NAME

fi_cq - Completion queue operations

fi_cq_open / fi_close: Open/close a completion queue
fi_control: Control CQ operation or attributes.
fi_cq_read / fi_cq_readfrom / fi_cq_readerr: Read a completion from a completion queue
fi_cq_sread / fi_cq_sreadfrom: A synchronous (blocking) read that waits until a specified condition has been met before reading a completion from a completion queue.
fi_cq_signal: Unblock any thread waiting in fi_cq_sread or fi_cq_sreadfrom.
fi_cq_strerror: Converts provider specific error information into a printable string

SYNOPSIS

#include <rdma/fi_domain.h>

int fi_cq_open(struct fid_domain *domain, struct fi_cq_attr *attr,
    struct fid_cq **cq, void *context);

int fi_close(struct fid *cq);

int fi_control(struct fid *cq, int command, void *arg);

ssize_t fi_cq_read(struct fid_cq *cq, void *buf, size_t count);

ssize_t fi_cq_readfrom(struct fid_cq *cq, void *buf, size_t count,
    fi_addr_t *src_addr);

ssize_t fi_cq_readerr(struct fid_cq *cq, struct fi_cq_err_entry *buf,
    uint64_t flags);

ssize_t fi_cq_sread(struct fid_cq *cq, void *buf, size_t count,
    const void *cond, int timeout);

ssize_t fi_cq_sreadfrom(struct fid_cq *cq, void *buf, size_t count,
    fi_addr_t *src_addr, const void *cond, int timeout);

int fi_cq_signal(struct fid_cq *cq);

const char * fi_cq_strerror(struct fid_cq *cq, int prov_errno,
      const void *err_data, char *buf, size_t len);

ARGUMENTS

domain: Open resource domain
cq: Completion queue
attr: Completion queue attributes
context: User specified context associated with the completion queue.
buf: For read calls, the data buffer to write completions into. For write calls, a completion to insert into the completion queue. For fi_cq_strerror, an optional buffer that receives printable error information.
count: Number of CQ entries.
len: Length of data buffer
src_addr: Source address of a completed receive operation
flags: Additional flags to apply to the operation
command: Command of control operation to perform on CQ.
arg: Optional control argument
cond: Condition that must be met before a completion is generated
timeout: Time in milliseconds to wait. A negative value indicates infinite timeout.
prov_errno: Provider specific error value
err_data: Provider specific error data related to a completion

DESCRIPTION

Completion queues are used to report events associated with data transfers. They are associated with message sends and receives, RMA, atomic, tagged messages, and triggered events. Reported events are usually associated with a fabric endpoint, but may also refer to memory regions used as the target of an RMA or atomic operation.

fi_cq_open

fi_cq_open allocates a new completion queue. Unlike event queues, completion queues are associated with a resource domain and may be offloaded entirely in provider hardware.

The properties and behavior of a completion queue are defined by struct fi_cq_attr.

struct fi_cq_attr {
	size_t               size;      /* # entries for CQ */
	uint64_t             flags;     /* operation flags */
	enum fi_cq_format    format;    /* completion format */
	enum fi_wait_obj     wait_obj;  /* requested wait object */
	int                  signaling_vector; /* interrupt affinity */
	enum fi_cq_wait_cond wait_cond; /* wait condition format */
	struct fid_wait     *wait_set;  /* optional wait set */
};

size: Specifies the minimum size of a completion queue. A value of 0 indicates that the provider may choose a default value.
flags: Flags that control the configuration of the CQ.

FI_AFFINITY

Indicates that the signaling_vector field (see below) is valid.

format: Completion queues allow the application to select the amount of detail that it must store and report. The format attribute allows the application to select one of several completion formats, indicating the structure of the data that the completion queue should return when read. Supported formats and the structures that correspond to each are listed below. The meaning of the CQ entry fields are defined in the Completion Fields section.

FI_CQ_FORMAT_UNSPEC

If an unspecified format is requested, then the CQ will use a provider selected default format.
FI_CQ_FORMAT_CONTEXT

Provides only user specified context that was associated with the completion.

struct fi_cq_entry {
	void     *op_context; /* operation context */
};

FI_CQ_FORMAT_MSG

Provides minimal data for processing completions, with expanded support for reporting information about received messages.

struct fi_cq_msg_entry {
	void     *op_context; /* operation context */
	uint64_t flags;       /* completion flags */
	size_t   len;         /* size of received data */
};

FI_CQ_FORMAT_DATA

Provides data associated with a completion. Includes support for received message length, remote CQ data, and multi-receive buffers.

struct fi_cq_data_entry {
	void     *op_context; /* operation context */
	uint64_t flags;       /* completion flags */
	size_t   len;         /* size of received data */
	void     *buf;        /* receive data buffer */
	uint64_t data;        /* completion data */
};

FI_CQ_FORMAT_TAGGED

Expands completion data to include support for the tagged message interfaces.

struct fi_cq_tagged_entry {
	void     *op_context; /* operation context */
	uint64_t flags;       /* completion flags */
	size_t   len;         /* size of received data */
	void     *buf;        /* receive data buffer */
	uint64_t data;        /* completion data */
	uint64_t tag;         /* received tag */
};

wait_obj: CQ’s may be associated with a specific wait object. Wait objects allow applications to block until the wait object is signaled, indicating that a completion is available to be read. Users may use fi_control to retrieve the underlying wait object associated with a CQ, in order to use it in other system calls. The following values may be used to specify the type of wait object associated with a CQ: FI_WAIT_NONE, FI_WAIT_UNSPEC, FI_WAIT_SET, FI_WAIT_FD, FI_WAIT_MUTEX_COND, and FI_WAIT_YIELD. The default is FI_WAIT_NONE.

FI_WAIT_NONE

Used to indicate that the user will not block (wait) for completions on the CQ. When FI_WAIT_NONE is specified, the application may not call fi_cq_sread or fi_cq_sreadfrom.
FI_WAIT_UNSPEC

Specifies that the user will only wait on the CQ using fabric interface calls, such as fi_cq_sread or fi_cq_sreadfrom. In this case, the underlying provider may select the most appropriate or highest performing wait object available, including custom wait mechanisms. Applications that select FI_WAIT_UNSPEC are not guaranteed to retrieve the underlying wait object.
FI_WAIT_SET

Indicates that the completion queue should use a wait set object to wait for completions. If specified, the wait_set field must reference an existing wait set object.
FI_WAIT_FD

Indicates that the CQ should use a file descriptor as its wait mechanism. A file descriptor wait object must be usable in select, poll, and epoll routines. However, a provider may signal an FD wait object by marking it as readable, writable, or with an error.
FI_WAIT_MUTEX_COND

Specifies that the CQ should use a pthread mutex and cond variable as a wait object.
FI_WAIT_YIELD

Indicates that the CQ will wait without a wait object but instead yield on every wait. Allows usage of fi_cq_sread and fi_cq_sreadfrom through a spin.

signaling_vector

If the FI_AFFINITY flag is set, this indicates the logical cpu number (0..max cpu - 1) that interrupts associated with the CQ should target. This field should be treated as a hint to the provider and may be ignored if the provider does not support interrupt affinity.

wait_cond

By default, when a completion is inserted into a CQ that supports blocking reads (fi_cq_sread/fi_cq_sreadfrom), the corresponding wait object is signaled. Users may specify a condition that must first be met before the wait is satisfied. This field indicates how the provider should interpret the cond field, which describes the condition needed to signal the wait object.

A wait condition should be treated as an optimization. Providers are not required to meet the requirements of the condition before signaling the wait object. Applications should not rely on the condition necessarily being true when a blocking read call returns.

If wait_cond is set to FI_CQ_COND_NONE, then no additional conditions are applied to the signaling of the CQ wait object, and the insertion of any new entry will trigger the wait condition. If wait_cond is set to FI_CQ_COND_THRESHOLD, then the cond field is interpreted as a size_t threshold value. The threshold indicates the number of entries that are to be queued before at the CQ before the wait is satisfied.

This field is ignored if wait_obj is set to FI_WAIT_NONE.

wait_set

If wait_obj is FI_WAIT_SET, this field references a wait object to which the completion queue should attach. When an event is inserted into the completion queue, the corresponding wait set will be signaled if all necessary conditions are met. The use of a wait_set enables an optimized method of waiting for events across multiple event and completion queues. This field is ignored if wait_obj is not FI_WAIT_SET.

fi_close

The fi_close call releases all resources associated with a completion queue. Any completions which remain on the CQ when it is closed are lost.

When closing the CQ, there must be no opened endpoints, transmit contexts, or receive contexts associated with the CQ. If resources are still associated with the CQ when attempting to close, the call will return -FI_EBUSY.

fi_control

The fi_control call is used to access provider or implementation specific details of the completion queue. Access to the CQ should be serialized across all calls when fi_control is invoked, as it may redirect the implementation of CQ operations. The following control commands are usable with a CQ.

FI_GETWAIT (void **): This command allows the user to retrieve the low-level wait object associated with the CQ. The format of the wait-object is specified during CQ creation, through the CQ attributes. The fi_control arg parameter should be an address where a pointer to the returned wait object will be written. See fi_eq.3 for addition details using fi_control with FI_GETWAIT.

fi_cq_read

The fi_cq_read operation performs a non-blocking read of completion data from the CQ. The format of the completion event is determined using the fi_cq_format option that was specified when the CQ was opened. Multiple completions may be retrieved from a CQ in a single call. The maximum number of entries to return is limited to the specified count parameter, with the number of entries successfully read from the CQ returned by the call. (See return values section below.) A count value of 0 may be used to drive progress on associated endpoints when manual progress is enabled.

CQs are optimized to report operations which have completed successfully. Operations which fail are reported ‘out of band’. Such operations are retrieved using the fi_cq_readerr function. When an operation that has completed with an unexpected error is encountered, it is placed into a temporary error queue. Attempting to read from a CQ while an item is in the error queue results in fi_cq_read failing with a return code of -FI_EAVAIL. Applications may use this return code to determine when to call fi_cq_readerr.

fi_cq_readfrom

The fi_cq_readfrom call behaves identical to fi_cq_read, with the exception that it allows the CQ to return source address information to the user for any received data. Source address data is only available for those endpoints configured with FI_SOURCE capability. If fi_cq_readfrom is called on an endpoint for which source addressing data is not available, the source address will be set to FI_ADDR_NOTAVAIL. The number of input src_addr entries must be the same as the count parameter.

Returned source addressing data is converted from the native address used by the underlying fabric into an fi_addr_t, which may be used in transmit operations. Under most circumstances, returning fi_addr_t requires that the source address already have been inserted into the address vector associated with the receiving endpoint. This is true for address vectors of type FI_AV_TABLE. In select providers when FI_AV_MAP is used, source addresses may be converted algorithmically into a usable fi_addr_t, even though the source address has not been inserted into the address vector. This is permitted by the API, as it allows the provider to avoid address look-up as part of receive message processing. In no case do providers insert addresses into an AV separate from an application calling fi_av_insert or similar call.

For endpoints allocated using the FI_SOURCE_ERR capability, if the source address cannot be converted into a valid fi_addr_t value, fi_cq_readfrom will return -FI_EAVAIL, even if the data were received successfully. The completion will then be reported through fi_cq_readerr with error code -FI_EADDRNOTAVAIL. See fi_cq_readerr for details.

If FI_SOURCE is specified without FI_SOURCE_ERR, source addresses which cannot be mapped to a usable fi_addr_t will be reported as FI_ADDR_NOTAVAIL.

fi_cq_sread / fi_cq_sreadfrom

The fi_cq_sread and fi_cq_sreadfrom calls are the blocking equivalent operations to fi_cq_read and fi_cq_readfrom. Their behavior is similar to the non-blocking calls, with the exception that the calls will not return until either a completion has been read from the CQ or an error or timeout occurs.

Threads blocking in this function will return to the caller if they are signaled by some external source. This is true even if the timeout has not occurred or was specified as infinite.

It is invalid for applications to call these functions if the CQ has been configured with a wait object of FI_WAIT_NONE or FI_WAIT_SET.

fi_cq_readerr

The read error function, fi_cq_readerr, retrieves information regarding any asynchronous operation which has completed with an unexpected error. fi_cq_readerr is a non-blocking call, returning immediately whether an error completion was found or not.

Error information is reported to the user through struct fi_cq_err_entry. The format of this structure is defined below.

struct fi_cq_err_entry {
	void     *op_context; /* operation context */
	uint64_t flags;       /* completion flags */
	size_t   len;         /* size of received data */
	void     *buf;        /* receive data buffer */
	uint64_t data;        /* completion data */
	uint64_t tag;         /* message tag */
	size_t   olen;        /* overflow length */
	int      err;         /* positive error code */
	int      prov_errno;  /* provider error code */
	void    *err_data;    /*  error data */
	size_t   err_data_size; /* size of err_data */
};

The general reason for the error is provided through the err field. Provider specific error information may also be available through the prov_errno and err_data fields. Users may call fi_cq_strerror to convert provider specific error information into a printable string for debugging purposes. See field details below for more information on the use of err_data and err_data_size.

Note that error completions are generated for all operations, including those for which a completion was not requested (e.g. an endpoint is configured with FI_SELECTIVE_COMPLETION, but the request did not have the FI_COMPLETION flag set). In such cases, providers will return as much information as made available by the underlying software and hardware about the failure, other fields will be set to NULL or 0. This includes the op_context value, which may not have been provided or was ignored on input as part of the transfer.

Notable completion error codes are given below.

FI_EADDRNOTAVAIL: This error code is used by CQs configured with FI_SOURCE_ERR to report completions for which a usable fi_addr_t source address could not be found. An error code of FI_EADDRNOTAVAIL indicates that the data transfer was successfully received and processed, with the fi_cq_err_entry fields containing information about the completion. The err_data field will be set to the source address data. The source address will be in the same format as specified through the fi_info addr_format field for the opened domain. This may be passed directly into an fi_av_insert call to add the source address to the address vector.

fi_cq_signal

The fi_cq_signal call will unblock any thread waiting in fi_cq_sread or fi_cq_sreadfrom. This may be used to wake-up a thread that is blocked waiting to read a completion operation. The fi_cq_signal operation is only available if the CQ was configured with a wait object.

COMPLETION FIELDS

The CQ entry data structures share many of the same fields. The meanings of these fields are the same for all CQ entry structure formats.

op_context: The operation context is the application specified context value that was provided with an asynchronous operation. The op_context field is valid for all completions that are associated with an asynchronous operation.
For completion events that are not associated with a posted operation, this field will be set to NULL. This includes completions generated at the target in response to RMA write operations that carry CQ data (FI_REMOTE_WRITE | FI_REMOTE_CQ_DATA flags set), when the FI_RX_CQ_DATA mode bit is not required.
flags: This specifies flags associated with the completed operation. The Completion Flags section below lists valid flag values. Flags are set for all relevant completions.
len: This len field only applies to completed receive operations (e.g. fi_recv, fi_trecv, etc.). It indicates the size of received message data – i.e. how many data bytes were placed into the associated receive buffer by a corresponding fi_send/fi_tsend/et al call. If an endpoint has been configured with the FI_MSG_PREFIX mode, the len also reflects the size of the prefix buffer.
buf: The buf field is only valid for completed receive operations, and only applies when the receive buffer was posted with the FI_MULTI_RECV flag. In this case, buf points to the starting location where the receive data was placed.
data: The data field is only valid if the FI_REMOTE_CQ_DATA completion flag is set, and only applies to receive completions. If FI_REMOTE_CQ_DATA is set, this field will contain the completion data provided by the peer as part of their transmit request. The completion data will be given in host byte order.
tag: A tag applies only to received messages that occur using the tagged interfaces. This field contains the tag that was included with the received message. The tag will be in host byte order.
olen: The olen field applies to received messages. It is used to indicate that a received message has overrun the available buffer space and has been truncated. The olen specifies the amount of data that did not fit into the available receive buffer and was discarded.
err: This err code is a positive fabric errno associated with a completion. The err value indicates the general reason for an error, if one occurred. See fi_errno.3 for a list of possible error codes.
prov_errno: On an error, prov_errno may contain a provider specific error code. The use of this field and its meaning is provider specific. It is intended to be used as a debugging aid. See fi_cq_strerror for additional details on converting this error value into a human readable string.
err_data: The err_data field is used to return provider specific information, if available, about the error. On input, err_data should reference a data buffer of size err_data_size. On output, the provider will fill in this buffer with any provider specific data which may help identify the cause of the error. The contents of the err_data field and its meaning is provider specific. It is intended to be used as a debugging aid. See fi_cq_strerror for additional details on converting this error data into a human readable string. See the compatibility note below on how this field is used for older libfabric releases.
err_data_size: On input, err_data_size indicates the size of the err_data buffer in bytes. On output, err_data_size will be set to the number of bytes copied to the err_data buffer. The err_data information is typically used with fi_cq_strerror to provide details about the type of error that occurred.
For compatibility purposes, the behavior of the err_data and err_data_size fields is may be modified from that listed above. If err_data_size is 0 on input, or the fabric was opened with release < 1.5, then any buffer referenced by err_data will be ignored on input. In this situation, on output err_data will be set to a data buffer owned by the provider. The contents of the buffer will remain valid until a subsequent read call against the CQ. Applications must serialize access to the CQ when processing errors to ensure that the buffer referenced by err_data does not change.

COMPLETION FLAGS

Completion flags provide additional details regarding the completed operation. The following completion flags are defined.

FI_SEND: Indicates that the completion was for a send operation. This flag may be combined with an FI_MSG or FI_TAGGED flag.
FI_RECV: Indicates that the completion was for a receive operation. This flag may be combined with an FI_MSG or FI_TAGGED flag.
FI_RMA: Indicates that an RMA operation completed. This flag may be combined with an FI_READ, FI_WRITE, FI_REMOTE_READ, or FI_REMOTE_WRITE flag.
FI_ATOMIC: Indicates that an atomic operation completed. This flag may be combined with an FI_READ, FI_WRITE, FI_REMOTE_READ, or FI_REMOTE_WRITE flag.
FI_MSG: Indicates that a message-based operation completed. This flag may be combined with an FI_SEND or FI_RECV flag.
FI_TAGGED: Indicates that a tagged message operation completed. This flag may be combined with an FI_SEND or FI_RECV flag.
FI_MULTICAST: Indicates that a multicast operation completed. This flag may be combined with FI_MSG and relevant flags. This flag is only guaranteed to be valid for received messages if the endpoint has been configured with FI_SOURCE.
FI_READ: Indicates that a locally initiated RMA or atomic read operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.
FI_WRITE: Indicates that a locally initiated RMA or atomic write operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.
FI_REMOTE_READ: Indicates that a remotely initiated RMA or atomic read operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.
FI_REMOTE_WRITE: Indicates that a remotely initiated RMA or atomic write operation has completed. This flag may be combined with an FI_RMA or FI_ATOMIC flag.
FI_REMOTE_CQ_DATA: This indicates that remote CQ data is available as part of the completion.
FI_MULTI_RECV: This flag applies to receive buffers that were posted with the FI_MULTI_RECV flag set. This completion flag indicates that the original receive buffer referenced by the completion has been consumed and was released by the provider. Providers may set this flag on the last message that is received into the multi- recv buffer, or may generate a separate completion that indicates that the buffer has been released.
Applications can distinguish between these two cases by examining the completion entry flags field. If additional flags, such as FI_RECV, are set, the completion is associated with a received message. In this case, the buf field will reference the location where the received message was placed into the multi-recv buffer. Other fields in the completion entry will be determined based on the received message. If other flag bits are zero, the provider is reporting that the multi-recv buffer has been released, and the completion entry is not associated with a received message.
FI_MORE: See the ‘Buffered Receives’ section in fi_msg(3) for more details. This flag is associated with receive completions on endpoints that have FI_BUFFERED_RECV mode enabled. When set to one, it indicates that the buffer referenced by the completion is limited by the FI_OPT_BUFFERED_LIMIT threshold, and additional message data must be retrieved by the application using an FI_CLAIM operation.
FI_CLAIM: See the ‘Buffered Receives’ section in fi_msg(3) for more details. This flag is set on completions associated with receive operations that claim buffered receive data. Note that this flag only applies to endpoints configured with the FI_BUFFERED_RECV mode bit.

COMPLETION EVENT SEMANTICS

Libfabric defines several completion ‘levels’, identified using operational flags. Each flag indicates the soonest that a completion event may be generated by a provider, and the assumptions that an application may make upon processing a completion. The operational flags are defined below, along with an example of how a provider might implement the semantic. Note that only meeting the semantic is required of the provider and not the implementation. Providers may implement stronger completion semantics than necessary for a given operation, but only the behavior defined by the completion level is guaranteed.

To help understand the conceptual differences in completion levels, consider mailing a letter. Placing the letter into the local mailbox for pick-up is similar to ‘inject complete’. Having the letter picked up and dropped off at the destination mailbox is equivalent to ‘transmit complete’. The ‘delivery complete’ semantic is a stronger guarantee, with a person at the destination signing for the letter. However, the person who signed for the letter is not necessarily the intended recipient. The ‘match complete’ option is similar to delivery complete, but requires the intended recipient to sign for the letter.

The ‘commit complete’ level has different semantics than the previously mentioned levels. Commit complete would be closer to the letter arriving at the destination and being placed into a fire proof safe.

The operational flags for the described completion levels are defined below.

FI_INJECT_COMPLETE

Indicates that a completion should be generated when the source buffer(s) may be reused. A completion guarantees that the buffers will not be read from again and the application may reclaim them. No other guarantees are made with respect to the state of the operation.

Example: A provider may generate this completion event after copying the source buffer into a network buffer, either in host memory or on the NIC. An inject completion does not indicate that the data has been transmitted onto the network, and a local error could occur after the completion event has been generated that could prevent it from being transmitted.

Inject complete allows for the fastest completion reporting (and, hence, buffer reuse), but provides the weakest guarantees against network errors.

Note: This flag is used to control when a completion entry is inserted into a completion queue. It does not apply to operations that do not generate a completion queue entry, such as the fi_inject operation, and is not subject to the inject_size message limit restriction.

FI_TRANSMIT_COMPLETE

Indicates that a completion should be generated when the transmit operation has completed relative to the local provider. The exact behavior is dependent on the endpoint type.

For reliable endpoints:

Indicates that a completion should be generated when the operation has been delivered to the peer endpoint. A completion guarantees that the operation is no longer dependent on the fabric or local resources. The state of the operation at the peer endpoint is not defined.

Example: A provider may generate a transmit complete event upon receiving an ack from the peer endpoint. The state of the message at the peer is unknown and may be buffered in the target NIC at the time the ack has been generated.

For unreliable endpoints:

Indicates that a completion should be generated when the operation has been delivered to the fabric. A completion guarantees that the operation is no longer dependent on local resources. The state of the operation within the fabric is not defined.

FI_DELIVERY_COMPLETE

Indicates that a completion should not be generated until an operation has been processed by the destination endpoint(s). A completion guarantees that the result of the operation is available; however, additional steps may need to be taken at the destination to retrieve the results. For example, an application may need to provide a receive buffers in order to retrieve messages that were buffered by the provider.

Delivery complete indicates that the message has been processed by the peer. If an application buffer was ready to receive the results of the message when it arrived, then delivery complete indicates that the data was placed into the application’s buffer.

This completion mode applies only to reliable endpoints. For operations that return data to the initiator, such as RMA read or atomic-fetch, the source endpoint is also considered a destination endpoint. This is the default completion mode for such operations.

FI_MATCH_COMPLETE

Indicates that a completion should be generated only after the operation has been matched with an application specified buffer. Operations using this completion semantic are dependent on the application at the target claiming the message or results. As a result, match complete may involve additional provider level acknowledgements or lengthy delays. However, this completion model enables peer applications to synchronize their execution. Many providers may not support this semantic.

FI_COMMIT_COMPLETE

Indicates that a completion should not be generated (locally or at the peer) until the result of an operation have been made persistent. A completion guarantees that the result is both available and durable, in the case of power failure.

This completion mode applies only to operations that target persistent memory regions over reliable endpoints. This completion mode is experimental.

FI_FENCE

This is not a completion level, but plays a role in the completion ordering between operations that would not normally be ordered. An operation that is marked with the FI_FENCE flag and all operations posted after the fenced operation are deferred until all previous operations targeting the same peer endpoint have completed. Additionally, the completion of the fenced operation indicates that prior operations have met the same completion level as the fenced operation. For example, if an operation is posted as FI_DELIVERY_COMPLETE | FI_FENCE, then its completion indicates prior operations have met the semantic required for FI_DELIVERY_COMPLETE. This is true even if the prior operation was posted with a lower completion level, such as FI_TRANSMIT_COMPLETE or FI_INJECT_COMPLETE.

Note that a completion generated for an operation posted prior to the fenced operation only guarantees that the completion level that was originally requested has been met. It is the completion of the fenced operation that guarantees that the additional semantics have been met.

The above completion semantics are defined with respect to the initiator of the operation. The different semantics are useful for describing when the initiator may re-use a data buffer, and guarantees what state a transfer must reach prior to a completion being generated. This allows applications to determine appropriate error handling in case of communication failures.

TARGET COMPLETION SEMANTICS

The completion semantic at the target is used to determine when data at the target is visible to the peer application. Visibility indicates that a memory read to the same address that was the target of a data transfer will return the results of the transfer. The target of a transfer can be identified by the initiator, as may be the case for RMA and atomic operations, or determined by the target, for example by providing a matching receive buffer. Global visibility indicates that the results are available regardless of where the memory read originates. For example, the read could come from a process running on a host CPU, it may be accessed by subsequent data transfer over the fabric, or read from a peer device such as a GPU.

In terms of completion semantics, visibility usually indicates that the transfer meets the FI_DELIVERY_COMPLETE requirements from the perspective of the target. The target completion semantic may be, but is not necessarily, linked with the completion semantic specified by the initiator of the transfer.

Often, target processes do not explicitly state a desired completion semantic and instead rely on the default semantic. The default behavior is based on several factors, including:

whether a completion even is generated at the target
the type of transfer involved (e.g. msg vs RMA)
endpoint data and message ordering guarantees
properties of the targeted memory buffer
the initiator’s specified completion semantic

Broadly, target completion semantics are grouped based on whether or not the transfer generates a completion event at the target. This includes writing a CQ entry or updating a completion counter. In common use cases, transfers that use a message interface (FI_MSG or FI_TAGGED) typically generate target events, while transfers involving an RMA interface (FI_RMA or FI_ATOMIC) often do not. There are exceptions to both these cases, depending on endpoint to CQ and counter bindings and operational flags. For example, RMA writes that carry remote CQ data will generate a completion event at the target, and are frequently used to convey visibility to the target application. The general guidelines for target side semantics are described below, followed by exceptions that modify that behavior.

By default, completions generated at the target indicate that the transferred data is immediately available to be read from the target buffer. That is, the target sees FI_DELIVERY_COMPLETE (or better) semantics, even if the initiator requested lower semantics. For applications using only data buffers allocated from host memory, this is often sufficient.

For operations that do not generate a completion event at the target, the visibility of the data at the target may need to be inferred based on subsequent operations that do generate target completions. Absent a target completion, when a completion of an operation is written at the initiator, the visibility semantic of the operation at the target aligns with the initiator completion semantic. For instance, if an RMA operation completes at the initiator as either FI_INJECT_COMPLETE or FI_TRANSMIT_COMPLETE, the data visibility at the target is not guaranteed.

One or more of the following mechanisms can be used by the target process to guarantee that the results of a data transfer that did not generate a completion at the target is now visible. This list is not inclusive of all options, but defines common uses. In the descriptions below, the first transfer does not result in a completion event at the target, but is eventually followed by a transfer which does.

If the endpoint guarantees message ordering between two transfers, the target completion of a second transfer will indicate that the data from the first transfer is available. For example, if the endpoint supports send after write ordering (FI_ORDER_SAW), then a receive completion corresponding to the send will indicate that the write data is available. This holds independent of the initiator’s completion semantic for either the write or send. When ordering is guaranteed, the second transfer can be queued with the provider immediately after queuing the first.
If the endpoint does not guarantee message ordering, the initiator must take additional steps to ensure visibility. If initiator requests FI_DELIVERY_COMPLETE semantics for the first operation, the initiator can wait for the operation to complete locally. Once the completion has been read, the target completion of a second transfer will indicate that the first transfer’s data is visible.
Alternatively, if message ordering is not guaranteed by the endpoint, the initiator can use the FI_FENCE and FI_DELIVERY_COMPLETE flags on the second data transfer to force the first transfers to meet the FI_DELIVERY_COMPLETE semantics. If the second transfer generates a completion at the target, that will indicate that the data is visible. Otherwise, a target completion for any transfer after the fenced operation will indicate that the data is visible.

The above semantics apply for transfers targeting traditional host memory buffers. However, the behavior may differ when device memory and/or persistent memory is involved (FI_HMEM and FI_PMEM capability bits). When heterogenous memory is involved, the concept of memory domains come into play. Memory domains identify the physical separation of memory, which may or may not be accessible through the same virtual address space. See the fi_mr(3) man page for further details on memory domains.

Completion ordering and data visibility are only well-defined for transfers that target the same memory domain. Applications need to be aware of ordering and visibility differences when transfers target different memory domains. Additionally, applications also need to be concerned with the memory domain that completions themselves are written and if it differs from the memory domain targeted by a transfer. In some situations, either the provider or application may need to call device specific APIs to synchronize or flush device memory caches in order to achieve the desired data visibility.

When heterogenous memory is in use, the default target completion semantic for transfers that generate a completion at the target is still FI_DELIVERY_COMPLETE, however, applications should be aware that there may be a negative impact on overall performance for providers to meet this requirement.

For example, a target process may be using a GPU to accelerate computations. A memory region mapping to memory on the GPU may be exposed to peers as either an RMA target or posted locally as a receive buffer. In this case, the application is concerned with two memory domains – system and GPU memory. Completions are written to system memory.

Continuing the example, a peer process sends a tagged message. That message is matched with the receive buffer located in GPU memory. The NIC copies the data from the network into the receive buffer and writes an entry into the completion queue. Note that both memory domains were accessed as part of this transfer. The message data was directed to the GPU memory, but the completion went to host memory. Because separate memory domains may not be synchronized with each other, it is possible for the host CPU to see and process the completion entry before the transfer to the GPU memory is visible to either the host GPU or even software running on the GPU. From the perspective of the provider, visibility of the completion does not imply visibility of data written to the GPU’s memory domain.

The default completion semantic at the target application for message operations is FI_DELIVERY_COMPLETE. An anticipated provider implementation in this situation is for the provider software running on the host CPU to intercept the CQ entry, detect that the data landed in heterogenous memory, and perform the necessary device synchronization or flush operation before reporting the completion up to the application. This ensures that the data is visible to CPU and GPU software prior to the application processing the completion.

In addition to the cost of provider software intercepting completions and checking if a transfer targeted heterogenous memory, device synchronization itself may impact performance. As a result, applications can request a lower completion semantic when posting receives. That indicates to the provider that the application will be responsible for handling any device specific flush operations that might be needed. See fi_msg(3) FLAGS.

For data transfers that do not generate a completion at the target, such as RMA or atomics, it is the responsibility of the application to ensure that all target buffers meet the necessary visibility requirements of the application. The previously mentioned bulleted methods for notifying the target that the data is visible may not be sufficient, as the provider software at the target could lack the context needed to ensure visibility. This implies that the application may need to call device synchronization/flush APIs directly.

For example, a peer application could perform several RMA writes that target GPU memory buffers. If the provider offloads RMA operations into the NIC, the provider software at the target will be unaware that the RMA operations have occurred. If the peer sends a message to the target application that indicates that the RMA operations are done, the application must ensure that the RMA data is visible to the host CPU or GPU prior to executing code that accesses the data. The target completion of having received the sent message is not sufficient, even if send-after-write ordering is supported.

Most target heterogenous memory completion semantics map to FI_TRANSMIT_COMPLETE or FI_DELIVERY_COMPLETE. Persistent memory (FI_PMEM capability), however, is often used with FI_COMMIT_COMPLETE semantics. Heterogenous completion concepts still apply.

For transfers flagged by the initiator with FI_COMMIT_COMPLETE, a completion at the target indicates that the results are visible and durable. For transfers targeting persistent memory, but using a different completion semantic at the initiator, the visibility at the target is similar to that described above. Durability is only associated with transfers marked with FI_COMMIT_COMPLETE.

For transfers targeting persistent memory that request FI_DELIVERY_COMPLETE, then a completion, at either the initiator or target, indicates that the data is visible. Visibility at the target can be conveyed using one of the above describe mechanism – generating a target completion, sending a message from the initiator, etc. Similarly, if the initiator requested FI_TRANSMIT_COMPLETE, then additional steps are needed to ensure visibility at the target. For example, the transfer can generate a completion at the target, which would indicate visibility, but not durability. The initiator can also follow the transfer with another operation that forces visibility, such as using FI_FENCE in conjunction with FI_DELIVERY_COMPLETE.

NOTES

A completion queue must be bound to at least one enabled endpoint before any operation such as fi_cq_read, fi_cq_readfrom, fi_cq_sread, fi_cq_sreadfrom etc. can be called on it.

Completion flags may be suppressed if the FI_NOTIFY_FLAGS_ONLY mode bit has been set. When enabled, only the following flags are guaranteed to be set in completion data when they are valid: FI_REMOTE_READ and FI_REMOTE_WRITE (when FI_RMA_EVENT capability bit has been set), FI_REMOTE_CQ_DATA, and FI_MULTI_RECV.

If a completion queue has been overrun, it will be placed into an ‘overrun’ state. Read operations will continue to return any valid, non-corrupted completions, if available. After all valid completions have been retrieved, any attempt to read the CQ will result in it returning an FI_EOVERRUN error event. Overrun completion queues are considered fatal and may not be used to report additional completions once the overrun occurs.

RETURN VALUES

fi_cq_open / fi_cq_signal: Returns 0 on success. On error, a negative value corresponding to fabric errno is returned.
fi_cq_read / fi_cq_readfrom / fi_cq_readerr
fi_cq_sread / fi_cq_sreadfrom: On success, returns the number of completion events retrieved from the completion queue. On error, a negative value corresponding to fabric errno is returned. If no completions are available to return from the CQ, -FI_EAGAIN will be returned.
fi_cq_sread / fi_cq_sreadfrom: On success, returns the number of completion events retrieved from the completion queue. On error, a negative value corresponding to fabric errno is returned. If the timeout expires or the calling thread is signaled and no data is available to be read from the completion queue, -FI_EAGAIN is returned.
fi_cq_strerror: Returns a character string interpretation of the provider specific error returned with a completion.

Fabric errno values are defined in rdma/fi_errno.h.

fi_cq(3) Libfabric Programmer's Manual