Polish the setup Connection part from Nusrat's comment

example/ck_tile/15_cross_gpu_reduce/cross_gpu_reduce.cpp

@@ -34,8 +34,9 @@ template <class T>
 using DeviceHandle = mscclpp::DeviceHandle<T>;
 extern __constant__ DeviceHandle<mscclpp::SmChannel> constSlaveSmChannels[8]; // For SmChannel
 
-void setupConnection(
-    int rank, int slaveRank, int worldSize, void* src_data, void* dst_data, size_t dataSize)
+extern __constant__ DeviceHandle<mscclpp::SmChannel> constMasterSmChannel;
+
+void setupConnection(int rank, int slaveRank, int worldSize, void* dst_data, size_t dataSize)
 {
     // Initialize MSCCL++ Communicator
     auto bootstrap = std::make_shared<mscclpp::TcpBootstrap>(rank, worldSize);
@@ -43,40 +44,67 @@ void setupConnection(
     mscclpp::Communicator comm(bootstrap);
     mscclpp::Transport transport = mscclpp::Transport::CudaIpc;
 
+    // We'll register our local memory. For the slave, this might be the destination buffer.
+    // For senders, this might be the source buffer or a local buffer we expose to the slave.
+    mscclpp::RegisteredMemory localMemory = comm.registerMemory(dst_data, dataSize, transport);
+
     if(rank == slaveRank)
     {
-        std::vector<mscclpp::NonblockingFuture<mscclpp::RegisteredMemory>> remoteMemories;
-        std::vector<mscclpp::NonblockingFuture<std::shared_ptr<mscclpp::Connection>>> connections(
-            worldSize);
+        std::vector<mscclpp::NonblockingFuture<std::shared_ptr<mscclpp::Connection>>>
+            connectionFutures;
+        std::vector<mscclpp::NonblockingFuture<mscclpp::RegisteredMemory>> remoteMemFutures;
         std::vector<std::shared_ptr<mscclpp::SmDevice2DeviceSemaphore>> slave_semaphore_list(
             worldSize);
         for(size_t senderRank = 0; senderRank < static_cast<size_t>(worldSize); ++senderRank)
         {
             if(senderRank == static_cast<size_t>(rank))
                 continue;
-            connections[senderRank] = comm.connectOnSetup(senderRank, 0, transport);
-            remoteMemories.push_back(comm.recvMemoryOnSetup(senderRank, 0));
+            connectionFutures.push_back(comm.connectOnSetup(senderRank, 0, transport));
+            comm.sendMemoryOnSetup(localMemory, senderRank, 0);
+            remoteMemFutures.push_back(comm.recvMemoryOnSetup(senderRank, 0));
         }
         comm.setup();
-        for(size_t senderRank = 0; senderRank < static_cast<size_t>(worldSize); ++senderRank)
+        // Now retrieve all completed futures
+        std::vector<std::shared_ptr<mscclpp::Connection>> connections;
+        connections.reserve(connectionFutures.size());
+        for(auto& cf : connectionFutures)
         {
-            if(senderRank == static_cast<size_t>(rank))
-                continue;
-            auto connection = connections[senderRank].get();
-            slave_semaphore_list[senderRank] =
-                std::make_shared<mscclpp::SmDevice2DeviceSemaphore>(comm, connection);
+            connections.push_back(cf.get());
+        }
+
+        std::vector<mscclpp::RegisteredMemory> remoteMemories;
+        remoteMemories.reserve(remoteMemFutures.size());
+        for(auto& rmf : remoteMemFutures)
+        {
+            remoteMemories.push_back(rmf.get());
         }
+
+        // Create semaphores and channels
+        // One semaphore per connection
+        std::vector<std::shared_ptr<mscclpp::SmDevice2DeviceSemaphore>> slaveSemaphores;
+        slaveSemaphores.reserve(connections.size());
+        for(auto& conn : connections)
+        {
+            slaveSemaphores.push_back(
+                std::make_shared<mscclpp::SmDevice2DeviceSemaphore>(comm, conn));
+        }
+
+        // Create channels
         std::vector<DeviceHandle<mscclpp::SmChannel>> SmChannels;
-        for(size_t i = 0; i < slave_semaphore_list.size(); ++i)
+        SmChannels.reserve(slaveSemaphores.size());
+        for(size_t i = 0; i < slaveSemaphores.size(); ++i)
         {
             SmChannels.push_back(mscclpp::deviceHandle(
-                mscclpp::SmChannel(slave_semaphore_list[i], remoteMemories[i].get(), src_data)));
+                mscclpp::SmChannel(slaveSemaphores[i],
+                                   remoteMemories[i], // Remote buffer from the sender
+                                   dst_data           // Local buffer (this slave's buffer)
+                                   )));
         }
-        hipError_t error =
+        hipError_t error_slave =
             hipMemcpyToSymbol(constSlaveSmChannels,
                               SmChannels.data(),
                               sizeof(DeviceHandle<mscclpp::SmChannel>) * SmChannels.size());
-        if(error != hipSuccess)
+        if(error_slave != hipSuccess)
         {
             std::cerr << "Error locating data to constant memory" << std::endl;
             return;
@@ -84,15 +112,34 @@ void setupConnection(
     }
     else
     {
-        auto localMemory = comm.registerMemory(dst_data, dataSize, transport);
-        mscclpp::NonblockingFuture<std::shared_ptr<mscclpp::Connection>> connection =
+        // This is a sender:
+        // We only connect to the slave, send our memory handle, and receive the slave's memory
+        // handle.
+        mscclpp::NonblockingFuture<std::shared_ptr<mscclpp::Connection>> connectionFuture =
             comm.connectOnSetup(slaveRank, 0, transport);
+        // Send our memory to the slave
         comm.sendMemoryOnSetup(localMemory, slaveRank, 0);
+
+        // Receive slave's memory
+        mscclpp::NonblockingFuture<mscclpp::RegisteredMemory> remoteMemoryFuture =
+            comm.recvMemoryOnSetup(slaveRank, 0);
         comm.setup();
-        auto sender_semaphore =
-            std::make_shared<mscclpp::SmDevice2DeviceSemaphore>(comm, connection.get());
-        auto tempSmChannel = mscclpp::SmChannel(sender_semaphore, localMemory, src_data);
-        DeviceHandle<mscclpp::SmChannel> SenderSmChannel = mscclpp::deviceHandle(tempSmChannel);
+        std::shared_ptr<mscclpp::Connection> connection = connectionFuture.get();
+        mscclpp::RegisteredMemory remoteMemory          = remoteMemoryFuture.get();
+
+        auto senderSemaphore =
+            std::make_shared<mscclpp::SmDevice2DeviceSemaphore>(comm, connection);
+
+        auto senderChannel = mscclpp::SmChannel(senderSemaphore, localMemory, remoteMemory.data());
+        DeviceHandle<mscclpp::SmChannel> senderSmChannel = mscclpp::deviceHandle(senderChannel);
+
+        hipError_t error_master = hipMemcpyToSymbol(
+            constMasterSmChannel, &senderSmChannel, sizeof(DeviceHandle<mscclpp::SmChannel>));
+        if(error_master != hipSuccess)
+        {
+            std::cerr << "Error locating data to constant memory" << std::endl;
+            return;
+        }
     }
 }