Provided by: liburing-dev_2.2-2_amd64 
NAME
io_uring - Asynchronous I/O facility
SYNOPSIS
#include <linux/io_uring.h>
DESCRIPTION
io_uring is a Linux-specific API for asynchronous I/O. It allows the user to submit one
or more I/O requests, which are processed asynchronously without blocking the calling
process. io_uring gets its name from ring buffers which are shared between user space and
kernel space. This arrangement allows for efficient I/O, while avoiding the overhead of
copying buffers between them, where possible. This interface makes io_uring different
from other UNIX I/O APIs, wherein, rather than just communicate between kernel and user
space with system calls, ring buffers are used as the main mode of communication. This
arrangement has various performance benefits which are discussed in a separate section
below. This man page uses the terms shared buffers, shared ring buffers and queues
interchangeably.
The general programming model you need to follow for io_uring is outlined below
• Set up shared buffers with io_uring_setup(2) and mmap(2), mapping into user space
shared buffers for the submission queue (SQ) and the completion queue (CQ). You
place I/O requests you want to make on the SQ, while the kernel places the results
of those operations on the CQ.
• For every I/O request you need to make (like to read a file, write a file, accept a
socket connection, etc), you create a submission queue entry, or SQE, describe the
I/O operation you need to get done and add it to the tail of the submission queue
(SQ). Each I/O operation is, in essence, the equivalent of a system call you would
have made otherwise, if you were not using io_uring. You can add more than one SQE
to the queue depending on the number of operations you want to request.
• After you add one or more SQEs, you need to call io_uring_enter(2) to tell the
kernel to dequeue your I/O requests off the SQ and begin processing them.
• For each SQE you submit, once it is done processing the request, the kernel places
a completion queue event or CQE at the tail of the completion queue or CQ. The
kernel places exactly one matching CQE in the CQ for every SQE you submit on the
SQ. After you retrieve a CQE, minimally, you might be interested in checking the
res field of the CQE structure, which corresponds to the return value of the system
call's equivalent, had you used it directly without using io_uring. For instance,
a read operation under io_uring, started with the IORING_OP_READ operation, issues
the equivalent of the read(2) system call. In practice, it mixes the semantics of
pread(2) and preadv2(2) in that it takes an explicit offset, and supports using -1
for the offset to indicate that the current file position should be used instead of
passing in an explicit offset. See the opcode documentation for more details. Given
that io_uring is an async interface, errno is never used for passing back error
information. Instead, res will contain what the equivalent system call would have
returned in case of success, and in case of error res will contain -errno . For
example, if the normal read system call would have returned -1 and set errno to
EINVAL , then res would contain -EINVAL . If the normal system call would have
returned a read size of 1024, then res would contain 1024.
• Optionally, io_uring_enter(2) can also wait for a specified number of requests to
be processed by the kernel before it returns. If you specified a certain number of
completions to wait for, the kernel would have placed at least those many number of
CQEs on the CQ, which you can then readily read, right after the return from
io_uring_enter(2).
• It is important to remember that I/O requests submitted to the kernel can complete
in any order. It is not necessary for the kernel to process one request after
another, in the order you placed them. Given that the interface is a ring, the
requests are attempted in order, however that doesn't imply any sort of ordering on
their completion. When more than one request is in flight, it is not possible to
determine which one will complete first. When you dequeue CQEs off the CQ, you
should always check which submitted request it corresponds to. The most common
method for doing so is utilizing the user_data field in the request, which is
passed back on the completion side.
Adding to and reading from the queues:
• You add SQEs to the tail of the SQ. The kernel reads SQEs off the head of the
queue.
• The kernel adds CQEs to the tail of the CQ. You read CQEs off the head of the
queue.
Submission queue polling
One of the goals of io_uring is to provide a means for efficient I/O. To this end,
io_uring supports a polling mode that lets you avoid the call to io_uring_enter(2), which
you use to inform the kernel that you have queued SQEs on to the SQ. With SQ Polling,
io_uring starts a kernel thread that polls the submission queue for any I/O requests you
submit by adding SQEs. With SQ Polling enabled, there is no need for you to call
io_uring_enter(2), letting you avoid the overhead of system calls. A designated kernel
thread dequeues SQEs off the SQ as you add them and dispatches them for asynchronous
processing.
Setting up io_uring
The main steps in setting up io_uring consist of mapping in the shared buffers with
mmap(2) calls. In the example program included in this man page, the function
app_setup_uring() sets up io_uring with a QUEUE_DEPTH deep submission queue. Pay
attention to the 2 mmap(2) calls that set up the shared submission and completion queues.
If your kernel is older than version 5.4, three mmap(2) calls are required.
Submitting I/O requests
The process of submitting a request consists of describing the I/O operation you need to
get done using an io_uring_sqe structure instance. These details describe the equivalent
system call and its parameters. Because the range of I/O operations Linux supports are
very varied and the io_uring_sqe structure needs to be able to describe them, it has
several fields, some packed into unions for space efficiency. Here is a simplified
version of struct io_uring_sqe with some of the most often used fields:
struct io_uring_sqe {
__u8 opcode; /* type of operation for this sqe */
__s32 fd; /* file descriptor to do IO on */
__u64 off; /* offset into file */
__u64 addr; /* pointer to buffer or iovecs */
__u32 len; /* buffer size or number of iovecs */
__u64 user_data; /* data to be passed back at completion time */
__u8 flags; /* IOSQE_ flags */
...
};
Here is struct io_uring_sqe in full:
struct io_uring_sqe {
__u8 opcode; /* type of operation for this sqe */
__u8 flags; /* IOSQE_ flags */
__u16 ioprio; /* ioprio for the request */
__s32 fd; /* file descriptor to do IO on */
union {
__u64 off; /* offset into file */
__u64 addr2;
};
union {
__u64 addr; /* pointer to buffer or iovecs */
__u64 splice_off_in;
};
__u32 len; /* buffer size or number of iovecs */
union {
__kernel_rwf_t rw_flags;
__u32 fsync_flags;
__u16 poll_events; /* compatibility */
__u32 poll32_events; /* word-reversed for BE */
__u32 sync_range_flags;
__u32 msg_flags;
__u32 timeout_flags;
__u32 accept_flags;
__u32 cancel_flags;
__u32 open_flags;
__u32 statx_flags;
__u32 fadvise_advice;
__u32 splice_flags;
};
__u64 user_data; /* data to be passed back at completion time */
union {
struct {
/* pack this to avoid bogus arm OABI complaints */
union {
/* index into fixed buffers, if used */
__u16 buf_index;
/* for grouped buffer selection */
__u16 buf_group;
} __attribute__((packed));
/* personality to use, if used */
__u16 personality;
__s32 splice_fd_in;
};
__u64 __pad2[3];
};
};
To submit an I/O request to io_uring, you need to acquire a submission queue entry (SQE)
from the submission queue (SQ), fill it up with details of the operation you want to
submit and call io_uring_enter(2). There are helper functions of the form io_uring_prep_X
to enable proper setup of the SQE. If you want to avoid calling io_uring_enter(2), you
have the option of setting up Submission Queue Polling.
SQEs are added to the tail of the submission queue. The kernel picks up SQEs off the head
of the SQ. The general algorithm to get the next available SQE and update the tail is as
follows.
struct io_uring_sqe *sqe;
unsigned tail, index;
tail = *sqring->tail;
index = tail & (*sqring->ring_mask);
sqe = &sqring->sqes[index];
/* fill up details about this I/O request */
describe_io(sqe);
/* fill the sqe index into the SQ ring array */
sqring->array[index] = index;
tail++;
atomic_store_release(sqring->tail, tail);
To get the index of an entry, the application must mask the current tail index with the
size mask of the ring. This holds true for both SQs and CQs. Once the SQE is acquired,
the necessary fields are filled in, describing the request. While the CQ ring directly
indexes the shared array of CQEs, the submission side has an indirection array between
them. The submission side ring buffer is an index into this array, which in turn contains
the index into the SQEs.
The following code snippet demonstrates how a read operation, an equivalent of a
preadv2(2) system call is described by filling up an SQE with the necessary parameters.
struct iovec iovecs[16];
...
sqe->opcode = IORING_OP_READV;
sqe->fd = fd;
sqe->addr = (unsigned long) iovecs;
sqe->len = 16;
sqe->off = offset;
sqe->flags = 0;
Memory ordering
Modern compilers and CPUs freely reorder reads and writes without affecting the
program's outcome to optimize performance. Some aspects of this need to be kept in
mind on SMP systems since io_uring involves buffers shared between kernel and user
space. These buffers are both visible and modifiable from kernel and user space.
As heads and tails belonging to these shared buffers are updated by kernel and user
space, changes need to be coherently visible on either side, irrespective of
whether a CPU switch took place after the kernel-user mode switch happened. We use
memory barriers to enforce this coherency. Being significantly large subjects on
their own, memory barriers are out of scope for further discussion on this man
page.
Letting the kernel know about I/O submissions
Once you place one or more SQEs on to the SQ, you need to let the kernel know that
you've done so. You can do this by calling the io_uring_enter(2) system call.
This system call is also capable of waiting for a specified count of events to
complete. This way, you can be sure to find completion events in the completion
queue without having to poll it for events later.
Reading completion events
Similar to the submission queue (SQ), the completion queue (CQ) is a shared buffer between
the kernel and user space. Whereas you placed submission queue entries on the tail of the
SQ and the kernel read off the head, when it comes to the CQ, the kernel places completion
queue events or CQEs on the tail of the CQ and you read off its head.
Submission is flexible (and thus a bit more complicated) since it needs to be able to
encode different types of system calls that take various parameters. Completion, on the
other hand is simpler since we're looking only for a return value back from the kernel.
This is easily understood by looking at the completion queue event structure, struct
io_uring_cqe:
struct io_uring_cqe {
__u64 user_data; /* sqe->data submission passed back */
__s32 res; /* result code for this event */
__u32 flags;
};
Here, user_data is custom data that is passed unchanged from submission to completion.
That is, from SQEs to CQEs. This field can be used to set context, uniquely identifying
submissions that got completed. Given that I/O requests can complete in any order, this
field can be used to correlate a submission with a completion. res is the result from the
system call that was performed as part of the submission; its return value. The flags
field could carry request-specific metadata in the future, but is currently unused.
The general sequence to read completion events off the completion queue is as follows:
unsigned head;
head = *cqring->head;
if (head != atomic_load_acquire(cqring->tail)) {
struct io_uring_cqe *cqe;
unsigned index;
index = head & (cqring->mask);
cqe = &cqring->cqes[index];
/* process completed CQE */
process_cqe(cqe);
/* CQE consumption complete */
head++;
}
atomic_store_release(cqring->head, head);
It helps to be reminded that the kernel adds CQEs to the tail of the CQ, while you need to
dequeue them off the head. To get the index of an entry at the head, the application must
mask the current head index with the size mask of the ring. Once the CQE has been
consumed or processed, the head needs to be updated to reflect the consumption of the CQE.
Attention should be paid to the read and write barriers to ensure successful read and
update of the head.
io_uring performance
Because of the shared ring buffers between kernel and user space, io_uring can be a zero-
copy system. Copying buffers to and from becomes necessary when system calls that
transfer data between kernel and user space are involved. But since the bulk of the
communication in io_uring is via buffers shared between the kernel and user space, this
huge performance overhead is completely avoided.
While system calls may not seem like a significant overhead, in high performance
applications, making a lot of them will begin to matter. While workarounds the operating
system has in place to deal with Spectre and Meltdown are ideally best done away with,
unfortunately, some of these workarounds are around the system call interface, making
system calls not as cheap as before on affected hardware. While newer hardware should not
need these workarounds, hardware with these vulnerabilities can be expected to be in the
wild for a long time. While using synchronous programming interfaces or even when using
asynchronous programming interfaces under Linux, there is at least one system call
involved in the submission of each request. In io_uring, on the other hand, you can batch
several requests in one go, simply by queueing up multiple SQEs, each describing an I/O
operation you want and make a single call to io_uring_enter(2). This is possible due to
io_uring's shared buffers based design.
While this batching in itself can avoid the overhead associated with potentially multiple
and frequent system calls, you can reduce even this overhead further with Submission Queue
Polling, by having the kernel poll and pick up your SQEs for processing as you add them to
the submission queue. This avoids the io_uring_enter(2) call you need to make to tell the
kernel to pick SQEs up. For high-performance applications, this means even fewer system
call overheads.
CONFORMING TO
io_uring is Linux-specific.
EXAMPLES
The following example uses io_uring to copy stdin to stdout. Using shell redirection, you
should be able to copy files with this example. Because it uses a queue depth of only
one, this example processes I/O requests one after the other. It is purposefully kept
this way to aid understanding. In real-world scenarios however, you'll want to have a
larger queue depth to parallelize I/O request processing so as to gain the kind of
performance benefits io_uring provides with its asynchronous processing of requests.
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <linux/fs.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdatomic.h>
#include <linux/io_uring.h>
#define QUEUE_DEPTH 1
#define BLOCK_SZ 1024
/* Macros for barriers needed by io_uring */
#define io_uring_smp_store_release(p, v) \
atomic_store_explicit((_Atomic typeof(*(p)) *)(p), (v), \
memory_order_release)
#define io_uring_smp_load_acquire(p) \
atomic_load_explicit((_Atomic typeof(*(p)) *)(p), \
memory_order_acquire)
int ring_fd;
unsigned *sring_tail, *sring_mask, *sring_array,
*cring_head, *cring_tail, *cring_mask;
struct io_uring_sqe *sqes;
struct io_uring_cqe *cqes;
char buff[BLOCK_SZ];
off_t offset;
/*
* System call wrappers provided since glibc does not yet
* provide wrappers for io_uring system calls.
* */
int io_uring_setup(unsigned entries, struct io_uring_params *p)
{
return (int) syscall(__NR_io_uring_setup, entries, p);
}
int io_uring_enter(int ring_fd, unsigned int to_submit,
unsigned int min_complete, unsigned int flags)
{
return (int) syscall(__NR_io_uring_enter, ring_fd, to_submit,
min_complete, flags, NULL, 0);
}
int app_setup_uring(void) {
struct io_uring_params p;
void *sq_ptr, *cq_ptr;
/* See io_uring_setup(2) for io_uring_params.flags you can set */
memset(&p, 0, sizeof(p));
ring_fd = io_uring_setup(QUEUE_DEPTH, &p);
if (ring_fd < 0) {
perror("io_uring_setup");
return 1;
}
/*
* io_uring communication happens via 2 shared kernel-user space ring
* buffers, which can be jointly mapped with a single mmap() call in
* kernels >= 5.4.
*/
int sring_sz = p.sq_off.array + p.sq_entries * sizeof(unsigned);
int cring_sz = p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe);
/* Rather than check for kernel version, the recommended way is to
* check the features field of the io_uring_params structure, which is a
* bitmask. If IORING_FEAT_SINGLE_MMAP is set, we can do away with the
* second mmap() call to map in the completion ring separately.
*/
if (p.features & IORING_FEAT_SINGLE_MMAP) {
if (cring_sz > sring_sz)
sring_sz = cring_sz;
cring_sz = sring_sz;
}
/* Map in the submission and completion queue ring buffers.
* Kernels < 5.4 only map in the submission queue, though.
*/
sq_ptr = mmap(0, sring_sz, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_POPULATE,
ring_fd, IORING_OFF_SQ_RING);
if (sq_ptr == MAP_FAILED) {
perror("mmap");
return 1;
}
if (p.features & IORING_FEAT_SINGLE_MMAP) {
cq_ptr = sq_ptr;
} else {
/* Map in the completion queue ring buffer in older kernels separately */
cq_ptr = mmap(0, cring_sz, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_POPULATE,
ring_fd, IORING_OFF_CQ_RING);
if (cq_ptr == MAP_FAILED) {
perror("mmap");
return 1;
}
}
/* Save useful fields for later easy reference */
sring_tail = sq_ptr + p.sq_off.tail;
sring_mask = sq_ptr + p.sq_off.ring_mask;
sring_array = sq_ptr + p.sq_off.array;
/* Map in the submission queue entries array */
sqes = mmap(0, p.sq_entries * sizeof(struct io_uring_sqe),
PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE,
ring_fd, IORING_OFF_SQES);
if (sqes == MAP_FAILED) {
perror("mmap");
return 1;
}
/* Save useful fields for later easy reference */
cring_head = cq_ptr + p.cq_off.head;
cring_tail = cq_ptr + p.cq_off.tail;
cring_mask = cq_ptr + p.cq_off.ring_mask;
cqes = cq_ptr + p.cq_off.cqes;
return 0;
}
/*
* Read from completion queue.
* In this function, we read completion events from the completion queue.
* We dequeue the CQE, update and head and return the result of the operation.
* */
int read_from_cq() {
struct io_uring_cqe *cqe;
unsigned head;
/* Read barrier */
head = io_uring_smp_load_acquire(cring_head);
/*
* Remember, this is a ring buffer. If head == tail, it means that the
* buffer is empty.
* */
if (head == *cring_tail)
return -1;
/* Get the entry */
cqe = &cqes[head & (*cring_mask)];
if (cqe->res < 0)
fprintf(stderr, "Error: %s\n", strerror(abs(cqe->res)));
head++;
/* Write barrier so that update to the head are made visible */
io_uring_smp_store_release(cring_head, head);
return cqe->res;
}
/*
* Submit a read or a write request to the submission queue.
* */
int submit_to_sq(int fd, int op) {
unsigned index, tail;
/* Add our submission queue entry to the tail of the SQE ring buffer */
tail = *sring_tail;
index = tail & *sring_mask;
struct io_uring_sqe *sqe = &sqes[index];
/* Fill in the parameters required for the read or write operation */
sqe->opcode = op;
sqe->fd = fd;
sqe->addr = (unsigned long) buff;
if (op == IORING_OP_READ) {
memset(buff, 0, sizeof(buff));
sqe->len = BLOCK_SZ;
}
else {
sqe->len = strlen(buff);
}
sqe->off = offset;
sring_array[index] = index;
tail++;
/* Update the tail */
io_uring_smp_store_release(sring_tail, tail);
/*
* Tell the kernel we have submitted events with the io_uring_enter()
* system call. We also pass in the IOURING_ENTER_GETEVENTS flag which
* causes the io_uring_enter() call to wait until min_complete
* (the 3rd param) events complete.
* */
int ret = io_uring_enter(ring_fd, 1,1,
IORING_ENTER_GETEVENTS);
if(ret < 0) {
perror("io_uring_enter");
return -1;
}
return ret;
}
int main(int argc, char *argv[]) {
int res;
/* Setup io_uring for use */
if(app_setup_uring()) {
fprintf(stderr, "Unable to setup uring!\n");
return 1;
}
/*
* A while loop that reads from stdin and writes to stdout.
* Breaks on EOF.
*/
while (1) {
/* Initiate read from stdin and wait for it to complete */
submit_to_sq(STDIN_FILENO, IORING_OP_READ);
/* Read completion queue entry */
res = read_from_cq();
if (res > 0) {
/* Read successful. Write to stdout. */
submit_to_sq(STDOUT_FILENO, IORING_OP_WRITE);
read_from_cq();
} else if (res == 0) {
/* reached EOF */
break;
}
else if (res < 0) {
/* Error reading file */
fprintf(stderr, "Error: %s\n", strerror(abs(res)));
break;
}
offset += res;
}
return 0;
}
SEE ALSO
io_uring_enter(2) io_uring_register(2) io_uring_setup(2)