NAME

fi_psm2 - The PSM2 Fabric Provider

OVERVIEW

The psm2 provider runs over the PSM 2.x interface that is supported by the Intel Omni-Path Fabric. PSM 2.x has all the PSM 1.x features plus a set of new functions with enhanced capabilities. Since PSM 1.x and PSM 2.x are not ABI compatible the psm2 provider only works with PSM 2.x and doesn’t support Intel TrueScale Fabric.

LIMITATIONS

The psm2 provider doesn’t support all the features defined in the libfabric API. Here are some of the limitations:

Endpoint types

Only support non-connection based types FI_DGRAM and FI_RDM

Endpoint capabilities

Endpoints can support any combination of data transfer capabilities FI_TAGGED, FI_MSG, FI_ATOMICS, and FI_RMA. These capabilities can be further refined by FI_SEND, FI_RECV, FI_READ, FI_WRITE, FI_REMOTE_READ, and FI_REMOTE_WRITE to limit the direction of operations.

FI_MULTI_RECV is supported for non-tagged message queue only.

Scalable endpoints are supported if the underlying PSM2 library supports multiple endpoints. This condition must be satisfied both when the provider is built and when the provider is used. See the Scalable endpoints section for more information.

Other supported capabilities include FI_TRIGGER, FI_REMOTE_CQ_DATA, FI_RMA_EVENT, FI_SOURCE, and FI_SOURCE_ERR. Furthermore, FI_NAMED_RX_CTX is supported when scalable endpoints are enabled.

Modes

FI_CONTEXT is required for the FI_TAGGED and FI_MSG capabilities. That means, any request belonging to these two categories that generates a completion must pass as the operation context a valid pointer to type struct fi_context, and the space referenced by the pointer must remain untouched until the request has completed. If none of FI_TAGGED and FI_MSG is asked for, the FI_CONTEXT mode is not required.

Progress

The psm2 provider requires manual progress. The application is expected to call fi_cq_read or fi_cntr_read function from time to time when no other libfabric function is called to ensure progress is made in a timely manner. The provider does support auto progress mode. However, the performance can be significantly impacted if the application purely depends on the provider to make auto progress.

Scalable endpoints

Scalable endpoints support depends on the multi-EP feature of the PSM2 library. If the PSM2 library supports this feature, the availability is further controlled by an environment variable PSM2_MULTI_EP. The psm2 provider automatically sets this variable to 1 if it is not set. The feature can be disabled explicitly by setting PSM2_MULTI_EP to 0.

When creating a scalable endpoint, the exact number of contexts requested should be set in the “fi_info” structure passed to the fi_scalable_ep function. This number should be set in “fi_info->ep_attr->tx_ctx_cnt” or “fi_info->ep_attr->rx_ctx_cnt” or both, whichever greater is used. The psm2 provider allocates all requested contexts upfront when the scalable endpoint is created. The same context is used for both Tx and Rx.

For optimal performance, it is advised to avoid having multiple threads accessing the same context, either directly by posting send/recv/read/write request, or indirectly by polling associated completion queues or counters.

Shared Tx contexts

In order to achieve the purpose of saving PSM context by using shared Tx context, the endpoints bound to the shared Tx contexts need to be Tx only. The reason is that Rx capability always requires a PSM context, which can also be automatically used for Tx. As the result, allocating a shared Tx context for Rx capable endpoints actually consumes one extra context instead of saving some.

Unsupported features

These features are unsupported: connection management, passive endpoint, and shared receive context.

RUNTIME PARAMETERS

The psm2 provider checks for the following environment variables:

FI_PSM2_UUID

PSM requires that each job has a unique ID (UUID). All the processes in the same job need to use the same UUID in order to be able to talk to each other. The PSM reference manual advises to keep UUID unique to each job. In practice, it generally works fine to reuse UUID as long as (1) no two jobs with the same UUID are running at the same time; and (2) previous jobs with the same UUID have exited normally. If running into “resource busy” or “connection failure” issues with unknown reason, it is advisable to manually set the UUID to a value different from the default.

The default UUID is 00FF00FF-0000-0000-0000-00FF0F0F00FF.

FI_PSM2_NAME_SERVER

The psm2 provider has a simple built-in name server that can be used to resolve an IP address or host name into a transport address needed by the fi_av_insert call. The main purpose of this name server is to allow simple client-server type applications (such as those in fabtests) to be written purely with libfabric, without using any out-of-band communication mechanism. For such applications, the server would run first to allow endpoints be created and registered with the name server, and then the client would call fi_getinfo with the node parameter set to the IP address or host name of the server. The resulting fi_info structure would have the transport address of the endpoint created by the server in the dest_addr field. Optionally the service parameter can be used in addition to node. Notice that the service number is interpreted by the provider and is not a TCP/IP port number.

The name server is on by default. It can be turned off by setting the variable to 0. This may save a small amount of resource since a separate thread is created when the name server is on.

The provider detects OpenMPI and MPICH runs and changes the default setting to off.

FI_PSM2_TAGGED_RMA

The RMA functions are implemented on top of the PSM Active Message functions. The Active Message functions have limit on the size of data can be transferred in a single message. Large transfers can be divided into small chunks and be pipe-lined. However, the bandwidth is sub-optimal by doing this way.

The psm2 provider use PSM tag-matching message queue functions to achieve higher bandwidth for large size RMA. It takes advantage of the extra tag bits available in PSM2 to separate the RMA traffic from the regular tagged message queue.

The option is on by default. To turn it off set the variable to 0.

FI_PSM2_DELAY

Time (seconds) to sleep before closing PSM endpoints. This is a workaround for a bug in some versions of PSM library.

The default setting is 0.

FI_PSM2_TIMEOUT

Timeout (seconds) for gracefully closing PSM endpoints. A forced closing will be issued if timeout expires.

The default setting is 5.

FI_PSM2_PROG_INTERVAL

When auto progress is enabled (asked via the hints to fi_getinfo), a progress thread is created to make progress calls from time to time. This option set the interval (microseconds) between progress calls.

The default setting is 1 if affinity is set, or 1000 if not. See FI_PSM2_PROG_AFFINITY.

FI_PSM2_PROG_AFFINITY

When set, specify the set of CPU cores to set the progress thread affinity to. The format is <start>[:<end>[:<stride>]][,<start>[:<end>[:<stride>]]]*, where each triplet <start>:<end>:<stride> defines a block of core_ids. Both <start> and <end> can be either the core_id (when >=0) or core_id - num_cores (when <0).

By default affinity is not set.

FI_PSM2_INJECT_SIZE

Maximum message size allowed for fi_inject and fi_tinject calls. This is an experimental feature to allow some applications to override default inject size limitation. When the inject size is larger than the default value, some inject calls might block.

The default setting is 64.

FI_PSM2_LOCK_LEVEL

When set, dictate the level of locking being used by the provider. Level 2 means all locks are enabled. Level 1 disables some locks and is suitable for runs that limit the access to each PSM2 context to a single thread. Level 0 disables all locks and thus is only suitable for single threaded runs.

To use level 0 or level 1, wait object and auto progress mode cannot be used because they introduce internal threads that may break the conditions needed for these levels.

The default setting is 2.

FI_PSM2_DISCONNECT

The provider has a mechanism to automatically send disconnection notifications to all connected peers before the local endpoint is closed. As the response, the peers call psm2_ep_disconnect to clean up the connection state at their side. This allows the same PSM2 epid be used by different dynamically started processes (clients) to communicate with the same peer (server). This mechanism, however, introduce extra overhead to the finalization phase. For applications that never reuse epids within the same session such overhead is unnecessary.

This option controls whether the automatic disconnection notification mechanism should be enabled. For client-server application mentioned above, the client side should set this option to 1, but the server should set it to 0.

The default setting is 0.

FI_PSM2_TAG_LAYOUT

Select how the 96-bit PSM2 tag bits are organized. Currently three choices are available: tag60 means 32-4-60 partitioning for CQ data, internal protocol flags, and application tag. tag64 means 4-28-64 partitioning for internal protocol flags, CQ data, and application tag. auto means to choose either tag60 or tag64 based on the the hints passed to fi_getinfo – tag60 is used if remote CQ data support is requested explicitly, either by passing non-zero value via hints->domain_attr->cq_data_size or by including FI_REMOTE_CQ_DATA in hints->caps, otherwise tag64 is used. If tag64 is the result of automatic selection, fi_getinfo also returns a second instance of the provider with tag60 layout.

The default setting is auto.

Notice that if the provider is compiled with macro PSMX2_TAG_LAYOUT defined to 1 (means tag60) or 2 (means tag64), the choice is fixed at compile time and this runtime option will be disabled.

SEE ALSO

fabric(7), fi_provider(7), fi_psm(7),