NAME

fi_shm - The SHM Fabric Provider

OVERVIEW

The SHM provider is a complete provider that can be used on Linux systems supporting shared memory and process_vm_readv/process_vm_writev calls. The provider is intended to provide high-performance communication between processes on the same system.

SUPPORTED FEATURES

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

Endpoint types

The provider supports only endpoint type FI_EP_RDM.

Endpoint capabilities

Endpoints cna support any combinations of the following data transfer capabilities: FI_MSG, FI_TAGGED, FI_RMA, amd FI_ATOMICS. These capabilities can be further defined by FI_SEND, FI_RECV, FI_READ, FI_WRITE, FI_REMOTE_READ, and FI_REMOTE_WRITE to limit the direction of operations.

Modes

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

Progress

The SHM provider supports FI_PROGRESS_MANUAL. Receive side data buffers are not modified outside of completion processing routines. The provider processes messages using three different methods, based on the size of the message. For messages smaller than 4096 bytes, tx completions are generated immediately after the send. For larger messages, tx completions are not generated until the receiving side has processed the message.

Address Format

The SHM provider uses the address format FI_ADDR_STR, which follows the general format pattern “[prefix]://[addr]”. The application can provide addresses through the node or hints parameter. As long as the address is in a valid FI_ADDR_STR format (contains “://”), the address will be used as is. If the application input is incorrectly formatted or no input was provided, the SHM provider will resolve it according to the following SHM provider standards:

(flags & FI_SOURCE) ? src_addr : dest_addr =

• if (node && service) : “fi_ns://node:service”
• if (service) : “fi_ns://service”
• if (node && !service) : “fi_shm://node”
• if (!node && !service) : “fi_shm://PID”

!(flags & FI_SOURCE)

• src_addr = “fi_shm://PID”

In other words, if the application provides a source and/or destination address in an acceptable FI_ADDR_STR format (contains “://”), the call to util_getinfo will successfully fill in src_addr and dest_addr with the provided input. If the input is not in an ADDR_STR format, the shared memory provider will then create a proper FI_ADDR_STR address with either the “fi_ns://” (node/service format) or “fi_shm://” (shm format) prefixes signaling whether the addr is a “unique” address and does or does not need an extra endpoint name identifier appended in order to make it unique. For the shared memory provider, we assume that the service (with or without a node) is enough to make it unique, but a node alone is not sufficient. If only a node is provided, the “fi_shm://” prefix is used to signify that it is not a unique address. If no node or service are provided (and in the case of setting the src address without FI_SOURCE and no hints), the process ID will be used as a default address. On endpoint creation, if the src_addr has the “fi_shm://” prefix, the provider will append “:[uid]:[ep_idx]” as a unique endpoint name (essentially, in place of a service). In the case of the “fi_ns://” prefix (or any other prefix if one was provided by the application), no supplemental information is required to make it unique and it will remain with only the application-defined address. Note that the actual endpoint name will not include the FI_ADDR_STR “*://” prefix since it cannot be included in any shared memory region names. The provider will strip off the prefix before setting the endpoint name. As a result, the addresses “fi_prefix1://my_node:my_service” and “fi_prefix2://my_node:my_service” would result in endpoints and regions of the same name. The application can also override the endpoint name after creating an endpoint using setname() without any address format restrictions.

Msg flags The provider currently only supports the FI_REMOTE_CQ_DATA msg flag.

MR registration mode The provider implements FI_MR_VIRT_ADDR memory mode.

Atomic operations The provider supports all combinations of datatype and operations as long as the message is less than 4096 bytes (or 2048 for compare operations).

DSA

Intel Data Streaming Accelerator (DSA) is an integrated accelerator in Intel Xeon processors starting with Sapphire Rapids generation. One of the capabilities of DSA is to offload memory copy operations from the CPU. A system may have one or more DSA devices. Each DSA device may have one or more work queues. The DSA specification can be found here.

The SAR protocol of SHM provider is enabled to take advantage of DSA to offload memory copy operations into and out of SAR buffers in shared memory regions. To fully take advantage of the DSA offload capability, memory copy operations are performed asynchronously. Copy initiator thread constructs the DSA commands and submits to work queues. A copy operation may consists of more than one DSA commands. In such case, commands are spread across all available work queues in round robin fashion. The progress thread checks for DSA command completions. If the copy command successfully completes, it then notifies the peer to consume the data. If DSA encountered a page fault during command execution, the page fault is reported via completion records. In such case, the progress thread accesses the page to resolve the page fault and resubmits the command after adjusting for partial completions. One of the benefits of making memory copy operations asynchronous is that now data transfers between different target endpoints can be initiated in parallel. Use of Intel DSA in SAR protocol is disabled by default and can be enabled using an environment variable. Note that CMA must be disabled, e.g. FI_SHM_DISABLE_CMA=0, in order for DSA to be used. See the RUNTIME PARAMETERS section.

Compiling with DSA capabilities depends on the accel-config library which can be found here. Running with DSA requires using Linux Kernel 5.19.0-rc3 or later.

DSA devices need to be setup just once before runtime. This configuration file can be used as a template with accel-config utility to configure the DSA devices.

LIMITATIONS

The SHM provider has hard-coded maximums for supported queue sizes and data transfers. These values are reflected in the related fabric attribute structures

EPs must be bound to both RX and TX CQs.

No support for counters.

RUNTIME PARAMETERS

The shm provider checks for the following environment variables:

FI_SHM_SAR_THRESHOLD

Maximum message size to use segmentation protocol before switching to mmap (only valid when CMA is not available). Default: SIZE_MAX (18446744073709551615)

FI_SHM_TX_SIZE

Maximum number of outstanding tx operations. Default 1024

FI_SHM_RX_SIZE

Maximum number of outstanding rx operations. Default 1024

FI_SHM_DISABLE_CMA

Manually disables CMA. Default false

FI_SHM_USE_DSA_SAR

Enables memory copy offload to Intel DSA in SAR protocol. Default false

FI_SHM_USE_XPMEM

SHM can use SAR, CMA or XPMEM for host memory transfer. If FI_SHM_USE_XPMEM is set to 1, the provider will select XPMEM over CMA if XPMEM is available. Otherwise, if neither CMA nor XPMEM are available SHM shall default to the SAR protocol. Default 0

FI_XPMEM_MEMCPY_CHUNKSIZE

The maximum size which will be used with a single memcpy call. XPMEM copy performance improves when buffers are divided into smaller chunks. This environment variable is provided to fine tune performance on different systems. Default 262144

SEE ALSO

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