NCCL源码解析⑦：机器间Channel连接

作者｜KIDGINBROOK

上节中完成了单机内部的channel搜索，仍然以ringGraph为例的话，相当于在单台机器内部搜索出来了一系列的环，接下来需要将机器之间的环连接起来。

为了方便理解，假设两机十六卡的情况下第一台机器的一个ring为：

makefile 复制代码

graph->intra: GPU/0 GPU/7 GPU/6 GPU/3 GPU/2 GPU/5 GPU/4 GPU/1
graph->inter: NET/0 NET/0

第二个机器对应的ring为：

makefile 复制代码

graph->intra: GPU/10 GPU/9 GPU/8 GPU/13 GPU/12 GPU/15 GPU/14 GPU/11
graph->inter: NET/0 NET/0

allGather3Data用于rank间聚合channel的信息，ncclGraphInfo记录了环的信息，比如speed和type

ini 复制代码

struct ncclGraphInfo {
    int sameChannels;
    float speedIntra;
    float speedInter;
    int typeIntra;
  };
 
  struct {
    int cudaCompCap;
    int fullCudaCompCap;
    int nChannels;
    struct ncclGraphInfo tree;
    struct ncclGraphInfo ring;
    struct ncclGraphInfo collNet;
    struct ncclTopoRanks topoRanks;
  } *allGather3Data;
 
  NCCLCHECK(ncclCalloc(&allGather3Data, nranks));
  allGather3Data[rank].cudaCompCap = ncclCudaCompCap();
  allGather3Data[rank].nChannels = comm->nChannels = treeGraph.nChannels = ringGraph.nChannels =
    std::min(treeGraph.nChannels, ringGraph.nChannels);
  ...
  allGather3Data[rank].ring.sameChannels = ringGraph.sameChannels;
  allGather3Data[rank].ring.speedIntra = ringGraph.speedIntra;
  allGather3Data[rank].ring.speedInter = ringGraph.speedInter;
  allGather3Data[rank].ring.typeIntra = ringGraph.typeIntra;
  ...

然后开始设置ncclTopoRanks，获取当前rank在ring中的prev和next，其中第一个rank的prev和最后一个rank的next为-1，如rank6的prev为7，next为3；获取当前ring的ringRecv和ringSend，即ring的第一个节点和最后一个节点，最后将搜索到的环复制了一遍，这里在官方issue中看到相关解释是为了进一步的并行以充分利用带宽。

ini 复制代码

struct ncclTopoRanks {
  int ringRecv[MAXCHANNELS];
  int ringSend[MAXCHANNELS];
  int ringPrev[MAXCHANNELS];
  int ringNext[MAXCHANNELS];
  int treeUpRecv[MAXCHANNELS];
  int treeUpSend[MAXCHANNELS];
  int treeDnRecv[MAXCHANNELS];
  int treeDnSend[MAXCHANNELS];
};
 
ncclResult_t ncclTopoPreset(struct ncclComm* comm,
    struct ncclTopoGraph* treeGraph, struct ncclTopoGraph* ringGraph, struct ncclTopoGraph* collNetGraph,
    struct ncclTopoRanks* topoRanks) {
  int rank = comm->rank;
  int localRanks = comm->localRanks;
  int nChannels = comm->nChannels;
 
  for (int c=0; c<nChannels; c++) {
    struct ncclChannel* channel = comm->channels+c;
    channel->ring.prev = channel->ring.next = -1;
    ...
 
    int* ringIntra = ringGraph->intra+c*localRanks;
    int* treeIntra = treeGraph->intra+c*localRanks;
    int* collNetIntra = collNetGraph->intra+c*localRanks;
 
    for (int i=0; i<localRanks; i++) {
      if (ringIntra[i] == rank) {
        topoRanks->ringRecv[c] = ringIntra[0];
        topoRanks->ringSend[c] = ringIntra[localRanks-1];
        channel->ring.prev = (i == 0) ? -1 : ringIntra[i-1];
        channel->ring.next = (i == localRanks-1) ? -1 : ringIntra[i+1];
      }
      ...
    }
    topoRanks->ringPrev[c] = channel->ring.prev;
    topoRanks->ringNext[c] = channel->ring.next;
  }
  // Duplicate channels rings/trees
  struct ncclChannel* channel0 = comm->channels;
  struct ncclChannel* channel1 = channel0+nChannels;
  memcpy(channel1, channel0, nChannels*sizeof(struct ncclChannel));
  return ncclSuccess;
}

然后通过bootstrapAllGather获取全局的allGather3Data信息，计算出当前rank所在的node保存在comm->node，以及每个node的第一个rank保存在nodesFirstRank，因此例子中：

ini 复制代码

nodesFirstRank[0]: 0
nodesFirstRank[1]: 10

然后开始将每个机器的环首尾相连组成大环。

scss 复制代码

ncclResult_t ncclTopoPostset(struct ncclComm* comm, int* firstRanks, struct ncclTopoRanks** allTopoRanks, int* rings) {
  // Gather data from all ranks
  int *ringRecv, *ringSend, *ringPrev, *ringNext, *treeUpRecv, *treeUpSend, *treeDnRecv,*treeDnSend;
  int nranks = comm->nRanks;
  int nChannels = comm->nChannels;
  NCCLCHECK(ncclCalloc(&ringRecv, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&ringSend, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&ringPrev, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&ringNext, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&treeUpRecv, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&treeUpSend, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&treeDnRecv, nranks*MAXCHANNELS));
  NCCLCHECK(ncclCalloc(&treeDnSend, nranks*MAXCHANNELS));
  for (int i=0; i<nranks; i++) {
    for (int c=0; c<nChannels;c++) {
      ringRecv[c*nranks+i] = allTopoRanks[i]->ringRecv[c];
      ringSend[c*nranks+i] = allTopoRanks[i]->ringSend[c];
      ringPrev[c*nranks+i] = allTopoRanks[i]->ringPrev[c];
      ringNext[c*nranks+i] = allTopoRanks[i]->ringNext[c];
      treeUpRecv[c*nranks+i] = allTopoRanks[i]->treeUpRecv[c];
      treeUpSend[c*nranks+i] = allTopoRanks[i]->treeUpSend[c];
      treeDnRecv[c*nranks+i] = allTopoRanks[i]->treeDnRecv[c];
      treeDnSend[c*nranks+i] = allTopoRanks[i]->treeDnSend[c];
    }
  }
 
  // Connect rings and trees. This should also duplicate the channels.
  NCCLCHECK(connectRings(comm, ringRecv, ringSend, ringPrev, ringNext, firstRanks));
  NCCLCHECK(connectTrees(comm, treeUpRecv, treeUpSend, treeDnRecv, treeDnSend, firstRanks));
 
  // Duplicate ringPrev/ringNext for ncclBuildRing
  memcpy(ringPrev+nChannels*nranks, ringPrev, nChannels*nranks*sizeof(int));
  memcpy(ringNext+nChannels*nranks, ringNext, nChannels*nranks*sizeof(int));
 
  // Duplication should be complete now
  nChannels = comm->nChannels = std::min(MAXCHANNELS,nChannels*2);
 
  // Honor NCCL_MIN_NRINGS/NCCL_MAX_NRINGS.
  // We permit combining max, then min, to only use the first channels, then duplicate them.
  nChannels = comm->nChannels = std::min((int)ncclMaxNchannels(), nChannels);
  int c;
  for (c=nChannels; c<ncclMinNchannels(); c++) {
    memcpy(ringPrev+c*nranks, ringPrev+(c-nChannels)*nranks, nranks*sizeof(int));
    memcpy(ringNext+c*nranks, ringNext+(c-nChannels)*nranks, nranks*sizeof(int));
    memcpy(comm->channels+c, comm->channels+c-nChannels, sizeof(struct ncclChannel));
  }
  nChannels = comm->nChannels = c;
 
  // Create rings array and check all is fine
  NCCLCHECK(ncclBuildRings(nChannels, rings, comm->rank, comm->nRanks, ringPrev, ringNext));
 
  free(ringRecv);
  free(ringSend);
  free(ringPrev);
  free(ringNext);
  free(treeUpRecv);
  free(treeUpSend);
  free(treeDnRecv);
  free(treeDnSend);
 
  return ncclSuccess;
}

这里将所有channel的prev，next，send，recv信息打平到数组中，例如recv[0]表示第一个ring中rank0的recv是哪个rank，然后开始计算当前机器第一个rank的prev和最后一个rank的next。

ini 复制代码

static ncclResult_t connectRings(struct ncclComm* comm, int* ringRecv, int* ringSend, int* ringPrev, int* ringNext, int* firstRanks) {
  int nChannels = comm->nChannels;
  int nNodes = comm->nNodes;
  for (int c=0; c<nChannels; c++) {
    int* recv = ringRecv+c*comm->nRanks;
    int* send = ringSend+c*comm->nRanks;
    int* prev = ringPrev+c*comm->nRanks;
    int* next = ringNext+c*comm->nRanks;
    struct ncclChannel* channel0 = comm->channels+c;
    struct ncclChannel* channel1 = channel0+nChannels;
    for (int n=0; n<nNodes; n++) {
      int recvRank = recv[firstRanks[n]];
      int prevSendRank = send[firstRanks[(n-1+nNodes)%nNodes]];
      prev[recvRank] = prevSendRank;
      if (comm->rank == recvRank) {
        channel0->ring.prev = prevSendRank;
        channel1->ring.prev = prevSendRank;
      }
      int sendRank = send[firstRanks[n]];
      int nextRecvRank = recv[firstRanks[(n+1)%nNodes]];
      next[sendRank] = nextRecvRank;
      if (comm->rank == sendRank) {
        channel0->ring.next = nextRecvRank;
        channel1->ring.next = nextRecvRank;
      }
    }
    TRACE(NCCL_GRAPH, "Ring %d : %d -> %d -> %d", c, channel0->ring.prev, comm->rank, channel0->ring.next);
    TRACE(NCCL_GRAPH, "Ring %d : %d -> %d -> %d", c+nChannels, channel1->ring.prev, comm->rank, channel1->ring.next);
  }
  return ncclSuccess;
}

如上所示，当前机器recv rank的prev就是前一个机器的send rank，当前机器send rank的next就是下一个机器的recv rank。然后执行ncclBuildRings按照大环的顺序依次记录rank到rings。

ini 复制代码

ncclResult_t ncclBuildRings(int nrings, int* rings, int rank, int nranks, int* prev, int* next) {
  for (int r=0; r<nrings; r++) {
    char prefix[30];
 
    int current = rank;
    for (int i=0; i<nranks; i++) {
      rings[r*nranks+i] = current;
      current = next[r*nranks+current];
    }
    ...
    // Check that all ranks are there
    for (int i=0; i<nranks; i++) {
      int found = 0;
      for (int j=0; j<nranks; j++) {
        if (rings[r*nranks+j] == i) {
          found = 1;
          break;
        }
      }
      if (found == 0) {
        WARN("Error : ring %d does not contain rank %d", r, i);
        return ncclInternalError;
      }
    }
  }
  return ncclSuccess;
}

还是以上述为例，其中rank6记录的rings的第一个大环为：

GPU/6 GPU/3 GPU/2 GPU/5 GPU/4 GPU/1 GPU/10 GPU/9 GPU/8 GPU/13 GPU/12 GPU/15 GPU/14 GPU/11 GPU/0 GPU/7

到这里就完成了机器之间大环建立，每个rank都知道自己的上一个和下一个rank是谁，那么就可以建立实际的通信链路了。

接下来每个rank都要为通信分配一些内存，为了提高性能，这里会在分配buffer之前设置cpu亲和性，使得分配的内存尽量是当前numa本地的。

ini 复制代码

  cpu_set_t affinitySave;
  sched_getaffinity(0, sizeof(cpu_set_t), &affinitySave);
  NCCLCHECK(ncclTopoSetAffinity(comm->topo, comm->rank));
 
ncclResult_t ncclTopoSetAffinity(struct ncclTopoSystem* system, int rank) {
  struct ncclTopoNode* cpu = NULL, *gpu = NULL;
  for (int g=0; g<system->nodes[GPU].count; g++) {
    if (system->nodes[GPU].nodes[g].gpu.rank == rank) {
      gpu = system->nodes[GPU].nodes+g;
      // Find closer CPU
      int cpuIndex = -1, minHops = 0;
      for (int c=0; c<system->nodes[CPU].count; c++) {
        int nHops = system->nodes[GPU].nodes[g].paths[CPU][c].count;
        if (cpuIndex == -1 || nHops < minHops) {
          cpuIndex = c;
          minHops = nHops;
        }
      }
      cpu = system->nodes[CPU].nodes+cpuIndex;
    }
  }
  if (cpu == NULL) {
    WARN("Set CPU affinity : unable to find GPU/CPU for rank %d", rank);
    return ncclInternalError;
  }
 
  // Query the CPU affinity set we were provided
  cpu_set_t mask;
  SYSCHECK(sched_getaffinity(0, sizeof(cpu_set_t), &mask), "sched_getaffinity");
 
  // Get the affinity of the CPU close to our GPU.
  cpu_set_t cpuMask = cpu->cpu.affinity;
 
  cpu_set_t finalMask;
  if (ncclParamIgnoreCpuAffinity())
    // Ignore the CPU affinity set and use the GPU one instead
    finalMask = cpuMask;
  else
    // Use a subset of the GPU affinity set
    CPU_AND(&finalMask, &mask, &cpuMask);
 
  // If there is a non empty set, use it to set affinity
  if (CPU_COUNT(&finalMask)) {
    char affinityStr[sizeof(cpu_set_t)*2];
    NCCLCHECK(ncclCpusetToStr(&finalMask, affinityStr));
    INFO(NCCL_INIT, "Setting affinity for GPU %d to %s", gpu->gpu.dev, affinityStr);
    SYSCHECK(sched_setaffinity(0, sizeof(cpu_set_t), &finalMask), "sched_setaffinity");
  }
  return ncclSuccess;
}

首先获取当前线程的cpu亲和性保存到affinitySave，分配好buffer之后会用affinitySave来恢复亲和性。

然后通过ncclTopoSetAffinity设置cpu亲和性，找到当前rank对应的cpu节点之后，可以获取到该cpu对应的core，即cpuMask，然后获取当前线程对应的亲和性，即mask，默认会取cpuMask和mask的交集finalMask，如果交集不为空的话，会将finalMask设置给当前线程。

ini 复制代码

struct ncclConnect {
  char data[CONNECT_SIZE];
};  
 
  struct ncclConnect *connect;
  NCCLCHECKGOTO(ncclCalloc(&connect, 2), ret, affinity_restore);
  for (int c=0; c<comm->nChannels; c++) {
    struct ncclChannel* channel = comm->channels+c;
    NCCLCHECKGOTO(setupChannel(comm, c, rank, nranks, rings+c*nranks), ret, affinity_restore);
    if (comm->nRanks == 1) continue;
    NCCLCHECKGOTO(ncclTransportP2pSetup(comm, &ringGraph, channel, 1, &channel->ring.prev, 1, &channel->ring.next), ret, affinity_restore);
    ...
  }

然后简单看下ncclChannel数据结构，其中collectives保存了用户向nccl提交的通信操作，比如ncclSend，ncclRecv等都会向collectives里加一项，ncclColl则保存了这些操作对应的参数；collectives是一个环形队列，所以collStart指向了开始位置，collCount表示队列中操作数量；FifoHead和FifoTail用于协调kernel产出数据和NET发送数据，其实就是生产者消费者，ncclPeer保存了通信相关的信息，后续再具体介绍。

arduino 复制代码

struct ncclRing {
  // Shortcuts for userRanks[1] and userRanks[n-1]
  int prev;  // 记录环中当前rank的上一个rank
  int next;  // 记录环中当前rank的下一个rank
 
  // Maps an internal nccl index to user-specified rank order. This is necessary
  // since we need to know how the user expects data to be ordered across
  // devices. Ordered from current device.
  int* userRanks;  // 以当前rank为起点记录整个环
  int* devUserRanks;  // device断的userRanks
};
 
struct ncclChannel {
  union {
    struct {
      struct ncclRing ring;
      struct ncclTree treeUp;
      struct ncclTree treeDn;
      struct ncclTree collTreeUp;
      struct ncclTree collTreeDn;
 
      int id; 
 
      // Communication structures
      struct ncclPeer* peers;
      struct ncclPeer* devPeers;
 
      // Operation list for aggregation
      struct ncclColl* collectives;
      int collStart;
      int collCount;
      int collFifoHead; // Only used by GPU
      int collFifoTail; // Only used by CPU
    };  
    int data[0x80];
  };  
};

然后开始初始化channel，initChannel主要是buffer的分配，分配userRanks和devUserRanks，设置ncclPeer，分配collectives，因为host和device都会访问collectives这个数据结构，所以需要通过cudaHostAlloc分配host端的锁页内存，并通过flag cudaHostAllocMapped将其映射到cuda的地址空间。不过在uva系统上，cudaMallocHost，cudaHostAlloc + cudaHostAllocDefault以及cudaHostAlloc + cudaHostAllocMapped这三种方式没啥区别，host和device都可以访问。

scss 复制代码

ncclResult_t initChannel(struct ncclComm* comm, int channelid) {
  struct ncclChannel* channel = comm->channels+channelid;
  if (channel->id != -1) return ncclSuccess;
  channel->id = channelid;
 
  // Ring index to user rank table.
  NCCLCHECK(ncclCudaCalloc(&channel->ring.devUserRanks, comm->nRanks));
  NCCLCHECK(ncclCalloc(&channel->ring.userRanks, comm->nRanks));
 
  // Communication structures with peers.
  NCCLCHECK(ncclCudaCalloc(&channel->devPeers, comm->nRanks+1)); // The extra one rank is for collnet root (i.e. network)
  NCCLCHECK(ncclCalloc(&channel->peers, comm->nRanks+1));
  for (size_t i=0; i<comm->nRanks+1; ++i) {
    channel->peers[i].send.comm = comm;
    channel->peers[i].recv.comm = comm;
  }
 
  // Per-channel operation list.
  NCCLCHECK(ncclCudaHostCalloc(&channel->collectives, NCCL_MAX_OPS));
  return ncclSuccess;
}
 
template <typename T>
static ncclResult_t ncclCudaHostCalloc(T** ptr, size_t nelem) {
  CUDACHECK(cudaHostAlloc(ptr, nelem*sizeof(T), cudaHostAllocMapped));
  memset(*ptr, 0, nelem*sizeof(T)); 
  return ncclSuccess; 
}

然后从当前rank为起点，将环写到userRanks。

perl 复制代码

static ncclResult_t setupChannel(struct ncclComm* comm, int channelId, int rank, int nranks, int* ringRanks) {
  TRACE(NCCL_INIT, "rank %d nranks %d", rank, nranks);
  NCCLCHECK(initChannel(comm, channelId));
 
  struct ncclRing* ring = &comm->channels[channelId].ring;
  // Reorganize ranks to start with rank.
  int shift;
  for (shift = 0; shift<nranks; shift++) {
    if (ringRanks[shift] == rank) {
      break;
    }
  }
  for (int i=0; i<nranks; i++) {
    ring->userRanks[i] = ringRanks[(i+shift)%nranks];
  }
  return ncclSuccess;
}

然后执行ncclTransportP2pSetup建立当前rank和prev，next的通信链路。

到这里就完成了机器之间channel的连接，下节会了解到通信链路的建立过程。

（本文经授权后由OneFlow发布。原文：blog.csdn.net/KIDGIN7439/...

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