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/