1io_uring_setup(2) Linux Programmer's Manual io_uring_setup(2)
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6 io_uring_setup - setup a context for performing asynchronous I/O
7
9 #include <liburing.h>
10 int io_uring_setup(u32 entries, struct io_uring_params *p);
12
14 The io_uring_setup(2) system call sets up a submission queue (SQ) and
15 completion queue (CQ) with at least entries entries, and returns a file
16 descriptor which can be used to perform subsequent operations on the
17 io_uring instance. The submission and completion queues are shared be‐
18 tween userspace and the kernel, which eliminates the need to copy data
19 when initiating and completing I/O.
20 params is used by the application to pass options to the kernel, and by
22 the kernel to convey information about the ring buffers.
23 struct io_uring_params {
25 __u32 sq_entries;
26 __u32 cq_entries;
27 __u32 flags;
28 __u32 sq_thread_cpu;
29 __u32 sq_thread_idle;
30 __u32 features;
31 __u32 wq_fd;
32 __u32 resv[3];
33 struct io_sqring_offsets sq_off;
34 struct io_cqring_offsets cq_off;
35 };
36 The flags, sq_thread_cpu, and sq_thread_idle fields are used to config‐
38 ure the io_uring instance. flags is a bit mask of 0 or more of the
39 following values ORed together:
40 IORING_SETUP_IOPOLL
42 Perform busy-waiting for an I/O completion, as opposed to get‐
43 ting notifications via an asynchronous IRQ (Interrupt Request).
44 The file system (if any) and block device must support polling
45 in order for this to work. Busy-waiting provides lower latency,
46 but may consume more CPU resources than interrupt driven I/O.
47 Currently, this feature is usable only on a file descriptor
48 opened using the O_DIRECT flag. When a read or write is submit‐
49 ted to a polled context, the application must poll for comple‐
50 tions on the CQ ring by calling io_uring_enter(2). It is ille‐
51 gal to mix and match polled and non-polled I/O on an io_uring
52 instance.
53 This is only applicable for storage devices for now, and the
55 storage device must be configured for polling. How to do that
56 depends on the device type in question. For NVMe devices, the
57 nvme driver must be loaded with the poll_queues parameter set to
58 the desired number of polling queues. The polling queues will be
59 shared appropriately between the CPUs in the system, if the num‐
60 ber is less than the number of online CPU threads.
61 IORING_SETUP_SQPOLL
64 When this flag is specified, a kernel thread is created to per‐
65 form submission queue polling. An io_uring instance configured
66 in this way enables an application to issue I/O without ever
67 context switching into the kernel. By using the submission
68 queue to fill in new submission queue entries and watching for
69 completions on the completion queue, the application can submit
70 and reap I/Os without doing a single system call.
71 If the kernel thread is idle for more than sq_thread_idle mil‐
73 liseconds, it will set the IORING_SQ_NEED_WAKEUP bit in the
74 flags field of the struct io_sq_ring. When this happens, the
75 application must call io_uring_enter(2) to wake the kernel
76 thread. If I/O is kept busy, the kernel thread will never
77 sleep. An application making use of this feature will need to
78 guard the io_uring_enter(2) call with the following code se‐
79 quence:
80 /*
82 * Ensure that the wakeup flag is read after the tail pointer
83 * has been written. It's important to use memory load acquire
84 * semantics for the flags read, as otherwise the application
85 * and the kernel might not agree on the consistency of the
86 * wakeup flag.
87 */
88 unsigned flags = atomic_load_relaxed(sq_ring->flags);
89 if (flags & IORING_SQ_NEED_WAKEUP)
90 io_uring_enter(fd, 0, 0, IORING_ENTER_SQ_WAKEUP);
91 where sq_ring is a submission queue ring setup using the struct
93 io_sqring_offsets described below.
94 Note that, when using a ring setup with IORING_SETUP_SQPOLL, you
96 never directly call the io_uring_enter(2) system call. That is
97 usually taken care of by liburing's io_uring_submit(3) function.
98 It automatically determines if you are using polling mode or not
99 and deals with when your program needs to call io_uring_enter(2)
100 without you having to bother about it.
101 Before version 5.11 of the Linux kernel, to successfully use
103 this feature, the application must register a set of files to be
104 used for IO through io_uring_register(2) using the IORING_REGIS‐
105 TER_FILES opcode. Failure to do so will result in submitted IO
106 being errored with EBADF. The presence of this feature can be
107 detected by the IORING_FEAT_SQPOLL_NONFIXED feature flag. In
108 version 5.11 and later, it is no longer necessary to register
109 files to use this feature. 5.11 also allows using this as non-
110 root, if the user has the CAP_SYS_NICE capability. In 5.13 this
111 requirement was also relaxed, and no special privileges are
112 needed for SQPOLL in newer kernels. Certain stable kernels older
113 than 5.13 may also support unprivileged SQPOLL.
114 IORING_SETUP_SQ_AFF
116 If this flag is specified, then the poll thread will be bound to
117 the cpu set in the sq_thread_cpu field of the struct io_ur‐
118 ing_params. This flag is only meaningful when IOR‐
119 ING_SETUP_SQPOLL is specified. When cgroup setting cpuset.cpus
120 changes (typically in container environment), the bounded cpu
121 set may be changed as well.
122 IORING_SETUP_CQSIZE
124 Create the completion queue with struct io_uring_params.cq_en‐
125 tries entries. The value must be greater than entries, and may
126 be rounded up to the next power-of-two.
127 IORING_SETUP_CLAMP
129 If this flag is specified, and if entries exceeds IORING_MAX_EN‐
130 TRIES , then entries will be clamped at IORING_MAX_ENTRIES . If
131 the flag IORING_SETUP_SQPOLL is set, and if the value of struct
132 io_uring_params.cq_entries exceeds IORING_MAX_CQ_ENTRIES , then
133 it will be clamped at IORING_MAX_CQ_ENTRIES .
134 IORING_SETUP_ATTACH_WQ
136 This flag should be set in conjunction with struct io_ur‐
137 ing_params.wq_fd being set to an existing io_uring ring file de‐
138 scriptor. When set, the io_uring instance being created will
139 share the asynchronous worker thread backend of the specified
140 io_uring ring, rather than create a new separate thread pool.
141 IORING_SETUP_R_DISABLED
143 If this flag is specified, the io_uring ring starts in a dis‐
144 abled state. In this state, restrictions can be registered, but
145 submissions are not allowed. See io_uring_register(2) for de‐
146 tails on how to enable the ring. Available since 5.10.
147 IORING_SETUP_SUBMIT_ALL
149 Normally io_uring stops submitting a batch of request, if one of
150 these requests results in an error. This can cause submission of
151 less than what is expected, if a request ends in error while be‐
152 ing submitted. If the ring is created with this flag, io_ur‐
153 ing_enter(2) will continue submitting requests even if it en‐
154 counters an error submitting a request. CQEs are still posted
155 for errored request regardless of whether or not this flag is
156 set at ring creation time, the only difference is if the submit
157 sequence is halted or continued when an error is observed.
158 Available since 5.18.
159 IORING_SETUP_COOP_TASKRUN
161 By default, io_uring will interrupt a task running in userspace
162 when a completion event comes in. This is to ensure that comple‐
163 tions run in a timely manner. For a lot of use cases, this is
164 overkill and can cause reduced performance from both the inter-
165 processor interrupt used to do this, the kernel/user transition,
166 the needless interruption of the tasks userspace activities, and
167 reduced batching if completions come in at a rapid rate. Most
168 applications don't need the forceful interruption, as the events
169 are processed at any kernel/user transition. The exception are
170 setups where the application uses multiple threads operating on
171 the same ring, where the application waiting on completions
172 isn't the one that submitted them. For most other use cases,
173 setting this flag will improve performance. Available since
174 5.19.
175 IORING_SETUP_TASKRUN_FLAG
177 Used in conjunction with IORING_SETUP_COOP_TASKRUN, this pro‐
178 vides a flag, IORING_SQ_TASKRUN, which is set in the SQ ring
179 flags whenever completions are pending that should be processed.
180 liburing will check for this flag even when doing io_ur‐
181 ing_peek_cqe(3) and enter the kernel to process them, and appli‐
182 cations can do the same. This makes IORING_SETUP_TASKRUN_FLAG
183 safe to use even when applications rely on a peek style opera‐
184 tion on the CQ ring to see if anything might be pending to reap.
185 Available since 5.19.
186 IORING_SETUP_SQE128
188 If set, io_uring will use 128-byte SQEs rather than the normal
189 64-byte sized variant. This is a requirement for using certain
190 request types, as of 5.19 only the IORING_OP_URING_CMD
191 passthrough command for NVMe passthrough needs this. Available
192 since 5.19.
193 IORING_SETUP_CQE32
195 If set, io_uring will use 32-byte CQEs rather than the normal
196 16-byte sized variant. This is a requirement for using certain
197 request types, as of 5.19 only the IORING_OP_URING_CMD
198 passthrough command for NVMe passthrough needs this. Available
199 since 5.19.
200 IORING_SETUP_SINGLE_ISSUER
202 A hint to the kernel that only a single task (or thread) will
203 submit requests, which is used for internal optimisations. The
204 submission task is either the task that created the ring, or if
205 IORING_SETUP_R_DISABLED is specified then it is the task that
206 enables the ring through io_uring_register(2). The kernel en‐
207 forces this rule, failing requests with -EEXIST if the restric‐
208 tion is violated. Note that when IORING_SETUP_SQPOLL is set it
209 is considered that the polling task is doing all submissions on
210 behalf of the userspace and so it always complies with the rule
211 disregarding how many userspace tasks do io_uring_enter(2).
212 Available since 6.0.
213 IORING_SETUP_DEFER_TASKRUN
215 By default, io_uring will process all outstanding work at the
216 end of any system call or thread interrupt. This can delay the
217 application from making other progress. Setting this flag will
218 hint to io_uring that it should defer work until an io_uring_en‐
219 ter(2) call with the IORING_ENTER_GETEVENTS flag set. This al‐
220 lows the application to request work to run just before it wants
221 to process completions. This flag requires the IOR‐
222 ING_SETUP_SINGLE_ISSUER flag to be set, and also enforces that
223 the call to io_uring_enter(2) is called from the same thread
224 that submitted requests. Note that if this flag is set then it
225 is the application's responsibility to periodically trigger work
226 (for example via any of the CQE waiting functions) or else com‐
227 pletions may not be delivered. Available since 6.1.
228 If no flags are specified, the io_uring instance is setup for interrupt
230 driven I/O. I/O may be submitted using io_uring_enter(2) and can be
231 reaped by polling the completion queue.
232 The resv array must be initialized to zero.
234 features is filled in by the kernel, which specifies various features
236 supported by current kernel version.
237 IORING_FEAT_SINGLE_MMAP
239 If this flag is set, the two SQ and CQ rings can be mapped with
240 a single mmap(2) call. The SQEs must still be allocated sepa‐
241 rately. This brings the necessary mmap(2) calls down from three
242 to two. Available since kernel 5.4.
243 IORING_FEAT_NODROP
245 If this flag is set, io_uring supports almost never dropping
246 completion events. If a completion event occurs and the CQ ring
247 is full, the kernel stores the event internally until such a
248 time that the CQ ring has room for more entries. If this over‐
249 flow condition is entered, attempting to submit more IO will
250 fail with the -EBUSY error value, if it can't flush the over‐
251 flown events to the CQ ring. If this happens, the application
252 must reap events from the CQ ring and attempt the submit again.
253 If the kernel has no free memory to store the event internally
254 it will be visible by an increase in the overflow value on the
255 cqring. Available since kernel 5.5. Additionally io_uring_en‐
256 ter(2) will return -EBADR the next time it would otherwise sleep
257 waiting for completions (since kernel 5.19).
258 IORING_FEAT_SUBMIT_STABLE
261 If this flag is set, applications can be certain that any data
262 for async offload has been consumed when the kernel has consumed
263 the SQE. Available since kernel 5.5.
264 IORING_FEAT_RW_CUR_POS
266 If this flag is set, applications can specify offset == -1 with
267 IORING_OP_{READV,WRITEV} , IORING_OP_{READ,WRITE}_FIXED , and
268 IORING_OP_{READ,WRITE} to mean current file position, which be‐
269 haves like preadv2(2) and pwritev2(2) with offset == -1. It'll
270 use (and update) the current file position. This obviously comes
271 with the caveat that if the application has multiple reads or
272 writes in flight, then the end result will not be as expected.
273 This is similar to threads sharing a file descriptor and doing
274 IO using the current file position. Available since kernel 5.6.
275 IORING_FEAT_CUR_PERSONALITY
277 If this flag is set, then io_uring guarantees that both sync and
278 async execution of a request assumes the credentials of the task
279 that called io_uring_enter(2) to queue the requests. If this
280 flag isn't set, then requests are issued with the credentials of
281 the task that originally registered the io_uring. If only one
282 task is using a ring, then this flag doesn't matter as the cre‐
283 dentials will always be the same. Note that this is the default
284 behavior, tasks can still register different personalities
285 through io_uring_register(2) with IORING_REGISTER_PERSONALITY
286 and specify the personality to use in the sqe. Available since
287 kernel 5.6.
288 IORING_FEAT_FAST_POLL
290 If this flag is set, then io_uring supports using an internal
291 poll mechanism to drive data/space readiness. This means that
292 requests that cannot read or write data to a file no longer need
293 to be punted to an async thread for handling, instead they will
294 begin operation when the file is ready. This is similar to doing
295 poll + read/write in userspace, but eliminates the need to do
296 so. If this flag is set, requests waiting on space/data consume
297 a lot less resources doing so as they are not blocking a thread.
298 Available since kernel 5.7.
299 IORING_FEAT_POLL_32BITS
301 If this flag is set, the IORING_OP_POLL_ADD command accepts the
302 full 32-bit range of epoll based flags. Most notably EPOLLEXCLU‐
303 SIVE which allows exclusive (waking single waiters) behavior.
304 Available since kernel 5.9.
305 IORING_FEAT_SQPOLL_NONFIXED
307 If this flag is set, the IORING_SETUP_SQPOLL feature no longer
308 requires the use of fixed files. Any normal file descriptor can
309 be used for IO commands without needing registration. Available
310 since kernel 5.11.
311 IORING_FEAT_ENTER_EXT_ARG
313 If this flag is set, then the io_uring_enter(2) system call sup‐
314 ports passing in an extended argument instead of just the
315 sigset_t of earlier kernels. This. extended argument is of type
316 struct io_uring_getevents_arg and allows the caller to pass in
317 both a sigset_t and a timeout argument for waiting on events.
318 The struct layout is as follows:
319 struct io_uring_getevents_arg {
321 __u64 sigmask;
322 __u32 sigmask_sz;
323 __u32 pad;
324 __u64 ts;
325 };
326 and a pointer to this struct must be passed in if IORING_EN‐
328 TER_EXT_ARG is set in the flags for the enter system call.
329 Available since kernel 5.11.
330 IORING_FEAT_NATIVE_WORKERS
332 If this flag is set, io_uring is using native workers for its
333 async helpers. Previous kernels used kernel threads that as‐
334 sumed the identity of the original io_uring owning task, but
335 later kernels will actively create what looks more like regular
336 process threads instead. Available since kernel 5.12.
337 IORING_FEAT_RSRC_TAGS
339 If this flag is set, then io_uring supports a variety of fea‐
340 tures related to fixed files and buffers. In particular, it in‐
341 dicates that registered buffers can be updated in-place, whereas
342 before the full set would have to be unregistered first. Avail‐
343 able since kernel 5.13.
344 IORING_FEAT_CQE_SKIP
346 If this flag is set, then io_uring supports setting
347 IOSQE_CQE_SKIP_SUCCESS in the submitted SQE, indicating that no
348 CQE should be generated for this SQE if it executes normally. If
349 an error happens processing the SQE, a CQE with the appropriate
350 error value will still be generated. Available since kernel
351 5.17.
352 IORING_FEAT_LINKED_FILE
354 If this flag is set, then io_uring supports sane assignment of
355 files for SQEs that have dependencies. For example, if a chain
356 of SQEs are submitted with IOSQE_IO_LINK, then kernels without
357 this flag will prepare the file for each link upfront. If a
358 previous link opens a file with a known index, eg if direct de‐
359 scriptors are used with open or accept, then file assignment
360 needs to happen post execution of that SQE. If this flag is set,
361 then the kernel will defer file assignment until execution of a
362 given request is started. Available since kernel 5.17.
363 IORING_FEAT_REG_REG_RING
365 If this flag is set, then io_uring supports calling io_ur‐
366 ing_register(2) using a registered ring fd, via IORING_REGIS‐
367 TER_USE_REGISTERED_RING. Available since kernel 6.3.
368 The rest of the fields in the struct io_uring_params are filled in by
371 the kernel, and provide the information necessary to memory map the
372 submission queue, completion queue, and the array of submission queue
373 entries. sq_entries specifies the number of submission queue entries
374 allocated. sq_off describes the offsets of various ring buffer fields:
375 struct io_sqring_offsets {
377 __u32 head;
378 __u32 tail;
379 __u32 ring_mask;
380 __u32 ring_entries;
381 __u32 flags;
382 __u32 dropped;
383 __u32 array;
384 __u32 resv[3];
385 };
386 Taken together, sq_entries and sq_off provide all of the information
388 necessary for accessing the submission queue ring buffer and the sub‐
389 mission queue entry array. The submission queue can be mapped with a
390 call like:
391 ptr = mmap(0, sq_off.array + sq_entries * sizeof(__u32),
393 PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
394 ring_fd, IORING_OFF_SQ_RING);
395 where sq_off is the io_sqring_offsets structure, and ring_fd is the
397 file descriptor returned from io_uring_setup(2). The addition of
398 sq_off.array to the length of the region accounts for the fact that the
399 ring is located at the end of the data structure. As an example, the
400 ring buffer head pointer can be accessed by adding sq_off.head to the
401 address returned from mmap(2):
402 head = ptr + sq_off.head;
404 The flags field is used by the kernel to communicate state information
406 to the application. Currently, it is used to inform the application
407 when a call to io_uring_enter(2) is necessary. See the documentation
408 for the IORING_SETUP_SQPOLL flag above. The dropped member is incre‐
409 mented for each invalid submission queue entry encountered in the ring
410 buffer.
411 The head and tail track the ring buffer state. The tail is incremented
413 by the application when submitting new I/O, and the head is incremented
414 by the kernel when the I/O has been successfully submitted. Determin‐
415 ing the index of the head or tail into the ring is accomplished by ap‐
416 plying a mask:
417 index = tail & ring_mask;
419 The array of submission queue entries is mapped with:
421 sqentries = mmap(0, sq_entries * sizeof(struct io_uring_sqe),
423 PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
424 ring_fd, IORING_OFF_SQES);
425 The completion queue is described by cq_entries and cq_off shown here:
427 struct io_cqring_offsets {
429 __u32 head;
430 __u32 tail;
431 __u32 ring_mask;
432 __u32 ring_entries;
433 __u32 overflow;
434 __u32 cqes;
435 __u32 flags;
436 __u32 resv[3];
437 };
438 The completion queue is simpler, since the entries are not separated
440 from the queue itself, and can be mapped with:
441 ptr = mmap(0, cq_off.cqes + cq_entries * sizeof(struct io_uring_cqe),
443 PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, ring_fd,
444 IORING_OFF_CQ_RING);
445 Closing the file descriptor returned by io_uring_setup(2) will free all
447 resources associated with the io_uring context. Note that this may hap‐
448 pen asynchronously within the kernel, so it is not guaranteed that re‐
449 sources are freed immediately.
450
452 io_uring_setup(2) returns a new file descriptor on success. The appli‐
453 cation may then provide the file descriptor in a subsequent mmap(2)
454 call to map the submission and completion queues, or to the io_ur‐
455 ing_register(2) or io_uring_enter(2) system calls.
456 On error, a negative error code is returned. The caller should not rely
458 on errno variable.
459
461 EFAULT params is outside your accessible address space.
462 EINVAL The resv array contains non-zero data, p.flags contains an un‐
464 supported flag, entries is out of bounds, IORING_SETUP_SQ_AFF
465 was specified, but IORING_SETUP_SQPOLL was not, or IOR‐
466 ING_SETUP_CQSIZE was specified, but io_uring_params.cq_entries
467 was invalid.
468 EMFILE The per-process limit on the number of open file descriptors has
470 been reached (see the description of RLIMIT_NOFILE in getr‐
471 limit(2)).
472 ENFILE The system-wide limit on the total number of open files has been
474 reached.
475 ENOMEM Insufficient kernel resources are available.
477 EPERM IORING_SETUP_SQPOLL was specified, but the effective user ID of
479 the caller did not have sufficient privileges.
480
482 io_uring_register(2), io_uring_enter(2)
483Linux 2019-01-29 io_uring_setup(2)