NAME

fi_opx - The Omni-Path Express Fabric Provider

OVERVIEW

The OPX provider is a native implementation of the libfabric interfaces that makes direct use of Omni-Path fabrics as well as libfabric acceleration features. The purpose of this provider is to show the scalability and performance of libfabric, providing an “extreme scale” development environment for applications and middleware using the libfabric API, and to support a functional and performant version of MPI on Omni-Path fabrics.

SUPPORTED FEATURES

The OPX provider supports most features defined for the libfabric API.

Key features include:

Endpoint types: The Omni-Path HFI hardware is connectionless and reliable. The OPX provider only supports the FI_EP_RDM endpoint type.
Capabilities: Supported capabilities include FI_MSG, FI_RMA, *FI_TAGGED, FI_ATOMIC, FI_NAMED_RX_CTX, FI_SOURCE, FI_SEND, FI_RECV, FI_MULTI_RECV, FI_DIRECTED_RECV, FI_SOURCE.
Notes on FI_DIRECTED_RECV capability: The immediate data which is sent within the “senddata” call to support FI_DIRECTED_RECV for OPX must be exactly 4 bytes, which OPX uses to completely identify the source address to an exascale-level number of ranks for tag matching on the recv and can be managed within the MU packet. Therefore the domain attribute “cq_data_size” is set to 4 which is the OFI standard minimum.
Modes: Two modes are defined: FI_CONTEXT2 and FI_ASYNC_IOV. The OPX provider requires FI_CONTEXT2.
Additional features: Supported additional features include FABRIC_DIRECT, scalable endpoints, and counters.
Progress: Only FI_PROGRESS_MANUAL is supported.
Address vector: Only the FI_AV_MAP address vector format is supported.
Memory registration modes: Only FI_MR_SCALABLE is supported.

UNSUPPORTED FEATURES

Endpoint types: Unsupported endpoint types include FI_EP_DGRAM and FI_EP_MSG.
Capabilities: The OPX provider does not support FI_RMA_EVENT and FI_TRIGGER capabilities.
Address vector: The OPX provider does not support the FI_AV_TABLE address vector format. This may be added in the future.

LIMITATIONS

As OPX is under development this list of limitations is subject to change.

It runs under the following MPI versions:

Intel MPI from Parallel Studio 2020, update 4. Intel MPI from OneAPI 2021, update 3. Open MPI 4.1.2a1 (Older version of Open MPI will not work). MPICH 3.4.2.

Currently, this provider is PIO-only. SDMA is not supported at this time.

Usage:

If using with OpenMPI 4.1.x, disable UCX and openib transports. OPX is not compatible with Open MPI 4.1.x PML/BTL. DMA, RDMA and SDMA are not implemented. Performance falls off when using message sizes larger than 1 MTU (4K max size). Shared memory is not cleaned up after an application crashes. Use “rm -rf /dev/shm/*” to remove old shared-memory files.

RUNTIME PARAMETERS

FI_OPX_UUID

OPX requires a unique ID for each job. In order for all processes in a job to communicate with each other, they require to use the same UUID. This variable can be set with FI_OPX_UUID=${RANDOM} The default UUID is 00112233445566778899aabbccddeeff.

FI_OPX_RELIABILITY_SERVICE_USEC_MAX

This setting controls how frequently the reliability/replay function will issue PING requests to a remote connection. Reducing this value may improve performance at the expense of increased traffic on the OPX fabric. Default setting is 500.

FI_OPX_RELIABILITY_SERVICE_PRE_ACK_RATE

This setting controls how frequently a receiving rank will send ACKs for packets it has received without being prompted through a PING request. A non-zero value N tells the receiving rank to send an ACK for the last N packets every Nth packet. Used in conjunction with an increased value for FI_OPX_RELIABILITY_SERVICE_USEC_MAX may improve performance.

Valid values are 0 (disabled) and powers of 2 in the range of 1-32,768, inclusive.

Default setting is 64.

FI_OPX_HFI_SELECT

Controls how OPX chooses which HFI to use when opening a context. Has two forms:

<hfi-unit> Force OPX provider to use hfi-unit.
<selector1>[,<selector2>[,...,<selectorN>]] Select HFI based on first matching selector

Where selector is one of the following forms:

default to use the default logic
fixed:<hfi-unit> to fix to one hfi-unit
<selector-type>:<hfi-unit>:<selector-data>

The above fields have the following meaning:

selector-type The selector criteria the caller opening the context is evaluated against.
hfi-unit The HFI to use if the caller matches the selector.
selector-data Data the caller must match (e.g. NUMA node ID).

Where selector-type is one of the following:

numa True when caller is local to the NUMA node ID given by selector-data.
core True when caller is local to the CPU core given by selector-data.

And selector-data is one of the following:

value The specific value to match
<range-start>-<range-end> Matches with any value in that range

In the second form, when opening a context, OPX uses the hfi-unit of the first-matching selector. Selectors are evaluated left-to-right. OPX will return an error if the caller does not match any selector.

In either form, it is an error if the specified or selected HFI is not in the Active state. In this case, OPX will return an error and execution will not continue.

With this option, it is possible to cause OPX to try to open more contexts on an HFI than there are free contexts on that HFI. In this case, one or more of the context-opening calls will fail and OPX will return an error. For the second form, as which HFI is selected depends on properties of the caller, deterministic HFI selection requires deterministic caller properties. E.g. for the numa selector, if the caller can migrate between NUMA domains, then HFI selection will not be deterministic.

The logic used will always be the first valid in a selector list. For example, default and fixed will match all callers, so if either are in the beginning of a selector list, you will only use fixed or default regardles of if there are any more selectors.

Examples:

FI_OPX_HFI_SELECT=1 all callers will open contexts on HFI 0.
FI_OPX_HFI_SELECT=numa:0:0,numa:1:1,numa:0:2,numa:1:3 callers local to NUMA nodes 0 and 2 will use HFI 0, callers local to NUMA domains 1 and 3 will use HFI 1.
FI_OPX_HFI_SELECT=numa:0:0-3,default callers local to NUMA nodes 0 thru 3 (including 0 and 3) will use HFI 0, and all else will use default selection logic.
FI_OPX_HFI_SELECT=core:1:0,fixed:0 callers local to CPU core 0 will use HFI 1, and all others will use HFI 0.
FI_OPX_HFI_SELECT=default,core:1:0 all callers will use default HFI selection logic.