NCCL/RCCL tests

This example shows how to run NCCL or RCCL tests on a cluster using distributed tasks.

!!! info "Prerequisites" Before running a distributed task, make sure to create a fleet with placement set to cluster (can be a managed fleet or an SSH fleet).

Running as a task

Here's an example of a task that runs AllReduce test on 2 nodes, each with 4 GPUs (8 processes in total).

=== "NCCL tests"

<div editor-title="nccl-tests.dstack.yml">

```yaml
type: task
name: nccl-tests

nodes: 2

startup_order: workers-first
stop_criteria: master-done

env:
  - NCCL_DEBUG=INFO
commands:
  - |
    if [ $DSTACK_NODE_RANK -eq 0 ]; then
      mpirun \
        --allow-run-as-root \
        --hostfile $DSTACK_MPI_HOSTFILE \
        -n $DSTACK_GPUS_NUM \
        -N $DSTACK_GPUS_PER_NODE \
        --bind-to none \
        /opt/nccl-tests/build/all_reduce_perf -b 8 -e 8G -f 2 -g 1
    else
      sleep infinity
    fi

# Uncomment if the `kubernetes` backend requires it for `/dev/infiniband` access
#privileged: true

resources:
  gpu: nvidia:1..8
  shm_size: 16GB
```

</div>

!!! info "Default image"
    If you don't specify `image`, `dstack` uses its [base](https://github.com/dstackai/dstack/tree/master/docker/base) Docker image pre-configured with 
    `uv`, `python`, `pip`, essential CUDA drivers, `mpirun`, and NCCL tests (under `/opt/nccl-tests/build`). 

=== "RCCL tests"

<div editor-title="rccl-tests.dstack.yml">

```yaml
type: task
name: rccl-tests

nodes: 2
startup_order: workers-first
stop_criteria: master-done

# Mount the system libraries folder from the host
volumes:
  - /usr/local/lib:/mnt/lib

image: rocm/dev-ubuntu-22.04:6.4-complete
env:
  - NCCL_DEBUG=INFO
  - OPEN_MPI_HOME=/usr/lib/x86_64-linux-gnu/openmpi
commands:
  # Setup MPI and build RCCL tests
  - apt-get install -y git libopenmpi-dev openmpi-bin
  - git clone https://github.com/ROCm/rccl-tests.git
  - cd rccl-tests
  - make MPI=1 MPI_HOME=$OPEN_MPI_HOME

  # Preload the RoCE driver library from the host (for Broadcom driver compatibility)
  - export LD_PRELOAD=/mnt/lib/libbnxt_re-rdmav34.so

  # Run RCCL tests via MPI
  - |
    if [ $DSTACK_NODE_RANK -eq 0 ]; then
      mpirun --allow-run-as-root \
        --hostfile $DSTACK_MPI_HOSTFILE \
        -n $DSTACK_GPUS_NUM \
        -N $DSTACK_GPUS_PER_NODE \
        --mca btl_tcp_if_include ens41np0 \
        -x LD_PRELOAD \
        -x NCCL_IB_HCA=mlx5_0/1,bnxt_re0,bnxt_re1,bnxt_re2,bnxt_re3,bnxt_re4,bnxt_re5,bnxt_re6,bnxt_re7 \
        -x NCCL_IB_GID_INDEX=3 \
        -x NCCL_IB_DISABLE=0 \
        ./build/all_reduce_perf -b 8M -e 8G -f 2 -g 1 -w 5 --iters 20 -c 0;
    else
      sleep infinity
    fi

resources:
  gpu: MI300X:8
```

</div>

!!! info "RoCE library"
    RCCL tests use the RDMA/RoCE interconnect for internode communication. To use the RDMA/RoCE interconnect on Broadcom `bnxt_re` devices, RCCL requires the Broadcom-specific userspace provider library `libbnxt_re-rdmav34.so` to be available inside the container at `/usr/lib/x86_64-linux-gnu/libibverbs/libbnxt_re-rdmav34.so`. We make this library available by mounting it from the host and using `LD_PRELOAD` when running MPI.

    Alternatively, you can avoid `LD_PRELOAD` and directly mount `/usr/lib/x86_64-linux-gnu/libibverbs/libbnxt_re-rdmav34.so` if you use a custom image with OpenMPI pre-installed.

!!! info "Privileged" In some cases, the backend (e.g., kubernetes) may require privileged: true to access the high-speed interconnect (e.g., InfiniBand).

Apply a configuration

To run a configuration, use the dstack apply command.

$ dstack apply -f nccl-tests.dstack.yml

 #  BACKEND  REGION     INSTANCE       RESOURCES                                   SPOT  PRICE
 1  aws      us-east-1  g4dn.12xlarge  48xCPU, 192GB, 4xT4 (16GB), 100.0GB (disk)  no    \$3.912
 2  aws      us-west-2  g4dn.12xlarge  48xCPU, 192GB, 4xT4 (16GB), 100.0GB (disk)  no    \$3.912
 3  aws      us-east-2  g4dn.12xlarge  48xCPU, 192GB, 4xT4 (16GB), 100.0GB (disk)  no    \$3.912

Submit the run nccl-tests? [y/n]: y

What's next?

1. Check dev environments, tasks, services, and fleets.