Using NVIDIA SHARP with NVIDIA NCCL

NVIDIA Scalable Hierarchical Aggregation and Reduction Protocol (SHARP) Rev 3.7.0

RDMA and SHARP collectives are enabled with NVIDIA NCCL (‘nickel’) collective communication library through the NCCL-SHARP plugin.

The NCCL-SHARP plugin is distributed through the following channels:

  • Binary distribution with HPC-X. The plugin will be loaded in the environment with HPC-X modules and NCCL will load it automatically. The plugin can be built from the source of other CUDA versions.

  • Source distribution: https://github.com/Mellanox/nccl-rdma-sharp-plugins
    User can build the plugin from the source and set LD_LIBRARY_PATH to use it by NCCL.

  • NVIDIA ConnectX-6 HDR and above

  • NVIDIA Quantum HDR switch and above

  • MNLX_OFED

  • GPUDirectRDMA

    It is important to verify that the GPUDirect RDMA kernel module is properly loaded on each of the computing systems where you plan to run the job that requires the GPUDirect RDMA.

    Procedure_Heading_Icon-version-1-modificationdate-1713274321463-api-v2.PNG

    To check whether the GPUDirect RDMA module is loaded, run:

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    # service nv_peer_mem status

    Procedure_Heading_Icon-version-1-modificationdate-1713274321463-api-v2.PNG

    To run this verification on other Linux flavors:

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    # lsmod | grep nv_peer_mem

  • NCCL version 2.7.3 or higher
    Please refer to NVDIA’s Developer Guide for more details: https://docs.nvidia.com/deeplearning/sdk/nccl-developer-guide/docs/index.html

The following environment variables enable the SHARP aggregation with NCCL when using the NCCL-SHARP plugin.

  • NCCL variables:

    • NCCL_COLLNET_ENABLE=1

    • NCCL_ALGO=CollNet (Required to overcome a bug in NCCL <= 2.7.8 )

  • SHARP variables:

    • For guaranteed SAT resources on initialization: These options are enabled by default with NCCL SHARP Plugin version >= 2.1.x. Users can enable explicitly using following variables:

      • SHARP_COLL_LOCK_ON_COMM_INIT=1 (

      • SHARP_COLL_NUM_COLL_GROUP_RESOURCE_ALLOC_THRESHOLD=0

    • [Optional] SHARP_COLL_LOG_LEVEL=3

  • NCCL SHARP Plugin variables:

    • NCCL_SHARP_DISABLE

      • NCCL SHARP Streaming aggregation is supported on a single NCCL communicator/process group (PG). Applications can selectively enable SHARP on specific Process Group (PG) by setting this variable in the application before creating the PG

    • NCCL_SHARP_GROUP_SIZE_THRESH

      • Application can set this code option to selectively enable SHARP on the PG based on the group size

    • NCCL_IBEXT_DISABLE

      • NCCL plugin will be disabled and NCCL native communication transports will be used instead

On systems with multiple GPUs and multiple HCAs, NCCL creates an aggregation streaming flow (NCCL Ring/Channel) per HCA rail. It is required to build the cluster topology in such a way that leaf level switches connected to same HCA rail from each server.

The sanity performance of the setup can be verified with NCCL tests. Please refer to NCCL tests here: https://github.com/NVIDIA/nccl-tests

Example:

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$ mpirun -np 1024 -map-by ppr:8:node -x UCX_TLS=dc,shm,self -x LD_LIBRARY_PATH=/sw/nccl/build/lib::/sw/nccl-rdma-sharp-plugins/install/lib:$LD_LIBRARY_PATH -x NCCL_COLLNET_ENABLE=1 all_reduce_perf -b 4 -e 2G -f 2 -g 1 -w 50 -n 50   4 1 float sum 44.53 0.00 0.00 3e-05 44.21 0.00 0.00 3e-05 8 2 float sum 45.42 0.00 0.00 3e-05 45.85 0.00 0.00 3e-05 16 4 float sum 46.34 0.00 0.00 3e-05 45.84 0.00 0.00 2e-05 32 8 float sum 46.20 0.00 0.00 2e-05 46.56 0.00 0.00 2e-05 64 16 float sum 46.00 0.00 0.00 2e-05 48.33 0.00 0.00 2e-05 128 32 float sum 48.77 0.00 0.01 2e-05 47.23 0.00 0.01 2e-05 256 64 float sum 47.88 0.01 0.01 2e-05 47.85 0.01 0.01 2e-05 512 128 float sum 51.44 0.01 0.02 3e-05 48.66 0.01 0.02 3e-05 1024 256 float sum 51.27 0.02 0.04 4e-05 51.78 0.02 0.04 4e-05 2048 512 float sum 57.93 0.04 0.07 4e-05 56.45 0.04 0.07 4e-05 4096 1024 float sum 57.32 0.07 0.14 4e-05 93.51 0.04 0.09 4e-05 8192 2048 float sum 106.4 0.08 0.15 4e-05 59.70 0.14 0.27 4e-05 16384 4096 float sum 103.0 0.16 0.32 4e-05 58.23 0.28 0.56 4e-05 32768 8192 float sum 74.85 0.44 0.87 4e-05 137.8 0.24 0.48 4e-05 65536 16384 float sum 96.71 0.68 1.35 4e-05 92.89 0.71 1.41 4e-05 131072 32768 float sum 115.6 1.13 2.27 4e-05 120.7 1.09 2.17 4e-05 262144 65536 float sum 197.7 1.33 2.65 4e-05 167.6 1.56 3.13 4e-05 524288 131072 float sum 222.7 2.35 4.70 4e-05 239.2 2.19 4.38 4e-05 1048576 262144 float sum 280.9 3.73 7.46 4e-05 197.7 5.30 10.60 4e-05 2097152 524288 float sum 218.0 9.62 19.22 4e-05 213.9 9.81 19.59 4e-05 4194304 1048576 float sum 257.6 16.28 32.53 4e-05 254.7 16.47 32.90 4e-05 8388608 2097152 float sum 354.3 23.68 47.31 4e-05 523.5 16.02 32.02 4e-05 16777216 4194304 float sum 505.9 33.16 66.26 4e-05 484.1 34.66 69.24 4e-05 33554432 8388608 float sum 639.2 52.50 104.89 4e-05 678.6 49.45 98.80 4e-05 67108864 16777216 float sum 1358.2 49.41 98.72 4e-05 1048.6 64.00 127.87 4e-05 134217728 33554432 float sum 1737.2 77.26 154.37 4e-05 1777.6 75.51 150.86 4e-05 268435456 67108864 float sum 4359.5 61.58 123.03 4e-05 4262.3 62.98 125.83 4e-05 536870912 134217728 float sum 5619.7 95.53 190.88 4e-05 5699.0 94.20 188.22 4e-05 1073741824 268435456 float sum 12169 88.23 176.30 4e-05 11508 93.30 186.42 4e-05 2147483648 536870912 float sum 22618 94.94 189.70 4e-05 21814 98.44 196.70 4e-05 # Out of bounds values : 0 OK # Avg bus bandwidth : 41.2497 #

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