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Considering that GPUs other than A100 (such as those without InfiniBand or NVLink hardware support) have lower efficiency in multi-node parallel computing compared to single-node parallel computing, we no longer support enabling internal IP addresses for multi-node parallel computing.

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Multi-Node Multi-GPU

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Check Network Interface and IP

If the ifconfig command is not available, install it using apt-get update && apt-get install -y net-tools.
bash
root@container-b8cd119052-124c28e1:~# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 172.17.0.4  netmask 255.255.0.0  broadcast 172.17.255.255
        ether 02:42:ac:11:00:04  txqueuelen 0  (Ethernet)
        RX packets 29682  bytes 41859048 (41.8 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 29010  bytes 8195761 (8.1 MB)
        TX errors 0  dropped 0  overruns 0  carrier 0  collisions 0

eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.0.0.34  netmask 255.255.255.0  broadcast 10.0.0.255
        ether 02:42:0a:00:00:22  txqueuelen 0  (Ethernet)
        RX packets 171  bytes 9306 (9.3 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 14  bytes 1260 (1.2 KB)
        TX errors 0  dropped 0  overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 3911  bytes 13589584 (13.5 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 3911  bytes 13589584 (13.5 MB)
        TX errors 0  dropped 0  overruns 0  carrier 0  collisions 0
The network interface names may vary across different instances, so it is best to check each instance individually. In the example above, the network interface is eth1 with an IP address of 10.0.0.34. (There may be multiple network interfaces. Please select the independent IP address of the enabled instance and its corresponding network interface. The network interface name is usually eth1.)

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Testing

Download the test script: 
wget http://gpuhub-public.ks3-cn-beijing.ksyun.com/debug/ddp.py
Execute on the master node:
bash
export NCCL_SOCKET_IFNAME=eth1   # Replace with your own network interface name
# Set nnodes to the number of nodes (i.e., instances), nproc_per_node to the number of processes per node (i.e., number of GPUs), node_rank to the node number, and master_addr to the IP address of the master node's network interface. Adjust these four variables according to your actual situation. The master_port does not need to be changed.
python -m torch.distributed.launch \
    --nproc_per_node=1 \
    --nnodes=2 \
    --node_rank=0 \
    --master_addr="10.0.0.2" \
    --master_port=55568 \
    ddp.py
Execute on the worker node:
bash
export NCCL_SOCKET_IFNAME=eth1
python -m torch.distributed.launch \
    --nproc_per_node=1 \
    --nnodes=2 \
    --node_rank=1 \
    --master_addr="10.0.0.2" \
    --master_port=55568 \
    ddp.py

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Common Issues

If errors or blocks occur, first set the environment variable export NCCL_DEBUG=INFO and then run the training command to observe the NCCL debug logs. If you encounter log messages indicating a connection refusal, such as:
bash
container-xxx:152731:152731 [1] NCCL INFO Bootstrap : Using eth0:172.17.0.4<0>
container-xxx:152731:152731 [1] NCCL INFO NET/Plugin : No plugin found (libnccl-net.so), using internal implementation
container-xxx:152731:152731 [1] NCCL INFO NET/IB : No device found.
container-xxx:152731:152731 [1] NCCL INFO NET/Socket : Using [0]eth0:172.17.0.4<0> [1]eth1:172.18.0.16<0>
container-xxx:152731:152731 [1] NCCL INFO Using network Socket
container-xxx:152730:152730 [0] NCCL INFO Bootstrap : Using eth0:172.17.0.4<0>
container-xxx:152730:152730 [0] NCCL INFO NET/Plugin : No plugin found (libnccl-net.so), using internal implementation
container-xxx:152730:152730 [0] NCCL INFO NET/IB : No device found.
container-xxx:152730:152730 [0] NCCL INFO NET/Socket : Using [0]eth0:172.17.0.4<0> [1]eth1:172.18.0.16<0>
container-3e581195ae-be8c3f28:152730:152730 [0] NCCL INFO Using network Socket
container-3e581195ae-be8c3f28:152731:152775 [1] NCCL INFO Call to connect returned Connection refused, retrying
container-3e581195ae-be8c3f28:152730:152783 [0] NCCL INFO Call to connect returned Connection refused, retrying
container-3e581195ae-be8c3f28:152731:152775 [1] NCCL INFO Call to connect returned Connection refused, retrying
container-3e581195ae-be8c3f28:152730:152783 [0] NCCL INFO Call to connect returned Connection refused, retrying
It is highly likely that the NCCL_SOCKET_IFNAME environment variable is not taking effect. In this case, you can write the following content into the /etc/nccl.conf configuration file, and you will no longer need to add the environment variable separately:
bash
NCCL_SOCKET_IFNAME=eth1  # Replace with your own network interface name
NCCL_DEBUG=WARN
Refer to the official documentation: Environment Variables — NCCL 2.25.1 documentation