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NCCL深度学习Bootstrap网络连接建立源码解析

作者：OneFlow一流科技　　发布时间：2022-02-25 22:28:10　

标签：NCCL,Bootstrap,网络连接,深度学习

引言

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，

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<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; 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;
}