10.5. InifiniBand / RoCE support

10.5.1. How are InfiniBand / RoCE devices supported in Open MPI?

Open MPI’s support for InfiniBand and RoCE devices has changed over time.

In the Open MPI v5.0.x series, InfiniBand and RoCE devices are supported via the UCX (ucx) PML.


Prior versions of Open MPI also included the openib BTL for InfiniBand and RoCE devices. Open MPI v5.0.x no longer includes the openib BTL.

10.5.2. What is UCX?

UCX is an open-source optimized communication library which supports multiple networks, including RoCE, InfiniBand, uGNI, TCP, shared memory, and others. UCX mixes-and-matches transports and protocols which are available on the system to provide optimal performance. It also has built-in support for GPU transports (with CUDA and RoCM providers) which lets RDMA-capable transports access the GPU memory directly.

10.5.3. How do I use UCX with Open MPI?

If Open MPI includes UCX support, then UCX is enabled and selected by default for InfiniBand and RoCE network devices; typically, no additional parameters are required. In this case, the network port with the highest bandwidth on the system will be used for inter-node communication, and shared memory will be used for intra-node communication. To select a specific network device to use (for example, mlx5_0 device port 1):

shell$ mpirun -x UCX_NET_DEVICES=mlx5_0:1 ...

It’s also possible to force using UCX for MPI point-to-point and one-sided operations:

shell$ mpirun --mca pml ucx --mca osc ucx ...

For OpenSHMEM, in addition to the above, it’s possible to force using UCX for remote memory access and atomic memory operations:

shell$ mpirun --mca pml ucx --mca osc ucx --mca scoll ucx --mca atomic ucx ...

10.5.4. What is RDMA over Converged Ethernet (RoCE)?

RoCE (which stands for RDMA over Converged Ethernet) provides InfiniBand native RDMA transport on top of lossless Ethernet data links.

Since we’re talking about Ethernet, there’s no Subnet Manager, no Subnet Administrator, no InfiniBand SL, nor any other InfiniBand Subnet Administration parameters.

Connection management in RoCE is based on the OFED RDMACM (RDMA Connection Manager) service:

  • The OS IP stack is used to resolve remote (IP,hostname) tuples to a DMAC.

  • The outgoing Ethernet interface and VLAN are determined according to this resolution.

  • The appropriate RoCE device is selected accordingly.

  • Network parameters (such as MTU, SL, timeout) are set locally by the RDMACM in accordance with kernel policy.

10.5.5. How do I know what MCA parameters are available for tuning MPI performance?

The ompi_info command can display all the parameters available for any Open MPI component. For example:

shell$ ompi_info --param pml ucx --level 9


Unlike most other Open MPI components, the UCX PML mainly uses environment variables for run-time tuning — not Open MPI MCA parameters. Consult the UCX documentation for details about what environment variables are available.

10.5.6. How do I tell Open MPI which IB Service Level to use?

In order to tell the UCX PML which SL to use, the IB SL must be specified using the UCX_IB_SL environment variable. For example:

shell$ mpirun --mca pml ucx -x UCX_IB_SL=N ...

The value of IB SL N should be between 0 and 15, where 0 is the default value.

10.5.7. How do I run Open MPI over RoCE?

In order to use RoCE with the UCX PML, the relevant Ethernet port must be specified using the UCX_NET_DEVICES environment variable. For example:

shell$ mpirun --mca pml ucx -x UCX_NET_DEVICES=mlx5_0:1 ...

UCX selects IPv4 RoCEv2 by default. If different behavior is needed, you can set a specific GID index:

shell$ mpirun --mca pml ucx -x UCX_NET_DEVICES=mlx5_0:1 -x UCX_IB_GID_INDEX=1 ...

10.5.8. I’m experiencing a problem with Open MPI on my InfiniBand / RoCE network; how do I troubleshoot and get help?

In order for us to help you, it is most helpful if you can run a few steps before sending an e-mail to both perform some basic troubleshooting and provide us with enough information about your environment to help you. Please include answers to the following questions in your e-mail:

  1. Which UCX and OpenFabrics version are you running? Please specify where you got the software from (e.g., from the OpenFabrics and/or UCX community web sites, already included in your Linux distribution, downloade from NVIDIA’s web site, etc.).

  2. What distro and version of Linux are you running? What is your kernel version?

  3. What is the output of the ibv_devinfo command on a known “good” node and a known “bad” node?


    There must be at least one port listed as “PORT_ACTIVE” for Open MPI to work. If there is not at least one PORT_ACTIVE port, something is wrong with your InfiniBand / RoCE environment and Open MPI will not be able to run.

  4. What is the output of the ifconfig command on a known “good” node and a known “bad” node?


    Note that some Linux distributions do not put ifconfig in the default path for normal users; look for it at /sbin/ifconfig or /usr/sbin/ifconfig.

  5. If running under Bourne shells, what is the output of the ulimit -l command?

    If running under C shells, what is the output of the limit | grep memorylocked command?


    If the value is not unlimited, ……………..


    TODO Would be good to point to some UCX/vendor docs here about setting memory limits (rather than reproducing this information ourselves).