io_uring - Man Page

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

Adding to and reading from the queues:

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). 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 fro 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 Specter 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 lesser 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, reaped = 0;

    /* 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)

Referenced By

io_uring_register(2).

2020-07-26 Linux Programmer's Manual