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.
To successfully build and install verbs provider as part of libfabric, it needs the following packages: * libibverbs * libibverbs-devel * librdmacm * librdmacm-devel
You may also want to look into any OS specific instructions for enabling RDMA. e.g. RHEL has instructions on their documentation for enabling RDMA.
The IPoIB interface should be configured with a valid IP address. This is a requirement from librdmacm.
The verbs provider supports a subset of OFI features.
FI_EP_MSG, FI_EP_DGRAM (beta), FI_EP_RDM.
FI_EP_RDM is supported via OFI RxM and RxD utility providers which are layered on top of verbs. To the app, the provider name string would appear as "verbs;ofi_rxm" or "verbs;ofi_rxd". Please refer the man pages for RxM (fi_rxm.7) and RxD (fi_rxd.7) to know about the capabilities and limitations for the FI_EP_RDM endpoint.
Endpoint capabilities and features¶
FI_MSG, FI_RMA, FI_ATOMIC and shared receive contexts.
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.
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.
Verbs provider supports FI_INJECT, FI_COMPLETION, FI_REMOTE_CQ_DATA, FI_TRANSMIT_COMPLETE.
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 does not provide fork safety by default. Fork safety can be requested by setting IBV_FORK_SAFE, or RDMAV_FORK_SAFE. If the system configuration supports the use of huge pages, it is recommended to set RDMAV_HUGEPAGES_SAFE. See ibv_fork_init(3) for additional details.
Memory Registration Cache¶
The verbs provider uses the common memory registration cache functionality that's part of libfabric utility code. This speeds up memory registration calls from applications by caching registrations of frequently used memory regions. Please refer to fi_mr(3): Memory Registration Cache section for more details.
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.
Only FI_WAIT_FD wait object is supported only for FI_EP_MSG endpoint type. Wait sets are not supported.
Application has to make sure CQs are not overrun as this cannot be detected by the provider.
The following features are not supported in verbs provider:
FI_NAMED_RX_CTX, FI_DIRECTED_RECV, FI_TRIGGER, FI_RMA_EVENT
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.
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).
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.
- 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)
- The number of entries to be read from the verbs completion queue at a time (default: 8).
- Prioritize XRC transport fi_info before RC transport fi_info (default: 0, RC fi_info will be before XRC fi_info)
- The GID index to use (default: 0)
- Specify a specific verbs device to use by name
Variables specific to MSG endpoints¶
- The prefix or the full name of the network interface associated with the verbs device (default: ib)
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¶
fi_getinfo returns -FI_ENODATA¶
- Set FI_LOG_LEVEL=info or FI_LOG_LEVEL=debug (if debug build of libfabric is available) and check if there any errors because of incorrect input parameters to fi_getinfo.
- Check if "fi_info -p verbs" is successful. If that fails the following checklist may help in ensuring that the RDMA verbs stack is functional:
- If libfabric was compiled, check if verbs provider was built. Building verbs provider would be skipped if its dependencies (listed in requirements) aren't available on the system.
- Verify verbs device is functional:
- Does ibv_rc_pingpong (available in libibverbs) test work?
- Does ibv_devinfo (available in libibverbs) show the device with PORT_ACTIVE status?
- Check if Subnet Manager (SM) is running on the switch or on one of the nodes in the cluster.
- Is the cable connected?
- Verify librdmacm is functional:
- Does ucmatose test (available in librdmacm) work?
- Is the IPoIB interface (e.g. ib0) up and configured with a valid IP address?
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.
|2020-04-14||Libfabric Programmer's Manual|