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作者|KIDGINBROOK
更新|潘丽晨
NCCL 是英伟达开源的 GPU 通信库，反对汇合通信和点对点通信。
看下官网给的一个 demo：
#include <stdio.h>
#include "cuda_runtime.h"
#include "nccl.h"
#include "mpi.h"
#include <unistd.h>
#include <stdint.h>
#define MPICHECK(cmd) do { \
int e = cmd; \
if(e != MPI_SUCCESS) { \
printf("Failed: MPI error %s:%d'%d'\n", \
__FILE__,__LINE__, e); \
exit(EXIT_FAILURE); \
} \
} while(0)
#define CUDACHECK(cmd) do { \
cudaError_t e = cmd; \
if(e != cudaSuccess) { \
printf("Failed: Cuda error %s:%d'%s'\n", \
__FILE__,__LINE__,cudaGetErrorString(e)); \
exit(EXIT_FAILURE); \
} \
} while(0)
#define NCCLCHECK(cmd) do { \
ncclResult_t r = cmd; \
if (r!= ncclSuccess) { \
printf("Failed, NCCL error %s:%d'%s'\n", \
__FILE__,__LINE__,ncclGetErrorString(r)); \
exit(EXIT_FAILURE); \
} \
} while(0)
static uint64_t getHostHash(const char* string) {
// Based on DJB2a, result = result * 33 ^ char
uint64_t result = 5381;
for (int c = 0; string != '\0'; c++){result = ((result << 5) + result) ^ string;
}
return result;
}
static void getHostName(char* hostname, int maxlen) {gethostname(hostname, maxlen);
for (int i=0; i< maxlen; i++) {if (hostname[i] == '.') {hostname[i] = '\0';
return;
}
}
}
int main(int argc, char* argv[])
{
int size = 32*1024*1024;
int myRank, nRanks, localRank = 0;
//initializing MPI
MPICHECK(MPI_Init(&argc, &argv));
MPICHECK(MPI_Comm_rank(MPI_COMM_WORLD, &myRank));
MPICHECK(MPI_Comm_size(MPI_COMM_WORLD, &nRanks));
//calculating localRank which is used in selecting a GPU
uint64_t hostHashs[nRanks];
char hostname[1024];
getHostName(hostname, 1024);
hostHashs[myRank] = getHostHash(hostname);
MPICHECK(MPI_Allgather(MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, hostHashs, sizeof(uint64_t), MPI_BYTE, MPI_COMM_WORLD));
for (int p=0; p<nRanks; p++) {if (p == myRank) break;
if (hostHashs[p] == hostHashs[myRank]) localRank++;
}
//each process is using two GPUs
int nDev = 2;
float** sendbuff = (float**)malloc(nDev * sizeof(float*));
float** recvbuff = (float**)malloc(nDev * sizeof(float*));
cudaStream_t* s = (cudaStream_t*)malloc(sizeof(cudaStream_t)*nDev);
//picking GPUs based on localRank
for (int i = 0; i < nDev; ++i) {CUDACHECK(cudaSetDevice(localRank*nDev + i));
CUDACHECK(cudaMalloc(sendbuff + i, size * sizeof(float)));
CUDACHECK(cudaMalloc(recvbuff + i, size * sizeof(float)));
CUDACHECK(cudaMemset(sendbuff[i], 1, size * sizeof(float)));
CUDACHECK(cudaMemset(recvbuff[i], 0, size * sizeof(float)));
CUDACHECK(cudaStreamCreate(s+i));
}
ncclUniqueId id;
ncclComm_t comms[nDev];
//generating NCCL unique ID at one process and broadcasting it to all
if (myRank == 0) ncclGetUniqueId(&id);
MPICHECK(MPI_Bcast((void *)&id, sizeof(id), MPI_BYTE, 0, MPI_COMM_WORLD));
//initializing NCCL, group API is required around ncclCommInitRank as it is
//called across multiple GPUs in each thread/process
NCCLCHECK(ncclGroupStart());
for (int i=0; i<nDev; i++) {CUDACHECK(cudaSetDevice(localRank*nDev + i));
NCCLCHECK(ncclCommInitRank(comms+i, nRanks*nDev, id, myRank*nDev + i));
}
NCCLCHECK(ncclGroupEnd());
//calling NCCL communication API. Group API is required when using
//multiple devices per thread/process
NCCLCHECK(ncclGroupStart());
for (int i=0; i<nDev; i++)
NCCLCHECK(ncclAllReduce((const void*)sendbuff[i], (void*)recvbuff[i], size, ncclFloat, ncclSum,
comms[i], s[i]));
NCCLCHECK(ncclGroupEnd());
//synchronizing on CUDA stream to complete NCCL communication
for (int i=0; i<nDev; i++)
CUDACHECK(cudaStreamSynchronize(s[i]));
//freeing device memory
for (int i=0; i<nDev; i++) {CUDACHECK(cudaFree(sendbuff[i]));
CUDACHECK(cudaFree(recvbuff[i]));
}
//finalizing NCCL
for (int i=0; i<nDev; i++) {ncclCommDestroy(comms[i]);
}
//finalizing MPI
MPICHECK(MPI_Finalize());
printf("[MPI Rank %d] Success \n", myRank);
return 0;
}在上边的示例中，rank0 会执行 ncclGetUniqueId 获取 Id，而后通过 mpi 播送给其余 rank，接下来看下 UniqueId 是怎么产生的。
ncclResult_t ncclGetUniqueId(ncclUniqueId* out) {NCCLCHECK(ncclInit());
NCCLCHECK(PtrCheck(out, "GetUniqueId", "out"));
return bootstrapGetUniqueId(out);
}而后看下 ncclInit。
首先执行 initEnv，设置环境变量。
而后执行 initNet，用来初始化 nccl 所须要的网络，包含两个，一个是 bootstrap 网络，另外一个是数据通信网络，bootstrap 网络次要用于初始化时替换一些简略的信息，比方每个机器的 ip 端口，因为数据量很小，而且次要是在初始化阶段执行一次，因而 bootstrap 应用的是 tcp；而通信网络是用于理论数据的传输，因而会优先应用 rdma（反对 gdr 的话会优先应用 gdr）。
ncclResult_t initNet() {
// Always initialize bootstrap network
NCCLCHECK(bootstrapNetInit());
NCCLCHECK(initNetPlugin(&ncclNet, &ncclCollNet));
if (ncclNet != NULL) return ncclSuccess;
if (initNet(&ncclNetIb) == ncclSuccess) {ncclNet = &ncclNetIb;} else {NCCLCHECK(initNet(&ncclNetSocket));
ncclNet = &ncclNetSocket;
}
return ncclSuccess;
}bootstrapNetInit 就是 bootstrap 网络的初始化，次要就是通过 findInterfaces 遍历机器上所有的网卡信息，通过 prefixList 匹配抉择应用哪些网卡，将可用网卡的信息保留下来，将 ifa_name 保留到全局的 bootstrapNetIfNames，ip 地址保留到全局 bootstrapNetIfAddrs，默认除了 docker 和 lo 其余的网卡都能够应用。
例如在测试机器上有三张网卡，别离是 xgbe0、xgbe1、xgbe2，那么就会把这三个 ifaname 和对应的 ip 地址保留下来，另外 nccl 提供了环境变量 NCCL_SOCKET_IFNAME 能够用来指定想用的网卡名，例如通过 export NCCL_SOCKET_IFNAME=xgbe0 来指定应用 xgbe0，其实就是通过 prefixList 来匹配做到的。
static int findInterfaces(const char* prefixList, char* names, union socketAddress *addrs, int sock_family, int maxIfNameSize, int maxIfs) {struct netIf userIfs[MAX_IFS];
bool searchNot = prefixList && prefixList[0] == '^';
if (searchNot) prefixList++;
bool searchExact = prefixList && prefixList[0] == '=';
if (searchExact) prefixList++;
int nUserIfs = parseStringList(prefixList, userIfs, MAX_IFS);
int found = 0;
struct ifaddrs *interfaces, *interface;
getifaddrs(&interfaces);
for (interface = interfaces; interface && found < maxIfs; interface = interface->ifa_next) {if (interface->ifa_addr == NULL) continue;
int family = interface->ifa_addr->sa_family;
if (family != AF_INET && family != AF_INET6)
continue;
if (sock_family != -1 && family != sock_family)
continue;
if (family == AF_INET6) {struct sockaddr_in6* sa = (struct sockaddr_in6*)(interface->ifa_addr);
if (IN6_IS_ADDR_LOOPBACK(&sa->sin6_addr)) continue;
}
if (!(matchIfList(interface->ifa_name, -1, userIfs, nUserIfs, searchExact) ^ searchNot)) {continue;}
bool duplicate = false;
for (int i = 0; i < found; i++) {if (strcmp(interface->ifa_name, names+i*maxIfNameSize) == 0) {duplicate = true; break;}
}
if (!duplicate) {strncpy(names+found*maxIfNameSize, interface->ifa_name, maxIfNameSize);
int salen = (family == AF_INET) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
memcpy(addrs+found, interface->ifa_addr, salen);
found++;
}
}
freeifaddrs(interfaces);
return found;
}开始初始化通信网络。
ncclNet_t 构造体是一系列的函数指针，比方初始化，发送，接管等；socket，IB 等通信形式都实现了本人的 ncclNet_t，如 ncclNetSocket，ncclNetIb，初始化通信网络的过程就是顺次看哪个通信模式可用，而后赋值给全局的 ncclNet。
首先执行 initNetPlugin，查看是否有 libnccl-net.so，测试环境没有这个 so，所以间接返回。
而后尝试应用 IB 网络：
首先执行 ncclNetIb 的 init 函数，就是 ncclIbInit。
ncclResult_t ncclIbInit(ncclDebugLogger_t logFunction) {
static int shownIbHcaEnv = 0;
if(wrap_ibv_symbols() != ncclSuccess) {return ncclInternalError;}
if (ncclParamIbDisable()) return ncclInternalError;
if (ncclNIbDevs == -1) {pthread_mutex_lock(&ncclIbLock);
wrap_ibv_fork_init();
if (ncclNIbDevs == -1) {
ncclNIbDevs = 0;
if (findInterfaces(ncclIbIfName, &ncclIbIfAddr, MAX_IF_NAME_SIZE, 1) != 1) {WARN("NET/IB : No IP interface found.");
return ncclInternalError;
}
// Detect IB cards
int nIbDevs;
struct ibv_device** devices;
// Check if user defined which IB device:port to use
char* userIbEnv = getenv("NCCL_IB_HCA");
if (userIbEnv != NULL && shownIbHcaEnv++ == 0) INFO(NCCL_NET|NCCL_ENV, "NCCL_IB_HCA set to %s", userIbEnv);
struct netIf userIfs[MAX_IB_DEVS];
bool searchNot = userIbEnv && userIbEnv[0] == '^';
if (searchNot) userIbEnv++;
bool searchExact = userIbEnv && userIbEnv[0] == '=';
if (searchExact) userIbEnv++;
int nUserIfs = parseStringList(userIbEnv, userIfs, MAX_IB_DEVS);
if (ncclSuccess != wrap_ibv_get_device_list(&devices, &nIbDevs)) return ncclInternalError;
for (int d=0; d<nIbDevs && ncclNIbDevs<MAX_IB_DEVS; d++) {
struct ibv_context * context;
if (ncclSuccess != wrap_ibv_open_device(&context, devices[d]) || context == NULL) {WARN("NET/IB : Unable to open device %s", devices[d]->name);
continue;
}
int nPorts = 0;
struct ibv_device_attr devAttr;
memset(&devAttr, 0, sizeof(devAttr));
if (ncclSuccess != wrap_ibv_query_device(context, &devAttr)) {WARN("NET/IB : Unable to query device %s", devices[d]->name);
if (ncclSuccess != wrap_ibv_close_device(context)) {return ncclInternalError;}
continue;
}
for (int port = 1; port <= devAttr.phys_port_cnt; port++) {
struct ibv_port_attr portAttr;
if (ncclSuccess != wrap_ibv_query_port(context, port, &portAttr)) {WARN("NET/IB : Unable to query port %d", port);
continue;
}
if (portAttr.state != IBV_PORT_ACTIVE) continue;
if (portAttr.link_layer != IBV_LINK_LAYER_INFINIBAND
&& portAttr.link_layer != IBV_LINK_LAYER_ETHERNET) continue;
// check against user specified HCAs/ports
if (! (matchIfList(devices[d]->name, port, userIfs, nUserIfs, searchExact) ^ searchNot)) {continue;}
TRACE(NCCL_INIT|NCCL_NET,"NET/IB: [%d] %s:%d/%s", d, devices[d]->name, port,
portAttr.link_layer == IBV_LINK_LAYER_INFINIBAND ? "IB" : "RoCE");
ncclIbDevs[ncclNIbDevs].device = d;
ncclIbDevs[ncclNIbDevs].guid = devAttr.sys_image_guid;
ncclIbDevs[ncclNIbDevs].port = port;
ncclIbDevs[ncclNIbDevs].link = portAttr.link_layer;
ncclIbDevs[ncclNIbDevs].speed = ncclIbSpeed(portAttr.active_speed) * ncclIbWidth(portAttr.active_width);
ncclIbDevs[ncclNIbDevs].context = context;
strncpy(ncclIbDevs[ncclNIbDevs].devName, devices[d]->name, MAXNAMESIZE);
NCCLCHECK(ncclIbGetPciPath(ncclIbDevs[ncclNIbDevs].devName, &ncclIbDevs[ncclNIbDevs].pciPath, &ncclIbDevs[ncclNIbDevs].realPort));
ncclIbDevs[ncclNIbDevs].maxQp = devAttr.max_qp;
ncclNIbDevs++;
nPorts++;
pthread_create(&ncclIbAsyncThread, NULL, ncclIbAsyncThreadMain, context);
}
if (nPorts == 0 && ncclSuccess != wrap_ibv_close_device(context)) {return ncclInternalError;}
}
if (nIbDevs && (ncclSuccess != wrap_ibv_free_device_list(devices))) {return ncclInternalError;};
}
if (ncclNIbDevs == 0) {INFO(NCCL_INIT|NCCL_NET, "NET/IB : No device found.");
} else {char line[1024];
line[0] = '\0';
for (int d=0; d<ncclNIbDevs; d++) {snprintf(line+strlen(line), 1023-strlen(line), "[%d]%s:%d/%s", d, ncclIbDevs[d].devName,
ncclIbDevs[d].port, ncclIbDevs[d].link == IBV_LINK_LAYER_INFINIBAND ? "IB" : "RoCE");
}
line[1023] = '\0';
char addrline[1024];
INFO(NCCL_INIT|NCCL_NET, "NET/IB : Using%s ; OOB %s:%s", line, ncclIbIfName, socketToString(&ncclIbIfAddr.sa, addrline));
}
pthread_mutex_unlock(&ncclIbLock);
}
return ncclSuccess;
}首先第三行通过 wrap_ibv_symbols 加载动静库 libibverbs.so，而后获取动静库的各个函数。
而后通过 wrap_ibv_fork_init 防止 fork 引起 rdma 网卡读写出错。
前面会讲到 ib 网络也会用到 socket 进行带外网络的传输，所以这里也通过 findInterfaces 获取一个可用的网卡保留到 ncclIbIfAddr。
通过 ibv_get_device_list 获取所有 rdma 设施到 devices 中，遍历 devices 的每个 device，因为每个 HCA 可能有多个物理 port，所以对每个 device 遍历每一个物理 port，获取每个 port 的信息。
而后将相干信息保留到全局的 ncclIbDevs 中，比方是哪个 device 的哪个 port，应用的是 IB 还是 ROCE，device 的 pci 门路，maxqp，device 的 name 等，留神这里也有相似 bootstrap 网络 NCCL_SOCKET_IFNAME 的环境变量，叫 NCCL_IB_HCA，能够指定应用哪个 IB HCA。
到这里整个初始化的过程就实现了，一句话总结就是，获取了以后机器上所有可用的 IB 网卡和一般以太网卡之后保留下来。
而后开始生成 UniqueId。
ncclResult_t bootstrapCreateRoot(ncclUniqueId* id, bool idFromEnv) {ncclNetHandle_t* netHandle = (ncclNetHandle_t*) id;
void* listenComm;
NCCLCHECK(bootstrapNetListen(idFromEnv ? dontCareIf : 0, netHandle, &listenComm));
pthread_t thread;
pthread_create(&thread, NULL, bootstrapRoot, listenComm);
return ncclSuccess;
}ncclNetHandle_t 也是一个字符数组，而后执行 bootstrapNetListen。
static ncclResult_t bootstrapNetListen(int dev, ncclNetHandle_t* netHandle, void** listenComm) {union socketAddress* connectAddr = (union socketAddress*) netHandle;
static_assert(sizeof(union socketAddress) < NCCL_NET_HANDLE_MAXSIZE, "union socketAddress size is too large");
// if dev >= 0, listen based on dev
if (dev >= 0) {NCCLCHECK(bootstrapNetGetSocketAddr(dev, connectAddr));
} else if (dev == findSubnetIf) {...} // Otherwise, handle stores a local address
struct bootstrapNetComm* comm;
NCCLCHECK(bootstrapNetNewComm(&comm));
NCCLCHECK(createListenSocket(&comm->fd, connectAddr));
*listenComm = comm;
return ncclSuccess;
}顺次看下这三个函数，通过 bootstrapNetGetSocketAddr 获取一个可用的 ip 地址。
static ncclResult_t bootstrapNetGetSocketAddr(int dev, union socketAddress* addr) {if (dev >= bootstrapNetIfs) return ncclInternalError;
memcpy(addr, bootstrapNetIfAddrs+dev, sizeof(*addr));
return ncclSuccess;
}此时 dev 是 0，bootstrapNetIfs 是初始化 bootstrap 网络的时候一共找到了几个可用的网卡，这里就是获取了第 0 个可用的 ip 地址。
而后通过 bootstrapNetNewComm 创立 bootstrapNetComm，bootstrapNetComm 其实就是 fd，bootstrapNetNewComm 其实就是 new 了一个 bootstrapNetComm。
struct bootstrapNetComm {int fd;};通过 createListenSocket 启动 socker server。
static ncclResult_t createListenSocket(int *fd, union socketAddress *localAddr) {
/* IPv4/IPv6 support */
int family = localAddr->sa.sa_family;
int salen = (family == AF_INET) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
/* Create socket and bind it to a port */
int sockfd = socket(family, SOCK_STREAM, 0);
if (sockfd == -1) {WARN("Net : Socket creation failed : %s", strerror(errno));
return ncclSystemError;
}
if (socketToPort(&localAddr->sa)) {
// Port is forced by env. Make sure we get the port.
int opt = 1;
#if defined(SO_REUSEPORT)
SYSCHECK(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt)), "setsockopt");
#else
SYSCHECK(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)), "setsockopt");
#endif
}
// localAddr port should be 0 (Any port)
SYSCHECK(bind(sockfd, &localAddr->sa, salen), "bind");
/* Get the assigned Port */
socklen_t size = salen;
SYSCHECK(getsockname(sockfd, &localAddr->sa, &size), "getsockname");
#ifdef ENABLE_TRACE
char line[1024];
TRACE(NCCL_INIT|NCCL_NET,"Listening on socket %s", socketToString(&localAddr->sa, line));
#endif
/* Put the socket in listen mode
* NB: The backlog will be silently truncated to the value in /proc/sys/net/core/somaxconn
*/
SYSCHECK(listen(sockfd, 16384), "listen");
*fd = sockfd;
return ncclSuccess;
}创立监听 fd，ip 由 localaddr 指定，初始端口为 0，bind 时随机找一个可用端口，并通过 getsockname(sockfd, &localAddr->sa, &size) 将 ip 端口写回到 localaddr，这里 localaddr 就是 UniqueId。
到这里 UniqueId 也就产生了，其实就是以后机器的 ip 和 port。
（本文经受权后由 OneFlow 公布。原文：https://blog.csdn.net/KIDGIN7439/article/details/126712106?sp…）
欢送 Star、试用 OneFlow 最新版本：https://github.com/Oneflow-Inc/oneflow/
正文完
