRoot,这个时候rank0就获取到其余所有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。这里能够看出,每个节点创立了两个listen comm,其中rank0应用extHandleListenRoot进行通信,其余节点之间通过extHandleListen进行通信。

而后再回去接着看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上接管新连贯创立tmpRecvComm,而后接管到以后rank的next的ip,port;而后连贯next创立bscomm到state->extBstrapRingSendComm,接管prev的连贯创立bscomm到state->extBstrapRingRecvComm,到当初bootstrap网络连接就齐全建设起来了,如下图:

最初gather所有rank的ip port,首先将本人的nethandle放到peerBstrapHandles的对应地位,如下所示。

而后执行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;
}

每一次将本人的data发送给对应的rank,而后接管其余rank发送过去的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。

最初总结一下,本节次要创立了bootstrap环形网络连接,并保留到ncclComm里。

欢送 Star、试用 OneFlow 最新版本:https://github.com/Oneflow-Inc/oneflow/

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