Scale-Out via Host-NIC

Multi-node scale-out for Intel® Gaudi® AI accelerator accelerator devices via host NIC interfaces is enabled. This functionality is provided by HCCL library and activated either by an internal auto-detection mechanism or explicit selection by the user. Data transfer between nodes is supported with Scale-Out via Host-NIC over OFI mode.

Scale-Out Auto-detection

By default, HCCL runs internal auto-detection and selection logic to select and use the most efficient method for scaling between nodes. Mode selection is done with the below priorities:


Scale-Out Mode



Gaudi NICs

Gaudi scale-out NICs are connected.


Host NIC Gaudi Direct (GDR)

  • Verbs or AWS EFA OFI provider is available:

    • EFA: libfabric version 1.16.1 or later

    • Verbs: libfabric version 1.20.0.

  • Linux kernel version 5.12 or later.


Host NICs using host memory

libfabric is available with one of the following providers: “efa”, “verbs” or “tcp”. To use “verbs” provider, refer to Enabling InfiniBand NICs (Verbs) for Host NIC Scaling.


No Scale-Out


  • HCCL auto-detection mechanism is disabled when setting HCCL_OVER_OFI env variable (force P2).

  • Host NIC GDR with Verbs provider is supported on Gaudi 2 only.

Host NIC Gaudi Direct Setup

  • To enable Host NIC Gaudi Direct on AWS EFA, set RDMAV_FORK_SAFE=1 and FI_EFA_USE_DEVICE_RDMA=1 environment variables.

  • To enable Host NIC Gaudi Direct on Verbs:

    • Set RDMAV_FORK_SAFE=1 and MLX5_SCATTER_TO_CQE=0 environment variables.

    • Disable PCIe Access Control (ACS).

    • Use libfabric version 1.20.0.

    • Build libfabric using --with-synapseai configuration option.

Scale-Out via Host-NIC over OFI

HCCL interacts with libFabric to utilize any underlying HW and networking mode.

Configuration Knobs

HCCL exports several environment variables that control the behavior of scale-out communication over libFabric. The table below lists the available environment variables.

Environment Variable



Identifies the network interface(s) that should be used for scale-out comms.


Identifies the root process (rank 0) of the global communicator group. Typically set to <IPaddress:port> the IP address of the network interface used by the root. This must be set for all HCCL processes when there is no alternate network to broadcast this.


  1. Download HCCL OFI Wrapper.

  2. Build and install libFabric.

  3. Build HCCL OFI Wrapper.


See additional instructions in HCCL OFI Wrapper page.


If you are using Intel Gaudi Containers, the above steps are not required as OFI Wrapper and libFabric are already installed by default.