fi_psm2(7) Libfabric Programmer's Manual
The PSM2 Fabric Provider
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.
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
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, and FI_SOURCE. Furthermore, FI_NAMED_RX_CTX is supported when scalable endpoints are enabled.
- 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.
- 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 has this feature, the availability is
further controlled by an environment variable PSM2_MULTI_EP, which by
default is 0 (disabled). It is important to set the environment variable
to 1 before using the scalable endpoint with the psm2 provider.
For convenience, the psm2 provider has a mechanism for application to turn on the PSM2 multi-EP feature programmatically, thus avoid the need of manually setting the environment variable. To use this feature, when the application calls fi_getinfo() the first time, it should pass a non-NULL “hints” parameter with non-NULL “ep_attr” field and with “hints->ep_attr->tx_ctx_cnt” set to be greater than 1. The psm2 provider treats this as a request to use scalable endpoint and thus sets the default value of PSM2_MULTI_EP to 1. This mechanism, however, has no effect if PSM2_MULTI_EP has already been set.
When creating a scalable endpoint, the actual number of contexts needed 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 asked contexts upfront when the scalable endpoint is created. The same context is used for both Tx and Rx.
It is important to notice that each context requires a dedicated completion queue or counter. Any attempt to bind to a context a completion queue or counter that has already been bound to another context or endpoint will fail. For optimal performance, it is also advised to avoid having multiple threads accessing the same context.
- Unsupported features
- These features are unsupported: connection management, passive endpoint, shared receive context, and send/inject with immediate data over tagged message queue.
The psm2 provider checks for the following environment variables:
- 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.
- 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,
and 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 server in the dest_addr field.
The name server won’t work properly if there are more than one processes from the same job (i.e. with the same UUID) running on the same node and acting as servers. For such scenario it is recommended to have each process getting local transport address with fi_getname and exchanging the addresses with out-of-band mechanism.
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.
- 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.
- 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 1.
- Timeout (seconds) for gracefully closing PSM endpoints. A forced closing
will be issued if timeout expires.
The default setting is 5.
- 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.
- 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
<end>can be either the
core_id(when >=0) or
core_id - num_cores(when <0).
By default affinity is not set.
- 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. This is only effective for messages. Inject size
for RMA operations is still limited to the default setting.
The default setting is 64.
- 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
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.