Provided by: libfabric-dev_2.1.0-1.1_amd64 
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 condi‐
tion 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 belong‐
ing 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 li‐
brary 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_at‐
tr->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, ei‐
ther directly by posting send/recv/read/write request, or indirectly by polling associated completion
queues or counters.
Using the scalable endpoint as a whole in communication functions is not supported. Instead, individual
tx context or rx context of the scalable endpoint should be used. Similarly, using the address of the
scalable endpoint as the source address or destination address doesn’t collectively address all the tx/rx
contexts. It addresses only the first tx/rx context, instead.
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 re‐
quires 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 con‐
text.
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.
It is possible to create endpoints with UUID different from the one set here. To achieve that, set `in‐
fo->ep_attr->auth_key' to the uuid value and `info->ep_attr->auth_key_size' to its size (16 bytes) when
calling fi_endpoint() or fi_scalable_ep(). It is still true that an endpoint can only communicate with
endpoints with the same UUID.
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 writ‐
ten purely with libfabric, without using any out-of-band communication mechanism. For such appli‐
cations, 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 do‐
ing 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 ver‐
sions 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 time‐
out expires.
The default setting is 5.
FI_PSM2_CONN_TIMEOUT
Timeout (seconds) for establishing connection between two PSM endpoints.
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 in‐
ternal threads that may break the conditions needed for these levels.
The default setting is 2.
FI_PSM2_LAZY_CONN
There are two strategies on when to establish connections between the PSM2 endpoints that OFI end‐
points are built on top of. In eager connection mode, connections are established when addresses
are inserted into the address vector. In lazy connection mode, connections are established when
addresses are used the first time in communication. Eager connection mode has slightly lower
critical path overhead but lazy connection mode scales better.
This option controls how the two connection modes are used. When set to 1, lazy connection mode is al‐
ways used. When set to 0, eager connection mode is used when required conditions are all met and lazy
connection mode is used otherwise. The conditions for eager connection mode are: (1) multiple endpoint
(and scalable endpoint) support is disabled by explicitly setting PSM2_MULTI_EP=0; and (2) the address
vector type is FI_AV_MAP.
The default setting is 0.
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 mecha‐
nism, 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 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_at‐
tr->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.
PSM2 EXTENSIONS
The psm2 provider supports limited low level parameter setting through the fi_set_val() and fi_get_val()
functions. Currently the following parameters can be set via the domain fid: • .RS 2
FI_PSM2_DISCONNECT *
Overwite the global runtime parameter FI_PSM2_DISCONNECT for this domain. See the RUNTIME PARAME‐
TERS section for details.
Valid parameter names are defined in the header file rdma/fi_ext_psm2.h.
SEE ALSO
fabric(7), fi_provider(7), fi_psm3(7),
AUTHORS
OpenFabrics.
Libfabric Programmer’s Manual 2024-12-10 fi_psm2(7)