NVIDIA NCCL 源码学习（四）- 建图过程

上次分析到nccl对机器PCI系统进行拓扑分析的过程，产出的结果为xml格式，接下来，nccl会根据这个xml进图的建立过程以便之后进行路径搜索。

ncclTopoGetSystem的最后会执行ncclTopoGetSystemFromXml将xml格式转成图格式

ncclResult_t ncclTopoGetSystemFromXml(struct ncclXml* xml, struct ncclTopoSystem** topoSystem) {NCCLCHECK(ncclCalloc(topoSystem, 1));struct ncclXmlNode* topNode;NCCLCHECK(xmlFindTag(xml, "system", &topNode));for (int s=0; s<topNode->nSubs; s++) {struct ncclXmlNode* node = topNode->subs[s];if (strcmp(node->name, "cpu") == 0) NCCLCHECK(ncclTopoAddCpu(node, *topoSystem));}NCCLCHECK(ncclTopoAddNvLinks(topNode, *topoSystem, NULL));NCCLCHECK(ncclTopoConnectCpus(*topoSystem));NCCLCHECK(ncclTopoSortSystem(*topoSystem));return ncclSuccess;
}

从xml中拿到根节点"system"，然后遍历子节点中的"cpu"，对每个cpu通过ncclTopoAddCpu进行建图，这里一个cpu其实就是一个numa。

ncclResult_t ncclTopoAddCpu(struct ncclXmlNode* xmlCpu, struct ncclTopoSystem* system) {int numaId;NCCLCHECK(xmlGetAttrInt(xmlCpu, "numaid", &numaId));struct ncclTopoNode* cpu;NCCLCHECK(ncclTopoCreateNode(system, &cpu, CPU, numaId));const char* str;NCCLCHECK(xmlGetAttr(xmlCpu, "affinity", &str));if (str != NULL) {NCCLCHECK(ncclStrToCpuset(str, &cpu->cpu.affinity));}NCCLCHECK(xmlGetAttrStr(xmlCpu, "arch", &str));NCCLCHECK(kvConvertToInt(str, &cpu->cpu.arch, kvDictCpuArch));if (cpu->cpu.arch == NCCL_TOPO_CPU_ARCH_X86) {NCCLCHECK(xmlGetAttrStr(xmlCpu, "vendor", &str));NCCLCHECK(kvConvertToInt(str, &cpu->cpu.vendor, kvDictCpuVendor));if (cpu->cpu.vendor == NCCL_TOPO_CPU_VENDOR_INTEL) {int familyId, modelId;NCCLCHECK(xmlGetAttrInt(xmlCpu, "familyid", &familyId));NCCLCHECK(xmlGetAttrInt(xmlCpu, "modelid", &modelId));cpu->cpu.model = (familyId == 6 && modelId >= 0x55) ? NCCL_TOPO_CPU_TYPE_SKL : NCCL_TOPO_CPU_INTEL_BDW;}   }for (int s=0; s<xmlCpu->nSubs; s++) {struct ncclXmlNode* node = xmlCpu->subs[s];if (strcmp(node->name, "pci") == 0) NCCLCHECK(ncclTopoAddPci(node, system, cpu));if (strcmp(node->name, "nic") == 0) {struct ncclTopoNode* nic = NULL;NCCLCHECK(ncclTopoGetNode(system, &nic, NIC, 0));if (nic == NULL) {NCCLCHECK(ncclTopoCreateNode(system, &nic, NIC, 0));NCCLCHECK(ncclTopoConnectNodes(cpu, nic, LINK_PCI, LOC_WIDTH));NCCLCHECK(ncclTopoConnectNodes(nic, cpu, LINK_PCI, LOC_WIDTH));}   NCCLCHECK(ncclTopoAddNic(node, system, nic));}   }return ncclSuccess;
}

接着创建一个cpu node，id为numaid，设置cpu的affinity，即该numa对应的核，设置cpu对应vendor等信息

然后遍历cpu node的子节点，根据不同的类型执行不同的函数，如果是PCI节点，则执行ncclTopoAddPci

ncclResult_t ncclTopoAddPci(struct ncclXmlNode* xmlPci, struct ncclTopoSystem* system, struct ncclTopoNode* parent) {const char* str;int type;NCCLCHECK(xmlGetAttrStr(xmlPci, "class", &str));NCCLCHECK(kvConvertToInt(str, &type, kvDictPciClass));int64_t busId;NCCLCHECK(xmlGetAttrStr(xmlPci, "busid", &str));NCCLCHECK(busIdToInt64(str, &busId));struct ncclTopoNode* node = NULL;if (type == GPU) {struct ncclXmlNode* xmlGpu;NCCLCHECK(xmlGetSub(xmlPci, "gpu", &xmlGpu));if (xmlGpu == NULL) return ncclSuccess;int index;NCCLCHECK(xmlGetAttrIndex(xmlGpu, "rank", &index));if (index == -1) return ncclSuccess;NCCLCHECK(ncclTopoCreateNode(system, &node, type, busId));NCCLCHECK(ncclTopoAddGpu(xmlGpu, system, node));}if (type == NIC) {struct ncclXmlNode* xmlNic;NCCLCHECK(xmlGetSub(xmlPci, "nic", &xmlNic));if (xmlNic == NULL) return ncclSuccess;// Ignore sub device ID and merge multi-port NICs into one PCI device.busId &= 0xfffffffffffffff0;struct ncclTopoNode* nicNode = NULL;NCCLCHECK(ncclTopoGetNode(system, &nicNode, type, busId));if (nicNode == NULL) {NCCLCHECK(ncclTopoCreateNode(system, &nicNode, type, busId));node = nicNode; // Connect it to parent later on}NCCLCHECK(ncclTopoAddNic(xmlNic, system, nicNode));} else if (type == PCI) {NCCLCHECK(ncclTopoCreateNode(system, &node, type, busId));for (int s=0; s<xmlPci->nSubs; s++) {struct ncclXmlNode* xmlSubPci = xmlPci->subs[s];NCCLCHECK(ncclTopoAddPci(xmlSubPci, system, node));}}if (node) {int width, speed;NCCLCHECK(xmlGetAttrInt(xmlPci, "link_width", &width));NCCLCHECK(xmlGetAttrStr(xmlPci, "link_speed", &str));// Manage cases where speed was not indicated in /sysif (width == 0) width = 16;NCCLCHECK(kvConvertToInt(str, &speed, kvDictPciGen)); // Values in 100Mbps, per lane (we want GB/s in the end)NCCLCHECK(ncclTopoConnectNodes(node, parent, LINK_PCI, width*speed/80.0));NCCLCHECK(ncclTopoConnectNodes(parent, node, LINK_PCI, width*speed/80.0));}return ncclSuccess;
}

首先获取pci的type和busId，然后判断type，如果是PCI，那么创建一个PCI node，递归执行ncclTopoAddPci，直到遇到NIC或者GPU xml节点。

如果遇到的是NIC，那么创建NIC节点，然后执行ncclTopoAddNic，这里会在xml nic下遍历xml net，对每个xml net创建net node，id为dev，然后设置speed，port，gdr等属性

ncclResult_t ncclTopoAddNet(struct ncclXmlNode* xmlNet, struct ncclTopoSystem* system, struct ncclTopoNode* nic) {int dev;NCCLCHECK(xmlGetAttrInt(xmlNet, "dev", &dev));struct ncclTopoNode* net;NCCLCHECK(ncclTopoCreateNode(system, &net, NET, dev));const char* str;NCCLCHECK(xmlGetAttr(xmlNet, "guid", &str));if (str) sscanf(str, "0x%lx", &net->net.asic);else net->net.asic = dev;ncclDebugNoWarn = NCCL_GRAPH;int mbps;if (xmlGetAttrInt(xmlNet, "speed", &mbps) != ncclSuccess) mbps = 0;if (mbps <= 0) mbps = 10000; // Some NICs define speed = -1net->net.width = mbps / 8000.0;if (xmlGetAttrInt(xmlNet, "port", &net->net.port) != ncclSuccess) net->net.port = 0;if (xmlGetAttrInt(xmlNet, "gdr", &net->net.gdrSupport) != ncclSuccess) net->net.gdrSupport = 0;if (xmlGetAttrInt(xmlNet, "maxconn", &net->net.maxChannels) != ncclSuccess) net->net.maxChannels = MAXCHANNELS;if (xmlGetAttrInt(xmlNet, "coll", &net->net.collSupport) != ncclSuccess) net->net.collSupport = 0;ncclDebugNoWarn = 0;NCCLCHECK(ncclTopoConnectNodes(nic, net, LINK_NET, net->net.width));NCCLCHECK(ncclTopoConnectNodes(net, nic, LINK_NET, net->net.width));return ncclSuccess;
}ncclResult_t ncclTopoAddNic(struct ncclXmlNode* xmlNic, struct ncclTopoSystem* system, struct ncclTopoNode* nic) {for (int s=0; s<xmlNic->nSubs; s++) {struct ncclXmlNode* xmlNet = xmlNic->subs[s];if (strcmp(xmlNet->name, "net") != 0) continue;int index;NCCLCHECK(xmlGetAttrIndex(xmlNet, "dev", &index));if (index == -1) continue;NCCLCHECK(ncclTopoAddNet(xmlNet, system, nic));}return ncclSuccess;
}

然后通过建立net node到nic node的正反向边，设置边的类型，边上累计带宽，并且当前节点的边按照带宽从大到小排序

ncclResult_t ncclTopoConnectNodes(struct ncclTopoNode* node, struct ncclTopoNode* remNode, int type, float width) {// Aggregate links into higher width for NVLinkstruct ncclTopoLink* link;for (link = node->links; link->remNode; link++) {if (link->remNode == remNode && link->type == type) break;}if (link->remNode == NULL) node->nlinks++;link->type = type;link->remNode = remNode;link->width += width;// Sort links in BW descending orderstruct ncclTopoLink linkSave;memcpy(&linkSave, link, sizeof(struct ncclTopoLink));while (link != node->links) {if ((link-1)->width >= linkSave.width) break;memcpy(link, link-1, sizeof(struct ncclTopoLink));link--;}memcpy(link, &linkSave, sizeof(struct ncclTopoLink));return ncclSuccess;
}

到这里就添加完成了NIC，回到ncclTopoAddPci里，如果是gpu的话则创建gpu node，然后设置gpu node的rank，dev，gdr等属性。最后通过ncclTopoConnectNodes建立当前节点到子节点的双向边。

到这里就完成了每个numa节点下的建图，然后开始添加nvlink和QPI以连接，先看下nvlink

ncclResult_t ncclTopoAddNvLinks(struct ncclXmlNode* node, struct ncclTopoSystem* system, const char* parentBusId) {if (strcmp(node->name, "nvlink") == 0) {struct ncclTopoNode* gpu = NULL;int64_t pBusId;NCCLCHECK(busIdToInt64(parentBusId, &pBusId));NCCLCHECK(ncclTopoGetNode(system, &gpu, GPU, pBusId));if (gpu == NULL) {WARN("Add NVLink error : could not find GPU %lx\n", pBusId);return ncclInternalError;}int count;NCCLCHECK(xmlGetAttrInt(node, "count", &count));const char* targetClass;NCCLCHECK(xmlGetAttrStr(node, "tclass", &targetClass));int targetType;NCCLCHECK(kvConvertToInt(targetClass, &targetType, kvDictPciClass));struct ncclTopoNode* remote = NULL;if (targetType == GPU) {// NVL P2P connection to another GPUconst char* target;NCCLCHECK(xmlGetAttrStr(node, "target", &target));int64_t busId;NCCLCHECK(busIdToInt64(target, &busId));NCCLCHECK(ncclTopoGetNode(system, &remote, GPU, busId));} else if (targetType == CPU) {// NVL connection to the local CPUNCCLCHECK(findLocalCpu(gpu, &remote));} else {if (system->nodes[NVS].count == 0) {NCCLCHECK(ncclTopoCreateNode(system, &remote, NVS, 0));} else {remote = system->nodes[NVS].nodes;}}if (remote) {int nvlSpeed = gpu->gpu.cudaCompCap == 60 ? PASCAL_NVLINK_WIDTH : VOLTA_NVLINK_WIDTH;NCCLCHECK(ncclTopoConnectNodes(gpu, remote, LINK_NVL, count*nvlSpeed));if (remote->type != GPU) {NCCLCHECK(ncclTopoConnectNodes(remote, gpu, LINK_NVL, count*nvlSpeed));}}} else {const char* busId;NCCLCHECK(xmlGetAttr(node, "busid", &busId));for (int s=0; s<node->nSubs; s++) {NCCLCHECK(ncclTopoAddNvLinks(node->subs[s], system, busId ? busId : parentBusId));}}return ncclSuccess;
}

从根节点递归遍历下去，直到遇到nvlink xml节点，然后拿到nvlink的父节点，即gpu节点，然后通过tclass获取对端PCI设备类型，如果是gpu或者cpu，直接返回对端node，如果是nvswitch，那就先创建nvswitch节点，然后创建当前gpu节点和对端的双向边。然后通过ncclTopoConnectCpus将cpu两两连接。

最后为了方便后续搜索channel，通过ncclTopoSort递归将每个PCI节点的边按照nvlink，向下的PCI连接，向上的PCI连接，QPI的顺序进行排序，因为建边的过程中已经按照带宽排序过，所以nvlink一定在最前边，QPI一定在最后，因此只需要对中间的PCI排序即可

static ncclResult_t ncclTopoSort(struct ncclTopoNode* node, struct ncclTopoNode* upNode) {// Shift all links to have upLink as last linkif (upNode) {int l=0;while (node->links[l].remNode != upNode) l++;struct ncclTopoLink upLink;memcpy(&upLink, node->links+l, sizeof(struct ncclTopoLink));while (node->links[l+1].remNode) {memcpy(node->links+l, node->links+l+1, sizeof(struct ncclTopoLink));l++;}   memcpy(node->links+l, &upLink, sizeof(struct ncclTopoLink));}// Recursively sort the PCI treefor (int l=0; l<node->nlinks; l++) {struct ncclTopoLink* link = node->links+l;if (link->type == LINK_PCI && link->remNode != upNode) NCCLCHECK(ncclTopoSort(link->remNode, node));}return ncclSuccess;
}

到这里就完成了整个的建图过程，总结下，由于拓扑分析产出的xml不便于进行后续的路径搜索，所以本节基于xml对PCI系统进行了建图

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