Provided by: libfabric-dev_1.17.0-3_amd64 
NAME
fi_msg - Message data transfer operations
fi_recv / fi_recvv / fi_recvmsg
Post a buffer to receive an incoming message
fi_send / fi_sendv / fi_sendmsg fi_inject / fi_senddata : Initiate an operation to send a
message
SYNOPSIS
#include <rdma/fi_endpoint.h>
ssize_t fi_recv(struct fid_ep *ep, void * buf, size_t len,
void *desc, fi_addr_t src_addr, void *context);
ssize_t fi_recvv(struct fid_ep *ep, const struct iovec *iov, void **desc,
size_t count, fi_addr_t src_addr, void *context);
ssize_t fi_recvmsg(struct fid_ep *ep, const struct fi_msg *msg,
uint64_t flags);
ssize_t fi_send(struct fid_ep *ep, const void *buf, size_t len,
void *desc, fi_addr_t dest_addr, void *context);
ssize_t fi_sendv(struct fid_ep *ep, const struct iovec *iov,
void **desc, size_t count, fi_addr_t dest_addr, void *context);
ssize_t fi_sendmsg(struct fid_ep *ep, const struct fi_msg *msg,
uint64_t flags);
ssize_t fi_inject(struct fid_ep *ep, const void *buf, size_t len,
fi_addr_t dest_addr);
ssize_t fi_senddata(struct fid_ep *ep, const void *buf, size_t len,
void *desc, uint64_t data, fi_addr_t dest_addr, void *context);
ssize_t fi_injectdata(struct fid_ep *ep, const void *buf, size_t len,
uint64_t data, fi_addr_t dest_addr);
ARGUMENTS
ep Fabric endpoint on which to initiate send or post receive buffer.
buf Data buffer to send or receive.
len Length of data buffer to send or receive, specified in bytes. Valid transfers are
from 0 bytes up to the endpoint’s max_msg_size.
iov Vectored data buffer.
count Count of vectored data entries.
desc Descriptor associated with the data buffer. See fi_mr(3).
data Remote CQ data to transfer with the sent message.
dest_addr
Destination address for connectionless transfers. Ignored for connected endpoints.
src_addr
Source address to receive from for connectionless transfers. Applies only to
connectionless endpoints with the FI_DIRECTED_RECV capability enabled, otherwise
this field is ignored. If set to FI_ADDR_UNSPEC, any source address may match.
msg Message descriptor for send and receive operations.
flags Additional flags to apply for the send or receive operation.
context
User specified pointer to associate with the operation. This parameter is ignored
if the operation will not generate a successful completion, unless an op flag
specifies the context parameter be used for required input.
DESCRIPTION
The send functions – fi_send, fi_sendv, fi_sendmsg, fi_inject, and fi_senddata – are used
to transmit a message from one endpoint to another endpoint. The main difference between
send functions are the number and type of parameters that they accept as input.
Otherwise, they perform the same general function. Messages sent using fi_msg operations
are received by a remote endpoint into a buffer posted to receive such messages.
The receive functions – fi_recv, fi_recvv, fi_recvmsg – post a data buffer to an endpoint
to receive inbound messages. Similar to the send operations, receive operations operate
asynchronously. Users should not touch the posted data buffer(s) until the receive
operation has completed.
An endpoint must be enabled before an application can post send or receive operations to
it. For connected endpoints, receive buffers may be posted prior to connect or accept
being called on the endpoint. This ensures that buffers are available to receive incoming
data immediately after the connection has been established.
Completed message operations are reported to the user through one or more event collectors
associated with the endpoint. Users provide context which are associated with each
operation, and is returned to the user as part of the event completion. See fi_cq for
completion event details.
fi_send
The call fi_send transfers the data contained in the user-specified data buffer to a
remote endpoint, with message boundaries being maintained.
fi_sendv
The fi_sendv call adds support for a scatter-gather list to fi_send. The fi_sendv
transfers the set of data buffers referenced by the iov parameter to a remote endpoint as
a single message.
fi_sendmsg
The fi_sendmsg call supports data transfers over both connected and connectionless
endpoints, with the ability to control the send operation per call through the use of
flags. The fi_sendmsg function takes a struct fi_msg as input.
struct fi_msg {
const struct iovec *msg_iov; /* scatter-gather array */
void **desc; /* local request descriptors */
size_t iov_count;/* # elements in iov */
fi_addr_t addr; /* optional endpoint address */
void *context; /* user-defined context */
uint64_t data; /* optional message data */
};
fi_inject
The send inject call is an optimized version of fi_send with the following
characteristics. The data buffer is available for reuse immediately on return from the
call, and no CQ entry will be written if the transfer completes successfully.
Conceptually, this means that the fi_inject function behaves as if the FI_INJECT transfer
flag were set, selective completions are enabled, and the FI_COMPLETION flag is not
specified. Note that the CQ entry will be suppressed even if the default behavior of the
endpoint is to write CQ entries for all successful completions. See the flags discussion
below for more details. The requested message size that can be used with fi_inject is
limited by inject_size.
fi_senddata
The send data call is similar to fi_send, but allows for the sending of remote CQ data
(see FI_REMOTE_CQ_DATA flag) as part of the transfer.
fi_injectdata
The inject data call is similar to fi_inject, but allows for the sending of remote CQ data
(see FI_REMOTE_CQ_DATA flag) as part of the transfer.
fi_recv
The fi_recv call posts a data buffer to the receive queue of the corresponding endpoint.
Posted receives are searched in the order in which they were posted in order to match
sends. Message boundaries are maintained. The order in which the receives complete is
dependent on the endpoint type and protocol. For connectionless endpoints, the src_addr
parameter can be used to indicate that a buffer should be posted to receive incoming data
from a specific remote endpoint.
fi_recvv
The fi_recvv call adds support for a scatter-gather list to fi_recv. The fi_recvv posts
the set of data buffers referenced by the iov parameter to a receive incoming data.
fi_recvmsg
The fi_recvmsg call supports posting buffers over both connected and connectionless
endpoints, with the ability to control the receive operation per call through the use of
flags. The fi_recvmsg function takes a struct fi_msg as input.
FLAGS
The fi_recvmsg and fi_sendmsg calls allow the user to specify flags which can change the
default message handling of the endpoint. Flags specified with fi_recvmsg / fi_sendmsg
override most flags previously configured with the endpoint, except where noted (see
fi_endpoint.3). The following list of flags are usable with fi_recvmsg and/or fi_sendmsg.
FI_REMOTE_CQ_DATA
Applies to fi_sendmsg and fi_senddata. Indicates that remote CQ data is available
and should be sent as part of the request. See fi_getinfo for additional details
on FI_REMOTE_CQ_DATA.
FI_CLAIM
Applies to posted receive operations for endpoints configured for FI_BUFFERED_RECV
or FI_VARIABLE_MSG. This flag is used to retrieve a message that was buffered by
the provider. See the Buffered Receives section for details.
FI_COMPLETION
Indicates that a completion entry should be generated for the specified operation.
The endpoint must be bound to a completion queue with FI_SELECTIVE_COMPLETION that
corresponds to the specified operation, or this flag is ignored.
FI_DISCARD
Applies to posted receive operations for endpoints configured for FI_BUFFERED_RECV
or FI_VARIABLE_MSG. This flag is used to free a message that was buffered by the
provider. See the Buffered Receives section for details.
FI_MORE
Indicates that the user has additional requests that will immediately be posted
after the current call returns. Use of this flag may improve performance by
enabling the provider to optimize its access to the fabric hardware.
FI_INJECT
Applies to fi_sendmsg. Indicates that the outbound data buffer should be returned
to user immediately after the send call returns, even if the operation is handled
asynchronously. This may require that the underlying provider implementation copy
the data into a local buffer and transfer out of that buffer. This flag can only
be used with messages smaller than inject_size.
FI_MULTI_RECV
Applies to posted receive operations. This flag allows the user to post a single
buffer that will receive multiple incoming messages. Received messages will be
packed into the receive buffer until the buffer has been consumed. Use of this
flag may cause a single posted receive operation to generate multiple events as
messages are placed into the buffer. The placement of received data into the
buffer may be subjected to provider specific alignment restrictions.
The buffer will be released by the provider when the available buffer space falls below
the specified minimum (see FI_OPT_MIN_MULTI_RECV). Note that an entry to the associated
receive completion queue will always be generated when the buffer has been consumed, even
if other receive completions have been suppressed (i.e. the Rx context has been configured
for FI_SELECTIVE_COMPLETION). See the FI_MULTI_RECV completion flag fi_cq(3).
FI_INJECT_COMPLETE
Applies to fi_sendmsg. Indicates that a completion should be generated when the
source buffer(s) may be reused.
FI_TRANSMIT_COMPLETE
Applies to fi_sendmsg and fi_recvmsg. For sends, indicates that a completion
should not be generated until the operation has been successfully transmitted and
is no longer being tracked by the provider. For receive operations, indicates that
a completion may be generated as soon as the message has been processed by the
local provider, even if the message data may not be visible to all processing
elements. See fi_cq(3) for target side completion semantics.
FI_DELIVERY_COMPLETE
Applies to fi_sendmsg. Indicates that a completion should be generated when the
operation has been processed by the destination.
FI_FENCE
Applies to transmits. Indicates that the requested operation, also known as the
fenced operation, and any operation posted after the fenced operation will be
deferred until all previous operations targeting the same peer endpoint have
completed. Operations posted after the fencing will see and/or replace the results
of any operations initiated prior to the fenced operation.
The ordering of operations starting at the posting of the fenced operation (inclusive) to
the posting of a subsequent fenced operation (exclusive) is controlled by the endpoint’s
ordering semantics.
FI_MULTICAST
Applies to transmits. This flag indicates that the address specified as the data
transfer destination is a multicast address. This flag must be used in all
multicast transfers, in conjunction with a multicast fi_addr_t.
Buffered Receives
Buffered receives indicate that the networking layer allocates and manages the data
buffers used to receive network data transfers. As a result, received messages must be
copied from the network buffers into application buffers for processing. However,
applications can avoid this copy if they are able to process the message in place
(directly from the networking buffers).
Handling buffered receives differs based on the size of the message being sent. In
general, smaller messages are passed directly to the application for processing. However,
for large messages, an application will only receive the start of the message and must
claim the rest. The details for how small messages are reported and large messages may be
claimed are described below.
When a provider receives a message, it will write an entry to the completion queue
associated with the receiving endpoint. For discussion purposes, the completion queue is
assumed to be configured for FI_CQ_FORMAT_DATA. Since buffered receives are not
associated with application posted buffers, the CQ entry op_context will point to a struct
fi_recv_context.
struct fi_recv_context {
struct fid_ep *ep;
void *context;
};
The `ep' field will point to the receiving endpoint or Rx context, and `context' will be
NULL. The CQ entry’s `buf' will point to a provider managed buffer where the start of the
received message is located, and `len' will be set to the total size of the message.
The maximum sized message that a provider can buffer is limited by an
FI_OPT_BUFFERED_LIMIT. This threshold can be obtained and may be adjusted by the
application using the fi_getopt and fi_setopt calls, respectively. Any adjustments must
be made prior to enabling the endpoint. The CQ entry `buf' will point to a buffer of
received data. If the sent message is larger than the buffered amount, the CQ entry
`flags' will have the FI_MORE bit set. When the FI_MORE bit is set, `buf' will reference
at least FI_OPT_BUFFERED_MIN bytes of data (see fi_endpoint.3 for more info).
After being notified that a buffered receive has arrived, applications must either claim
or discard the message. Typically, small messages are processed and discarded, while
large messages are claimed. However, an application is free to claim or discard any
message regardless of message size.
To claim a message, an application must post a receive operation with the FI_CLAIM flag
set. The struct fi_recv_context returned as part of the notification must be provided as
the receive operation’s context. The struct fi_recv_context contains a `context' field.
Applications may modify this field prior to claiming the message. When the claim
operation completes, a standard receive completion entry will be generated on the
completion queue. The `context' of the associated CQ entry will be set to the `context'
value passed in through the fi_recv_context structure, and the CQ entry flags will have
the FI_CLAIM bit set.
Buffered receives that are not claimed must be discarded by the application when it is
done processing the CQ entry data. To discard a message, an application must post a
receive operation with the FI_DISCARD flag set. The struct fi_recv_context returned as
part of the notification must be provided as the receive operation’s context. When the
FI_DISCARD flag is set for a receive operation, the receive input buffer(s) and length
parameters are ignored.
IMPORTANT: Buffered receives must be claimed or discarded in a timely manner. Failure to
do so may result in increased memory usage for network buffering or communication stalls.
Once a buffered receive has been claimed or discarded, the original CQ entry `buf' or
struct fi_recv_context data may no longer be accessed by the application.
The use of the FI_CLAIM and FI_DISCARD operation flags is also described with respect to
tagged message transfers in fi_tagged.3. Buffered receives of tagged messages will
include the message tag as part of the CQ entry, if available.
The handling of buffered receives follows all message ordering restrictions assigned to an
endpoint. For example, completions may indicate the order in which received messages
arrived at the receiver based on the endpoint attributes.
Variable Length Messages
Variable length messages, or simply variable messages, are transfers where the size of the
message is unknown to the receiver prior to the message being sent. It indicates that the
recipient of a message does not know the amount of data to expect prior to the message
arriving. It is most commonly used when the size of message transfers varies greatly,
with very large messages interspersed with much smaller messages, making receive side
message buffering difficult to manage. Variable messages are not subject to max message
length restrictions (i.e. struct fi_ep_attr::max_msg_size limits), and may be up to the
maximum value of size_t (e.g. SIZE_MAX) in length.
Variable length messages support requests that the provider allocate and manage the
network message buffers. As a result, the application requirements and provider behavior
is identical as those defined for supporting the FI_BUFFERED_RECV mode bit. See the
Buffered Receive section above for details. The main difference is that buffered receives
are limited by the fi_ep_attr::max_msg_size threshold, whereas variable length messages
are not.
Support for variable messages is indicated through the FI_VARIABLE_MSG capability bit.
NOTES
If an endpoint has been configured with FI_MSG_PREFIX, the application must include buffer
space of size msg_prefix_size, as specified by the endpoint attributes. The prefix buffer
must occur at the start of the data referenced by the buf parameter, or be referenced by
the first IO vector. Message prefix space cannot be split between multiple IO vectors.
The size of the prefix buffer should be included as part of the total buffer length.
RETURN VALUE
Returns 0 on success. On error, a negative value corresponding to fabric errno is
returned. Fabric errno values are defined in rdma/fi_errno.h.
See the discussion below for details handling FI_EAGAIN.
ERRORS
-FI_EAGAIN
Indicates that the underlying provider currently lacks the resources needed to
initiate the requested operation. The reasons for a provider returning FI_EAGAIN
are varied. However, common reasons include insufficient internal buffering or
full processing queues.
Insufficient internal buffering is often associated with operations that use FI_INJECT.
In such cases, additional buffering may become available as posted operations complete.
Full processing queues may be a temporary state related to local processing (for example,
a large message is being transferred), or may be the result of flow control. In the
latter case, the queues may remain blocked until additional resources are made available
at the remote side of the transfer.
In all cases, the operation may be retried after additional resources become available.
It is strongly recommended that applications check for transmit and receive completions
after receiving FI_EAGAIN as a return value, independent of the operation which failed.
This is particularly important in cases where manual progress is employed, as
acknowledgements or flow control messages may need to be processed in order to resume
execution.
SEE ALSO
fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_cq(3)
AUTHORS
OpenFabrics.