1fi_cq(3) Libfabric v1.17.0 fi_cq(3)
2
6 fi_cq - Completion queue operations
7 fi_cq_open / fi_close
9 Open/close a completion queue
10 fi_control
12 Control CQ operation or attributes.
13 fi_cq_read / fi_cq_readfrom / fi_cq_readerr
15 Read a completion from a completion queue
16 fi_cq_sread / fi_cq_sreadfrom
18 A synchronous (blocking) read that waits until a specified con‐
19 dition has been met before reading a completion from a comple‐
20 tion queue.
21 fi_cq_signal
23 Unblock any thread waiting in fi_cq_sread or fi_cq_sreadfrom.
24 fi_cq_strerror
26 Converts provider specific error information into a printable
27 string
28
30 #include <rdma/fi_domain.h>
31 int fi_cq_open(struct fid_domain *domain, struct fi_cq_attr *attr,
33 struct fid_cq **cq, void *context);
34 int fi_close(struct fid *cq);
36 int fi_control(struct fid *cq, int command, void *arg);
38 ssize_t fi_cq_read(struct fid_cq *cq, void *buf, size_t count);
40 ssize_t fi_cq_readfrom(struct fid_cq *cq, void *buf, size_t count,
42 fi_addr_t *src_addr);
43 ssize_t fi_cq_readerr(struct fid_cq *cq, struct fi_cq_err_entry *buf,
45 uint64_t flags);
46 ssize_t fi_cq_sread(struct fid_cq *cq, void *buf, size_t count,
48 const void *cond, int timeout);
49 ssize_t fi_cq_sreadfrom(struct fid_cq *cq, void *buf, size_t count,
51 fi_addr_t *src_addr, const void *cond, int timeout);
52 int fi_cq_signal(struct fid_cq *cq);
54 const char * fi_cq_strerror(struct fid_cq *cq, int prov_errno,
56 const void *err_data, char *buf, size_t len);
57
59 domain Open resource domain
60 cq Completion queue
62 attr Completion queue attributes
64 context
66 User specified context associated with the completion queue.
67 buf For read calls, the data buffer to write completions into. For
69 write calls, a completion to insert into the completion queue.
70 For fi_cq_strerror, an optional buffer that receives printable
71 error information.
72 count Number of CQ entries.
74 len Length of data buffer
76 src_addr
78 Source address of a completed receive operation
79 flags Additional flags to apply to the operation
81 command
83 Command of control operation to perform on CQ.
84 arg Optional control argument
86 cond Condition that must be met before a completion is generated
88 timeout
90 Time in milliseconds to wait. A negative value indicates infi‐
91 nite timeout.
92 prov_errno
94 Provider specific error value
95 err_data
97 Provider specific error data related to a completion
98
100 Completion queues are used to report events associated with data trans‐
101 fers. They are associated with message sends and receives, RMA, atom‐
102 ic, tagged messages, and triggered events. Reported events are usually
103 associated with a fabric endpoint, but may also refer to memory regions
104 used as the target of an RMA or atomic operation.
105 fi_cq_open
107 fi_cq_open allocates a new completion queue. Unlike event queues, com‐
108 pletion queues are associated with a resource domain and may be off‐
109 loaded entirely in provider hardware.
110 The properties and behavior of a completion queue are defined by struct
112 fi_cq_attr.
113 struct fi_cq_attr {
115 size_t size; /* # entries for CQ */
116 uint64_t flags; /* operation flags */
117 enum fi_cq_format format; /* completion format */
118 enum fi_wait_obj wait_obj; /* requested wait object */
119 int signaling_vector; /* interrupt affinity */
120 enum fi_cq_wait_cond wait_cond; /* wait condition format */
121 struct fid_wait *wait_set; /* optional wait set */
122 };
123 size Specifies the minimum size of a completion queue. A value of 0
125 indicates that the provider may choose a default value.
126 flags Flags that control the configuration of the CQ.
128 - FI_AFFINITY
130 Indicates that the signaling_vector field (see below) is valid.
131 format Completion queues allow the application to select the amount of
133 detail that it must store and report. The format attribute al‐
134 lows the application to select one of several completion for‐
135 mats, indicating the structure of the data that the completion
136 queue should return when read. Supported formats and the struc‐
137 tures that correspond to each are listed below. The meaning of
138 the CQ entry fields are defined in the Completion Fields sec‐
139 tion.
140 - FI_CQ_FORMAT_UNSPEC
142 If an unspecified format is requested, then the CQ will use a
143 provider selected default format.
144 - FI_CQ_FORMAT_CONTEXT
146 Provides only user specified context that was associated with
147 the completion.
148 struct fi_cq_entry {
150 void *op_context; /* operation context */
151 };
152 • .RS 2
153 FI_CQ_FORMAT_MSG
155 Provides minimal data for processing completions, with expanded
156 support for reporting information about received messages.
157 struct fi_cq_msg_entry {
159 void *op_context; /* operation context */
160 uint64_t flags; /* completion flags */
161 size_t len; /* size of received data */
162 };
163 • .RS 2
164 FI_CQ_FORMAT_DATA
166 Provides data associated with a completion. Includes support
167 for received message length, remote CQ data, and multi-receive
168 buffers.
169 struct fi_cq_data_entry {
171 void *op_context; /* operation context */
172 uint64_t flags; /* completion flags */
173 size_t len; /* size of received data */
174 void *buf; /* receive data buffer */
175 uint64_t data; /* completion data */
176 };
177 • .RS 2
178 FI_CQ_FORMAT_TAGGED
180 Expands completion data to include support for the tagged mes‐
181 sage interfaces.
182 struct fi_cq_tagged_entry {
184 void *op_context; /* operation context */
185 uint64_t flags; /* completion flags */
186 size_t len; /* size of received data */
187 void *buf; /* receive data buffer */
188 uint64_t data; /* completion data */
189 uint64_t tag; /* received tag */
190 };
191 wait_obj
193 CQ’s may be associated with a specific wait object. Wait ob‐
194 jects allow applications to block until the wait object is sig‐
195 naled, indicating that a completion is available to be read.
196 Users may use fi_control to retrieve the underlying wait object
197 associated with a CQ, in order to use it in other system calls.
198 The following values may be used to specify the type of wait ob‐
199 ject associated with a CQ: FI_WAIT_NONE, FI_WAIT_UNSPEC,
200 FI_WAIT_SET, FI_WAIT_FD, FI_WAIT_MUTEX_COND, and FI_WAIT_YIELD.
201 The default is FI_WAIT_NONE.
202 - FI_WAIT_NONE
204 Used to indicate that the user will not block (wait) for comple‐
205 tions on the CQ. When FI_WAIT_NONE is specified, the applica‐
206 tion may not call fi_cq_sread or fi_cq_sreadfrom.
207 - FI_WAIT_UNSPEC
209 Specifies that the user will only wait on the CQ using fabric
210 interface calls, such as fi_cq_sread or fi_cq_sreadfrom. In
211 this case, the underlying provider may select the most appropri‐
212 ate or highest performing wait object available, including cus‐
213 tom wait mechanisms. Applications that select FI_WAIT_UNSPEC
214 are not guaranteed to retrieve the underlying wait object.
215 - FI_WAIT_SET
217 Indicates that the completion queue should use a wait set object
218 to wait for completions. If specified, the wait_set field must
219 reference an existing wait set object.
220 - FI_WAIT_FD
222 Indicates that the CQ should use a file descriptor as its wait
223 mechanism. A file descriptor wait object must be usable in se‐
224 lect, poll, and epoll routines. However, a provider may signal
225 an FD wait object by marking it as readable, writable, or with
226 an error.
227 - FI_WAIT_MUTEX_COND
229 Specifies that the CQ should use a pthread mutex and cond vari‐
230 able as a wait object.
231 - FI_WAIT_YIELD
233 Indicates that the CQ will wait without a wait object but in‐
234 stead yield on every wait. Allows usage of fi_cq_sread and
235 fi_cq_sreadfrom through a spin.
236 signaling_vector
238 If the FI_AFFINITY flag is set, this indicates the logical cpu
239 number (0..max cpu - 1) that interrupts associated with the CQ
240 should target. This field should be treated as a hint to the
241 provider and may be ignored if the provider does not support in‐
242 terrupt affinity.
243 wait_cond
245 By default, when a completion is inserted into a CQ that sup‐
246 ports blocking reads (fi_cq_sread/fi_cq_sreadfrom), the corre‐
247 sponding wait object is signaled. Users may specify a condition
248 that must first be met before the wait is satisfied. This field
249 indicates how the provider should interpret the cond field,
250 which describes the condition needed to signal the wait object.
251 A wait condition should be treated as an optimization. Providers are
253 not required to meet the requirements of the condition before signaling
254 the wait object. Applications should not rely on the condition neces‐
255 sarily being true when a blocking read call returns.
256 If wait_cond is set to FI_CQ_COND_NONE, then no additional conditions
258 are applied to the signaling of the CQ wait object, and the insertion
259 of any new entry will trigger the wait condition. If wait_cond is set
260 to FI_CQ_COND_THRESHOLD, then the cond field is interpreted as a size_t
261 threshold value. The threshold indicates the number of entries that
262 are to be queued before at the CQ before the wait is satisfied.
263 This field is ignored if wait_obj is set to FI_WAIT_NONE.
265 wait_set
267 If wait_obj is FI_WAIT_SET, this field references a wait object
268 to which the completion queue should attach. When an event is
269 inserted into the completion queue, the corresponding wait set
270 will be signaled if all necessary conditions are met. The use
271 of a wait_set enables an optimized method of waiting for events
272 across multiple event and completion queues. This field is ig‐
273 nored if wait_obj is not FI_WAIT_SET.
274 fi_close
276 The fi_close call releases all resources associated with a completion
277 queue. Any completions which remain on the CQ when it is closed are
278 lost.
279 When closing the CQ, there must be no opened endpoints, transmit con‐
281 texts, or receive contexts associated with the CQ. If resources are
282 still associated with the CQ when attempting to close, the call will
283 return -FI_EBUSY.
284 fi_control
286 The fi_control call is used to access provider or implementation spe‐
287 cific details of the completion queue. Access to the CQ should be se‐
288 rialized across all calls when fi_control is invoked, as it may redi‐
289 rect the implementation of CQ operations. The following control com‐
290 mands are usable with a CQ.
291 FI_GETWAIT (void **)
293 This command allows the user to retrieve the low-level wait ob‐
294 ject associated with the CQ. The format of the wait-object is
295 specified during CQ creation, through the CQ attributes. The
296 fi_control arg parameter should be an address where a pointer to
297 the returned wait object will be written. See fi_eq.3 for addi‐
298 tion details using fi_control with FI_GETWAIT.
299 fi_cq_read
301 The fi_cq_read operation performs a non-blocking read of completion da‐
302 ta from the CQ. The format of the completion event is determined using
303 the fi_cq_format option that was specified when the CQ was opened.
304 Multiple completions may be retrieved from a CQ in a single call. The
305 maximum number of entries to return is limited to the specified count
306 parameter, with the number of entries successfully read from the CQ re‐
307 turned by the call. (See return values section below.) A count value
308 of 0 may be used to drive progress on associated endpoints when manual
309 progress is enabled.
310 CQs are optimized to report operations which have completed successful‐
312 ly. Operations which fail are reported `out of band'. Such operations
313 are retrieved using the fi_cq_readerr function. When an operation that
314 has completed with an unexpected error is encountered, it is placed in‐
315 to a temporary error queue. Attempting to read from a CQ while an item
316 is in the error queue results in fi_cq_read failing with a return code
317 of -FI_EAVAIL. Applications may use this return code to determine when
318 to call fi_cq_readerr.
319 fi_cq_readfrom
321 The fi_cq_readfrom call behaves identical to fi_cq_read, with the ex‐
322 ception that it allows the CQ to return source address information to
323 the user for any received data. Source address data is only available
324 for those endpoints configured with FI_SOURCE capability. If
325 fi_cq_readfrom is called on an endpoint for which source addressing da‐
326 ta is not available, the source address will be set to FI_ADDR_NO‐
327 TAVAIL. The number of input src_addr entries must be the same as the
328 count parameter.
329 Returned source addressing data is converted from the native address
331 used by the underlying fabric into an fi_addr_t, which may be used in
332 transmit operations. Under most circumstances, returning fi_addr_t re‐
333 quires that the source address already have been inserted into the ad‐
334 dress vector associated with the receiving endpoint. This is true for
335 address vectors of type FI_AV_TABLE. In select providers when
336 FI_AV_MAP is used, source addresses may be converted algorithmically
337 into a usable fi_addr_t, even though the source address has not been
338 inserted into the address vector. This is permitted by the API, as it
339 allows the provider to avoid address look-up as part of receive message
340 processing. In no case do providers insert addresses into an AV sepa‐
341 rate from an application calling fi_av_insert or similar call.
342 For endpoints allocated using the FI_SOURCE_ERR capability, if the
344 source address cannot be converted into a valid fi_addr_t value,
345 fi_cq_readfrom will return -FI_EAVAIL, even if the data were received
346 successfully. The completion will then be reported through fi_cq_read‐
347 err with error code -FI_EADDRNOTAVAIL. See fi_cq_readerr for details.
348 If FI_SOURCE is specified without FI_SOURCE_ERR, source addresses which
350 cannot be mapped to a usable fi_addr_t will be reported as FI_ADDR_NO‐
351 TAVAIL.
352 fi_cq_sread / fi_cq_sreadfrom
354 The fi_cq_sread and fi_cq_sreadfrom calls are the blocking equivalent
355 operations to fi_cq_read and fi_cq_readfrom. Their behavior is similar
356 to the non-blocking calls, with the exception that the calls will not
357 return until either a completion has been read from the CQ or an error
358 or timeout occurs.
359 Threads blocking in this function will return to the caller if they are
361 signaled by some external source. This is true even if the timeout has
362 not occurred or was specified as infinite.
363 It is invalid for applications to call these functions if the CQ has
365 been configured with a wait object of FI_WAIT_NONE or FI_WAIT_SET.
366 fi_cq_readerr
368 The read error function, fi_cq_readerr, retrieves information regarding
369 any asynchronous operation which has completed with an unexpected er‐
370 ror. fi_cq_readerr is a non-blocking call, returning immediately
371 whether an error completion was found or not.
372 Error information is reported to the user through struct fi_cq_err_en‐
374 try. The format of this structure is defined below.
375 struct fi_cq_err_entry {
377 void *op_context; /* operation context */
378 uint64_t flags; /* completion flags */
379 size_t len; /* size of received data */
380 void *buf; /* receive data buffer */
381 uint64_t data; /* completion data */
382 uint64_t tag; /* message tag */
383 size_t olen; /* overflow length */
384 int err; /* positive error code */
385 int prov_errno; /* provider error code */
386 void *err_data; /* error data */
387 size_t err_data_size; /* size of err_data */
388 };
389 The general reason for the error is provided through the err field.
391 Provider specific error information may also be available through the
392 prov_errno and err_data fields. Users may call fi_cq_strerror to con‐
393 vert provider specific error information into a printable string for
394 debugging purposes. See field details below for more information on
395 the use of err_data and err_data_size.
396 Note that error completions are generated for all operations, including
398 those for which a completion was not requested (e.g. an endpoint is
399 configured with FI_SELECTIVE_COMPLETION, but the request did not have
400 the FI_COMPLETION flag set). In such cases, providers will return as
401 much information as made available by the underlying software and hard‐
402 ware about the failure, other fields will be set to NULL or 0. This
403 includes the op_context value, which may not have been provided or was
404 ignored on input as part of the transfer.
405 Notable completion error codes are given below.
407 FI_EADDRNOTAVAIL
409 This error code is used by CQs configured with FI_SOURCE_ERR to
410 report completions for which a usable fi_addr_t source address
411 could not be found. An error code of FI_EADDRNOTAVAIL indicates
412 that the data transfer was successfully received and processed,
413 with the fi_cq_err_entry fields containing information about the
414 completion. The err_data field will be set to the source ad‐
415 dress data. The source address will be in the same format as
416 specified through the fi_info addr_format field for the opened
417 domain. This may be passed directly into an fi_av_insert call
418 to add the source address to the address vector.
419 fi_cq_signal
421 The fi_cq_signal call will unblock any thread waiting in fi_cq_sread or
422 fi_cq_sreadfrom. This may be used to wake-up a thread that is blocked
423 waiting to read a completion operation. The fi_cq_signal operation is
424 only available if the CQ was configured with a wait object.
425
427 The CQ entry data structures share many of the same fields. The mean‐
428 ings of these fields are the same for all CQ entry structure formats.
429 op_context
431 The operation context is the application specified context value
432 that was provided with an asynchronous operation. The op_con‐
433 text field is valid for all completions that are associated with
434 an asynchronous operation.
435 For completion events that are not associated with a posted operation,
437 this field will be set to NULL. This includes completions generated at
438 the target in response to RMA write operations that carry CQ data
439 (FI_REMOTE_WRITE | FI_REMOTE_CQ_DATA flags set), when the FI_RX_CQ_DATA
440 mode bit is not required.
441 flags This specifies flags associated with the completed operation.
443 The Completion Flags section below lists valid flag values.
444 Flags are set for all relevant completions.
445 len This len field only applies to completed receive operations
447 (e.g. fi_recv, fi_trecv, etc.). It indicates the size of re‐
448 ceived message data – i.e. how many data bytes were placed into
449 the associated receive buffer by a corresponding
450 fi_send/fi_tsend/et al call. If an endpoint has been configured
451 with the FI_MSG_PREFIX mode, the len also reflects the size of
452 the prefix buffer.
453 buf The buf field is only valid for completed receive operations,
455 and only applies when the receive buffer was posted with the
456 FI_MULTI_RECV flag. In this case, buf points to the starting
457 location where the receive data was placed.
458 data The data field is only valid if the FI_REMOTE_CQ_DATA completion
460 flag is set, and only applies to receive completions. If FI_RE‐
461 MOTE_CQ_DATA is set, this field will contain the completion data
462 provided by the peer as part of their transmit request. The
463 completion data will be given in host byte order.
464 tag A tag applies only to received messages that occur using the
466 tagged interfaces. This field contains the tag that was includ‐
467 ed with the received message. The tag will be in host byte or‐
468 der.
469 olen The olen field applies to received messages. It is used to in‐
471 dicate that a received message has overrun the available buffer
472 space and has been truncated. The olen specifies the amount of
473 data that did not fit into the available receive buffer and was
474 discarded.
475 err This err code is a positive fabric errno associated with a com‐
477 pletion. The err value indicates the general reason for an er‐
478 ror, if one occurred. See fi_errno.3 for a list of possible er‐
479 ror codes.
480 prov_errno
482 On an error, prov_errno may contain a provider specific error
483 code. The use of this field and its meaning is provider specif‐
484 ic. It is intended to be used as a debugging aid. See
485 fi_cq_strerror for additional details on converting this error
486 value into a human readable string.
487 err_data
489 The err_data field is used to return provider specific informa‐
490 tion, if available, about the error. On input, err_data should
491 reference a data buffer of size err_data_size. On output, the
492 provider will fill in this buffer with any provider specific da‐
493 ta which may help identify the cause of the error. The contents
494 of the err_data field and its meaning is provider specific. It
495 is intended to be used as a debugging aid. See fi_cq_strerror
496 for additional details on converting this error data into a hu‐
497 man readable string. See the compatibility note below on how
498 this field is used for older libfabric releases.
499 err_data_size
501 On input, err_data_size indicates the size of the err_data buf‐
502 fer in bytes. On output, err_data_size will be set to the num‐
503 ber of bytes copied to the err_data buffer. The err_data infor‐
504 mation is typically used with fi_cq_strerror to provide details
505 about the type of error that occurred.
506 For compatibility purposes, the behavior of the err_data and err_da‐
508 ta_size fields is may be modified from that listed above. If err_da‐
509 ta_size is 0 on input, or the fabric was opened with release < 1.5,
510 then any buffer referenced by err_data will be ignored on input. In
511 this situation, on output err_data will be set to a data buffer owned
512 by the provider. The contents of the buffer will remain valid until a
513 subsequent read call against the CQ. Applications must serialize ac‐
514 cess to the CQ when processing errors to ensure that the buffer refer‐
515 enced by err_data does not change.
516
518 Completion flags provide additional details regarding the completed op‐
519 eration. The following completion flags are defined.
520 FI_SEND
522 Indicates that the completion was for a send operation. This
523 flag may be combined with an FI_MSG or FI_TAGGED flag.
524 FI_RECV
526 Indicates that the completion was for a receive operation. This
527 flag may be combined with an FI_MSG or FI_TAGGED flag.
528 FI_RMA Indicates that an RMA operation completed. This flag may be
530 combined with an FI_READ, FI_WRITE, FI_REMOTE_READ, or FI_RE‐
531 MOTE_WRITE flag.
532 FI_ATOMIC
534 Indicates that an atomic operation completed. This flag may be
535 combined with an FI_READ, FI_WRITE, FI_REMOTE_READ, or FI_RE‐
536 MOTE_WRITE flag.
537 FI_MSG Indicates that a message-based operation completed. This flag
539 may be combined with an FI_SEND or FI_RECV flag.
540 FI_TAGGED
542 Indicates that a tagged message operation completed. This flag
543 may be combined with an FI_SEND or FI_RECV flag.
544 FI_MULTICAST
546 Indicates that a multicast operation completed. This flag may
547 be combined with FI_MSG and relevant flags. This flag is only
548 guaranteed to be valid for received messages if the endpoint has
549 been configured with FI_SOURCE.
550 FI_READ
552 Indicates that a locally initiated RMA or atomic read operation
553 has completed. This flag may be combined with an FI_RMA or
554 FI_ATOMIC flag.
555 FI_WRITE
557 Indicates that a locally initiated RMA or atomic write operation
558 has completed. This flag may be combined with an FI_RMA or
559 FI_ATOMIC flag.
560 FI_REMOTE_READ
562 Indicates that a remotely initiated RMA or atomic read operation
563 has completed. This flag may be combined with an FI_RMA or
564 FI_ATOMIC flag.
565 FI_REMOTE_WRITE
567 Indicates that a remotely initiated RMA or atomic write opera‐
568 tion has completed. This flag may be combined with an FI_RMA or
569 FI_ATOMIC flag.
570 FI_REMOTE_CQ_DATA
572 This indicates that remote CQ data is available as part of the
573 completion.
574 FI_MULTI_RECV
576 This flag applies to receive buffers that were posted with the
577 FI_MULTI_RECV flag set. This completion flag indicates that the
578 original receive buffer referenced by the completion has been
579 consumed and was released by the provider. Providers may set
580 this flag on the last message that is received into the multi-
581 recv buffer, or may generate a separate completion that indi‐
582 cates that the buffer has been released.
583 Applications can distinguish between these two cases by examining the
585 completion entry flags field. If additional flags, such as FI_RECV,
586 are set, the completion is associated with a received message. In this
587 case, the buf field will reference the location where the received mes‐
588 sage was placed into the multi-recv buffer. Other fields in the com‐
589 pletion entry will be determined based on the received message. If
590 other flag bits are zero, the provider is reporting that the multi-recv
591 buffer has been released, and the completion entry is not associated
592 with a received message.
593 FI_MORE
595 See the `Buffered Receives' section in fi_msg(3) for more de‐
596 tails. This flag is associated with receive completions on end‐
597 points that have FI_BUFFERED_RECV mode enabled. When set to
598 one, it indicates that the buffer referenced by the completion
599 is limited by the FI_OPT_BUFFERED_LIMIT threshold, and addition‐
600 al message data must be retrieved by the application using an
601 FI_CLAIM operation.
602 FI_CLAIM
604 See the `Buffered Receives' section in fi_msg(3) for more de‐
605 tails. This flag is set on completions associated with receive
606 operations that claim buffered receive data. Note that this
607 flag only applies to endpoints configured with the
608 FI_BUFFERED_RECV mode bit.
609
611 Libfabric defines several completion `levels', identified using opera‐
612 tional flags. Each flag indicates the soonest that a completion event
613 may be generated by a provider, and the assumptions that an application
614 may make upon processing a completion. The operational flags are de‐
615 fined below, along with an example of how a provider might implement
616 the semantic. Note that only meeting the semantic is required of the
617 provider and not the implementation. Providers may implement stronger
618 completion semantics than necessary for a given operation, but only the
619 behavior defined by the completion level is guaranteed.
620 To help understand the conceptual differences in completion levels,
622 consider mailing a letter. Placing the letter into the local mailbox
623 for pick-up is similar to `inject complete'. Having the letter picked
624 up and dropped off at the destination mailbox is equivalent to `trans‐
625 mit complete'. The `delivery complete' semantic is a stronger guaran‐
626 tee, with a person at the destination signing for the letter. However,
627 the person who signed for the letter is not necessarily the intended
628 recipient. The `match complete' option is similar to delivery com‐
629 plete, but requires the intended recipient to sign for the letter.
630 The `commit complete' level has different semantics than the previously
632 mentioned levels. Commit complete would be closer to the letter arriv‐
633 ing at the destination and being placed into a fire proof safe.
634 The operational flags for the described completion levels are defined
636 below.
637 FI_INJECT_COMPLETE
639 Indicates that a completion should be generated when the source
640 buffer(s) may be reused. A completion guarantees that the buf‐
641 fers will not be read from again and the application may reclaim
642 them. No other guarantees are made with respect to the state of
643 the operation.
644 Example: A provider may generate this completion event after copying
646 the source buffer into a network buffer, either in host memory or on
647 the NIC. An inject completion does not indicate that the data has been
648 transmitted onto the network, and a local error could occur after the
649 completion event has been generated that could prevent it from being
650 transmitted.
651 Inject complete allows for the fastest completion reporting (and,
653 hence, buffer reuse), but provides the weakest guarantees against net‐
654 work errors.
655 Note: This flag is used to control when a completion entry is inserted
657 into a completion queue. It does not apply to operations that do not
658 generate a completion queue entry, such as the fi_inject operation, and
659 is not subject to the inject_size message limit restriction.
660 FI_TRANSMIT_COMPLETE
662 Indicates that a completion should be generated when the trans‐
663 mit operation has completed relative to the local provider. The
664 exact behavior is dependent on the endpoint type.
665 For reliable endpoints:
667 Indicates that a completion should be generated when the operation has
669 been delivered to the peer endpoint. A completion guarantees that the
670 operation is no longer dependent on the fabric or local resources. The
671 state of the operation at the peer endpoint is not defined.
672 Example: A provider may generate a transmit complete event upon receiv‐
674 ing an ack from the peer endpoint. The state of the message at the
675 peer is unknown and may be buffered in the target NIC at the time the
676 ack has been generated.
677 For unreliable endpoints:
679 Indicates that a completion should be generated when the operation has
681 been delivered to the fabric. A completion guarantees that the opera‐
682 tion is no longer dependent on local resources. The state of the oper‐
683 ation within the fabric is not defined.
684 FI_DELIVERY_COMPLETE
686 Indicates that a completion should not be generated until an op‐
687 eration has been processed by the destination endpoint(s). A
688 completion guarantees that the result of the operation is avail‐
689 able; however, additional steps may need to be taken at the des‐
690 tination to retrieve the results. For example, an application
691 may need to provide a receive buffers in order to retrieve mes‐
692 sages that were buffered by the provider.
693 Delivery complete indicates that the message has been processed by the
695 peer. If an application buffer was ready to receive the results of the
696 message when it arrived, then delivery complete indicates that the data
697 was placed into the application’s buffer.
698 This completion mode applies only to reliable endpoints. For opera‐
700 tions that return data to the initiator, such as RMA read or atomic-
701 fetch, the source endpoint is also considered a destination endpoint.
702 This is the default completion mode for such operations.
703 FI_MATCH_COMPLETE
705 Indicates that a completion should be generated only after the
706 operation has been matched with an application specified buffer.
707 Operations using this completion semantic are dependent on the
708 application at the target claiming the message or results. As a
709 result, match complete may involve additional provider level ac‐
710 knowledgements or lengthy delays. However, this completion mod‐
711 el enables peer applications to synchronize their execution.
712 Many providers may not support this semantic.
713 FI_COMMIT_COMPLETE
715 Indicates that a completion should not be generated (locally or
716 at the peer) until the result of an operation have been made
717 persistent. A completion guarantees that the result is both
718 available and durable, in the case of power failure.
719 This completion mode applies only to operations that target persistent
721 memory regions over reliable endpoints. This completion mode is exper‐
722 imental.
723 FI_FENCE
725 This is not a completion level, but plays a role in the comple‐
726 tion ordering between operations that would not normally be or‐
727 dered. An operation that is marked with the FI_FENCE flag and
728 all operations posted after the fenced operation are deferred
729 until all previous operations targeting the same peer endpoint
730 have completed. Additionally, the completion of the fenced op‐
731 eration indicates that prior operations have met the same com‐
732 pletion level as the fenced operation. For example, if an oper‐
733 ation is posted as FI_DELIVERY_COMPLETE | FI_FENCE, then its
734 completion indicates prior operations have met the semantic re‐
735 quired for FI_DELIVERY_COMPLETE. This is true even if the prior
736 operation was posted with a lower completion level, such as
737 FI_TRANSMIT_COMPLETE or FI_INJECT_COMPLETE.
738 Note that a completion generated for an operation posted prior to the
740 fenced operation only guarantees that the completion level that was
741 originally requested has been met. It is the completion of the fenced
742 operation that guarantees that the additional semantics have been met.
743 The above completion semantics are defined with respect to the initia‐
745 tor of the operation. The different semantics are useful for describ‐
746 ing when the initiator may re-use a data buffer, and guarantees what
747 state a transfer must reach prior to a completion being generated.
748 This allows applications to determine appropriate error handling in
749 case of communication failures.
750
752 The completion semantic at the target is used to determine when data at
753 the target is visible to the peer application. Visibility indicates
754 that a memory read to the same address that was the target of a data
755 transfer will return the results of the transfer. The target of a
756 transfer can be identified by the initiator, as may be the case for RMA
757 and atomic operations, or determined by the target, for example by pro‐
758 viding a matching receive buffer. Global visibility indicates that the
759 results are available regardless of where the memory read originates.
760 For example, the read could come from a process running on a host CPU,
761 it may be accessed by subsequent data transfer over the fabric, or read
762 from a peer device such as a GPU.
763 In terms of completion semantics, visibility usually indicates that the
765 transfer meets the FI_DELIVERY_COMPLETE requirements from the perspec‐
766 tive of the target. The target completion semantic may be, but is not
767 necessarily, linked with the completion semantic specified by the ini‐
768 tiator of the transfer.
769 Often, target processes do not explicitly state a desired completion
771 semantic and instead rely on the default semantic. The default behav‐
772 ior is based on several factors, including:
773 • whether a completion even is generated at the target
775 • the type of transfer involved (e.g. msg vs RMA)
777 • endpoint data and message ordering guarantees
779 • properties of the targeted memory buffer
781 • the initiator’s specified completion semantic
783 Broadly, target completion semantics are grouped based on whether or
785 not the transfer generates a completion event at the target. This in‐
786 cludes writing a CQ entry or updating a completion counter. In common
787 use cases, transfers that use a message interface (FI_MSG or FI_TAGGED)
788 typically generate target events, while transfers involving an RMA in‐
789 terface (FI_RMA or FI_ATOMIC) often do not. There are exceptions to
790 both these cases, depending on endpoint to CQ and counter bindings and
791 operational flags. For example, RMA writes that carry remote CQ data
792 will generate a completion event at the target, and are frequently used
793 to convey visibility to the target application. The general guidelines
794 for target side semantics are described below, followed by exceptions
795 that modify that behavior.
796 By default, completions generated at the target indicate that the
798 transferred data is immediately available to be read from the target
799 buffer. That is, the target sees FI_DELIVERY_COMPLETE (or better) se‐
800 mantics, even if the initiator requested lower semantics. For applica‐
801 tions using only data buffers allocated from host memory, this is often
802 sufficient.
803 For operations that do not generate a completion event at the target,
805 the visibility of the data at the target may need to be inferred based
806 on subsequent operations that do generate target completions. Absent a
807 target completion, when a completion of an operation is written at the
808 initiator, the visibility semantic of the operation at the target
809 aligns with the initiator completion semantic. For instance, if an RMA
810 operation completes at the initiator as either FI_INJECT_COMPLETE or
811 FI_TRANSMIT_COMPLETE, the data visibility at the target is not guaran‐
812 teed.
813 One or more of the following mechanisms can be used by the target
815 process to guarantee that the results of a data transfer that did not
816 generate a completion at the target is now visible. This list is not
817 inclusive of all options, but defines common uses. In the descriptions
818 below, the first transfer does not result in a completion event at the
819 target, but is eventually followed by a transfer which does.
820 • If the endpoint guarantees message ordering between two transfers,
822 the target completion of a second transfer will indicate that the da‐
823 ta from the first transfer is available. For example, if the end‐
824 point supports send after write ordering (FI_ORDER_SAW), then a re‐
825 ceive completion corresponding to the send will indicate that the
826 write data is available. This holds independent of the initiator’s
827 completion semantic for either the write or send. When ordering is
828 guaranteed, the second transfer can be queued with the provider imme‐
829 diately after queuing the first.
830 • If the endpoint does not guarantee message ordering, the initiator
832 must take additional steps to ensure visibility. If initiator re‐
833 quests FI_DELIVERY_COMPLETE semantics for the first operation, the
834 initiator can wait for the operation to complete locally. Once the
835 completion has been read, the target completion of a second transfer
836 will indicate that the first transfer’s data is visible.
837 • Alternatively, if message ordering is not guaranteed by the endpoint,
839 the initiator can use the FI_FENCE and FI_DELIVERY_COMPLETE flags on
840 the second data transfer to force the first transfers to meet the
841 FI_DELIVERY_COMPLETE semantics. If the second transfer generates a
842 completion at the target, that will indicate that the data is visi‐
843 ble. Otherwise, a target completion for any transfer after the
844 fenced operation will indicate that the data is visible.
845 The above semantics apply for transfers targeting traditional host mem‐
847 ory buffers. However, the behavior may differ when device memory
848 and/or persistent memory is involved (FI_HMEM and FI_PMEM capability
849 bits). When heterogenous memory is involved, the concept of memory do‐
850 mains come into play. Memory domains identify the physical separation
851 of memory, which may or may not be accessible through the same virtual
852 address space. See the fi_mr(3) man page for further details on memory
853 domains.
854 Completion ordering and data visibility are only well-defined for
856 transfers that target the same memory domain. Applications need to be
857 aware of ordering and visibility differences when transfers target dif‐
858 ferent memory domains. Additionally, applications also need to be con‐
859 cerned with the memory domain that completions themselves are written
860 and if it differs from the memory domain targeted by a transfer. In
861 some situations, either the provider or application may need to call
862 device specific APIs to synchronize or flush device memory caches in
863 order to achieve the desired data visibility.
864 When heterogenous memory is in use, the default target completion se‐
866 mantic for transfers that generate a completion at the target is still
867 FI_DELIVERY_COMPLETE, however, applications should be aware that there
868 may be a negative impact on overall performance for providers to meet
869 this requirement.
870 For example, a target process may be using a GPU to accelerate computa‐
872 tions. A memory region mapping to memory on the GPU may be exposed to
873 peers as either an RMA target or posted locally as a receive buffer.
874 In this case, the application is concerned with two memory domains –
875 system and GPU memory. Completions are written to system memory.
876 Continuing the example, a peer process sends a tagged message. That
878 message is matched with the receive buffer located in GPU memory. The
879 NIC copies the data from the network into the receive buffer and writes
880 an entry into the completion queue. Note that both memory domains were
881 accessed as part of this transfer. The message data was directed to
882 the GPU memory, but the completion went to host memory. Because sepa‐
883 rate memory domains may not be synchronized with each other, it is pos‐
884 sible for the host CPU to see and process the completion entry before
885 the transfer to the GPU memory is visible to either the host GPU or
886 even software running on the GPU. From the perspective of the
887 provider, visibility of the completion does not imply visibility of da‐
888 ta written to the GPU’s memory domain.
889 The default completion semantic at the target application for message
891 operations is FI_DELIVERY_COMPLETE. An anticipated provider implemen‐
892 tation in this situation is for the provider software running on the
893 host CPU to intercept the CQ entry, detect that the data landed in het‐
894 erogenous memory, and perform the necessary device synchronization or
895 flush operation before reporting the completion up to the application.
896 This ensures that the data is visible to CPU and GPU software prior to
897 the application processing the completion.
898 In addition to the cost of provider software intercepting completions
900 and checking if a transfer targeted heterogenous memory, device syn‐
901 chronization itself may impact performance. As a result, applications
902 can request a lower completion semantic when posting receives. That
903 indicates to the provider that the application will be responsible for
904 handling any device specific flush operations that might be needed.
905 See fi_msg(3) FLAGS.
906 For data transfers that do not generate a completion at the target,
908 such as RMA or atomics, it is the responsibility of the application to
909 ensure that all target buffers meet the necessary visibility require‐
910 ments of the application. The previously mentioned bulleted methods
911 for notifying the target that the data is visible may not be suffi‐
912 cient, as the provider software at the target could lack the context
913 needed to ensure visibility. This implies that the application may
914 need to call device synchronization/flush APIs directly.
915 For example, a peer application could perform several RMA writes that
917 target GPU memory buffers. If the provider offloads RMA operations in‐
918 to the NIC, the provider software at the target will be unaware that
919 the RMA operations have occurred. If the peer sends a message to the
920 target application that indicates that the RMA operations are done, the
921 application must ensure that the RMA data is visible to the host CPU or
922 GPU prior to executing code that accesses the data. The target comple‐
923 tion of having received the sent message is not sufficient, even if
924 send-after-write ordering is supported.
925 Most target heterogenous memory completion semantics map to FI_TRANS‐
927 MIT_COMPLETE or FI_DELIVERY_COMPLETE. Persistent memory (FI_PMEM capa‐
928 bility), however, is often used with FI_COMMIT_COMPLETE semantics.
929 Heterogenous completion concepts still apply.
930 For transfers flagged by the initiator with FI_COMMIT_COMPLETE, a com‐
932 pletion at the target indicates that the results are visible and
933 durable. For transfers targeting persistent memory, but using a dif‐
934 ferent completion semantic at the initiator, the visibility at the tar‐
935 get is similar to that described above. Durability is only associated
936 with transfers marked with FI_COMMIT_COMPLETE.
937 For transfers targeting persistent memory that request FI_DELIVERY_COM‐
939 PLETE, then a completion, at either the initiator or target, indicates
940 that the data is visible. Visibility at the target can be conveyed us‐
941 ing one of the above describe mechanism – generating a target comple‐
942 tion, sending a message from the initiator, etc. Similarly, if the
943 initiator requested FI_TRANSMIT_COMPLETE, then additional steps are
944 needed to ensure visibility at the target. For example, the transfer
945 can generate a completion at the target, which would indicate visibili‐
946 ty, but not durability. The initiator can also follow the transfer
947 with another operation that forces visibility, such as using FI_FENCE
948 in conjunction with FI_DELIVERY_COMPLETE.
949
951 A completion queue must be bound to at least one enabled endpoint be‐
952 fore any operation such as fi_cq_read, fi_cq_readfrom, fi_cq_sread,
953 fi_cq_sreadfrom etc. can be called on it.
954 Completion flags may be suppressed if the FI_NOTIFY_FLAGS_ONLY mode bit
956 has been set. When enabled, only the following flags are guaranteed to
957 be set in completion data when they are valid: FI_REMOTE_READ and
958 FI_REMOTE_WRITE (when FI_RMA_EVENT capability bit has been set), FI_RE‐
959 MOTE_CQ_DATA, and FI_MULTI_RECV.
960 If a completion queue has been overrun, it will be placed into an
962 `overrun' state. Read operations will continue to return any valid,
963 non-corrupted completions, if available. After all valid completions
964 have been retrieved, any attempt to read the CQ will result in it re‐
965 turning an FI_EOVERRUN error event. Overrun completion queues are con‐
966 sidered fatal and may not be used to report additional completions once
967 the overrun occurs.
968
970 fi_cq_open / fi_cq_signal
971 : Returns 0 on success. On error, returns a negative fabric errno.
972 fi_cq_read / fi_cq_readfrom
974 : On success, returns the number of completions retrieved from the com‐
975 pletion queue. On error, returns a negative fabric errno, with these
976 two errors explicitly identified: If no completions are available to
977 read from the CQ, returns -FI_EAGAIN. If the topmost completion is for
978 a failed transfer (an error entry), returns -FI_EAVAIL.
979 fi_cq_sread / fi_cq_sreadfrom
981 : On success, returns the number of completions retrieved from the com‐
982 pletion queue. On error, returns a negative fabric errno, with these
983 two errors explicitly identified: If the timeout expires or the calling
984 thread is signaled and no data is available to be read from the comple‐
985 tion queue, returns -FI_EAGAIN. If the topmost completion is for a
986 failed transfer (an error entry), returns -FI_EAVAIL.
987 fi_cq_readerr
989 : On success, returns the positive value 1 (number of error entries re‐
990 turned). On error, returns a negative fabric errno, with this error
991 explicitly identified: If no error completions are available to read
992 from the CQ, returns -FI_EAGAIN.
993 fi_cq_strerror
995 : Returns a character string interpretation of the provider specific
996 error returned with a completion.
997 Fabric errno values are defined in rdma/fi_errno.h.
999
1001 fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_eq(3), fi_cntr(3),
1002 fi_poll(3)
1003
1005 OpenFabrics.
1006Libfabric Programmer’s Manual 2022-12-11 fi_cq(3)