fi_verbs - The Verbs Fabric Provider


The verbs provider enables applications using OFI to be run over any verbs hardware (Infiniband, iWarp, etc). It uses the Linux Verbs API for network transport and provides a translation of OFI calls to appropriate verbs API calls. It uses librdmacm for communication management and libibverbs for other control and data transfer operations.


The verbs provider supports a subset of OFI features.

Endpoint types


New change in libfabric v1.6: FI_EP_RDM is supported through the OFI RxM utility provider. This is done automatically when the app requests FI_EP_RDM endpoint. Please refer the man page for RxM provider to learn more. The provider’s internal support for RDM endpoints is deprecated and would be removed from libfabric v1.7 onwards. Till then apps can explicitly request the internal RDM support by disabling ofi_rxm provider through FI_PROVIDER env variable (FI_PROVIDER=^ofi_rxm).

Endpoint capabilities and features

MSG endpoints

FI_MSG, FI_RMA, FI_ATOMIC and shared receive contexts.

RDM endpoints (internal - deprecated)


DGRAM endpoints



Verbs provider requires applications to support the following modes:

FI_EP_MSG endpoint type

  • FI_LOCAL_MR / FI_MR_LOCAL mr mode.

  • FI_RX_CQ_DATA for applications that want to use RMA. Applications must take responsibility of posting receives for any incoming CQ data.

FI_EP_RDM endpoint type (internal - deprecated)


Addressing Formats

Supported addressing formats include

  • MSG and RDM (internal - deprecated) EPs support: FI_SOCKADDR, FI_SOCKADDR_IN, FI_SOCKADDR_IN6, FI_SOCKADDR_IB
  • DGRAM supports: FI_ADDR_IB_UD


Verbs provider supports FI_PROGRESS_AUTO: Asynchronous operations make forward progress automatically.

Operation flags


Msg Ordering

Verbs provider support the following message ordering:

  • Read after Read

  • Read after Write

  • Read after Send

  • Write after Write

  • Write after Send

  • Send after Write

  • Send after Send

and the following completion ordering:

  • TX contexts: FI_ORDER_STRICT
  • RX contexts: FI_ORDER_DATA


Verbs provider supports the fork system call by default. See the limitations section for restrictions. It can be turned off by setting the FI_FORK_UNSAFE environment variable to “yes”. This can improve the performance of memory registrations but it also makes the use of fork unsafe.

Memory Registration Cache

The verbs provider features a memory registration cache. This speeds up memory registration calls from applications by caching registrations of frequently used memory regions. The user can control the maximum combined size of all cache entries and the maximum number of cache entries with the environment variables FI_VERBS_MR_MAX_CACHED_SIZE and FI_VERBS_MR_MAX_CACHED_CNT respectively. Look below in the environment variables section for details.

Note: The memory registration cache framework hooks into alloc and free calls to monitor the memory regions. If this doesn’t work as expected caching would not be optimal.


Memory Regions

Only FI_MR_BASIC mode is supported. Adding regions via s/g list is supported only up to a s/g list size of 1. No support for binding memory regions to a counter.

Wait objects

Only FI_WAIT_FD wait object is supported only for FI_EP_MSG endpoint type. Wait sets are not supported.

Resource Management

Application has to make sure CQs are not overrun as this cannot be detected by the provider.

Unsupported Features

The following features are not supported in verbs provider:

Unsupported Capabilities


Other unsupported features

Scalable endpoints, FABRIC_DIRECT

Unsupported features specific to MSG endpoints

  • Counters, FI_SOURCE, FI_TAGGED, FI_PEEK, FI_CLAIM, fi_cancel, fi_ep_alias, shared TX context, cq_readfrom operations.
  • Completion flags are not reported if a request posted to an endpoint completes in error.

Unsupported features specific to RDM (internal - deprecated) endpoints

The RDM support for verbs have the following limitations:

  • Supports iovs of only size 1.

  • Wait objects are not supported.

  • Not thread safe.


The support for fork in the provider has the following limitations:

  • Fabric resources like endpoint, CQ, EQ, etc. should not be used in the forked process.
  • The memory registered using fi_mr_reg has to be page aligned since ibv_reg_mr marks the entire page that a memory region belongs to as not to be re-mapped when the process is forked (MADV_DONTFORK).

XRC Transport

The XRC transport is intended to be used when layered with the RXM provider and requires the use of shared receive contexts. See fi_rxm(7).


The verbs provider checks for the following environment variables.

Common variables:

Default maximum tx context size (default: 384)
Default maximum rx context size (default: 384)
Default maximum tx iov_limit (default: 4). Note: RDM (internal - deprecated) EP type supports only 1
Default maximum rx iov_limit (default: 4). Note: RDM (internal - deprecated) EP type supports only 1
Default maximum inline size. Actual inject size returned in fi_info may be greater (default: 64)
Set min_rnr_timer QP attribute (0 - 31) (default: 12)
Enable On-Demand-Paging (ODP) experimental feature. The feature is supported only on Mellanox OFED (default: 0)
The number of entries to be read from the verbs completion queue at a time (default: 8).
The prefix or the full name of the network interface associated with the verbs device (default: ib)
Enable Memory Registration caching (default: 0)
Maximum number of cache entries (default: 4096)
Maximum total size of cache entries (default: 4 GB)
Prioritize XRC transport fi_info before RC transport fi_info (default: 0, RC fi_info will be before XRC fi_info)

Variables specific to RDM (internal - deprecated) endpoints

The number of pre-registered buffers for buffered operations between the endpoints, must be a power of 2 (default: 8).
The maximum size of a buffered operation (bytes) (default: platform specific).
The segment size for zero copy protocols (bytes)(default: 1073741824).
The wake up timeout of the helper thread (usec) (default: 100).
The operation code that will be used for eager messaging. Only IBV_WR_SEND and IBV_WR_RDMA_WRITE_WITH_IMM are supported. The last one is not applicable for iWarp. (default: IBV_WR_SEND)
If specified, bind the CM thread to the indicated range(s) of Linux virtual processor ID(s). This option is currently not supported on OS X. Usage: id_start[-id_end[:stride]][,]

Variables specific to DGRAM endpoints

The option that enables/disables OFI Name Server thread. The NS thread is used to resolve IP-addresses to provider specific addresses (default: 1, if “OMPI_COMM_WORLD_RANK” and “PMI_RANK” environment variables aren’t defined)
The port on which Name Server thread listens incoming connections and requests (default: 5678)

Environment variables notes

The fi_info utility would give the up-to-date information on environment variables: fi_info -p verbs -e

Troubleshooting / Known issues

When running an app over verbs provider with Valgrind, there may be reports of memory leak in functions from dependent libraries (e.g. libibverbs, librdmacm). These leaks are safe to ignore.

The provider protects CQ overruns that may happen because more TX operations were posted to endpoints than CQ size. On the receive side, it isn’t expected to overrun the CQ. In case it happens the application developer should take care not to post excess receives without draining the CQ. CQ overruns can make the MSG endpoints unusable.


fabric(7), fi_provider(7),