NAME

fi_lnx - The LINKx (LNX) Provider

OVERVIEW

The LNX provider is designed to link two or more providers, allowing applications to seamlessly use multiple providers or NICs. This provider uses the libfabric peer infrastructure to aid in the use of the underlying providers. This version of the provider currently supports linking the libfabric shared memory provider for intra-node traffic and another provider for inter-node traffic. Future releases of the provider will allow linking any number of providers and provide the users with the ability to influence the way the providers are utilized for traffic load.

SUPPORTED FEATURES

This release contains an initial implementation of the LNX provider that offers the following support:

Endpoint types

The provider supports only endpoint type FI_EP_RDM.

Endpoint capabilities

LNX is a passthrough layer on the send path. On the receive path LNX utilizes the peer infrastructure to create shared receive queues (SRQ). Receive requests are placed on the SRQ instead of on the core provider receive queue. When the provider receives a message it queries the SRQ for a match. If one is found the receive request is completed, otherwise the message is placed on the LNX shared unexpected queue (SUQ). Further receive requests query the SUQ for matches. The first release of the provider only supports tagged and RMA operations. Other message types will be supported in future releases.

Modes

The provider does not require the use of any mode bits.

Progress

LNX utilizes the peer infrastructure to provide a shared completion queue. Each linked provider still needs to handle its own progress. Completion events will however be placed on the shared completion queue, which is passed to the application for access.

Address Format

LNX wraps the linked providers addresses in one common binary blob. It does not alter or change the linked providers address format. It wraps them into a LNX structure which is then flattened and returned to the application. This is passed between different nodes. The LNX provider is able to parse the flattened format and operate on the different links. This assumes that nodes in the same group are all using the same version of the provider with the exact same links. IE: you can’t have one node linking SHM+CXI while another linking SHM+RXM.

Message Operations

LNX is designed to intercept message operations such as fi_tsenddata and based on specific criteria forward the operation to the appropriate provider. For the first release, LNX will only support linking SHM provider for intra-node traffic and another provider (ex: CXI) for inter node traffic. LNX send operation looks at the destination and based on whether the destination is local or remote it will select the provider to forward the operation to. The receive case has been described earlier.

Using the Provider

In order to use the provider the user needs to set FI_LNX_PROV_LINKS environment variable to the linked providers in the following format shm+. This will allow LNX to report back to the application in the fi_getinfo() call the different links which can be selected. Since there are multiple domains per provider LNX reports a permutation of all the possible links. For example if there are two CXI interfaces on the machine LNX will report back shm+cxi0 and shm+cxi1. The application can then select based on its own criteria the link it wishes to use. The application typically uses the PCI information in the fi_info structure to select the interface to use. A common selection criteria is the interface nearest the core the process is bound to. In order to make this determination, the application requires the PCI information about the interface. For this reason LNX forwards the PCI information for the inter-node provider in the link to the application.

LIMITATIONS AND FUTURE WORK

Hardware Support

LNX doesn’t support hardware offload; ex hardware tag matching. This is an inherit limitation when using the peer infrastructure. Due to the use of a shared receive queue which linked providers need to query when a message is received, any hardware offload which requires sending the receive buffers to the hardware directly will not work with the shared receive queue. The shared receive queue provides two advantages; 1) reduce memory usage, 2) coordinate the receive operations. For #2 this is needed when receiving from FI_ADDR_UNSPEC. In this case both providers which are part of the link can race to gain access to the receive buffer. It is a future effort to determine a way to use hardware tag matching and other hardware offload capability with LNX

Limited Linking

This release of the provider supports linking SHM provider for intra-node operations and another provider which supports the FI_PEER capability for inter-node operations. It is a future effort to expand to link any multiple sets of providers.

Memory Registration

As part of the memory registration operation, varying hardware can perform hardware specific steps such as memory pinning. Due to the fact that memory registration APIs do not specify the source or destination addresses it is not possible for LNX to determine which provider to forward the memory registration to. LNX, therefore, registers the memory with all linked providers. This might not be efficient and might have unforeseen side effects. A better method is needed to support memory registration. One option is to have memory registration cache in lnx to avoid expensive operations.

Operation Types

This release of LNX supports tagged and RMA operations only. Future releases will expand the support to other operation types.

Multi-Rail

Future design effort is being planned to support utilizing multiple interfaces for traffic simultaneously. This can be over homogeneous interfaces or over heterogeneous interfaces.

RUNTIME PARAMETERS

The LNX provider checks for the following environment variables:

FI_LNX_PROV_LINKS

This environment variable is used to specify which providers to link. This must be set in order for the LNX provider to return a list of fi_info blocks in the fi_getinfo() call. The format which must be used is: <prov1>+<prov2>+… As mentioned earlier currently LNX supports linking only two providers the first of which is SHM followed by one other provider for inter-node operations

FI_LNX_DISABLE_SHM

By default this environment variable is set to 0. However, the user can set it to one and then the SHM provider will not be used. This can be useful for debugging and performance analysis. The SHM provider will naturally be used for all intra-node operations. Therefore, to test SHM in isolation with LNX, the processes can be limited to the same node only.

FI_LNX_USE_SRQ

Shared Receive Queues are integral part of the peer infrastructure, but they have the limitation of not using hardware offload, such as tag matching. SRQ is needed to support the FI_ADDR_UNSPEC case. If the application is sure this will never be the case, then it can turn off SRQ support by setting this environment variable to 0. It is 1 by default.

SEE ALSO

fabric(7), fi_provider(7), fi_getinfo(3)