io_uring_setup_reg_wait(3)

NAME
io_uring_setup_reg_wait - Sets up and registers fixed wait regions

SYNOPSIS
#include <liburing.h>

struct io_uring_reg_wait *io_uring_setup_reg_wait(struct io_uring *ring,
                                                  unsigned nentries,
                                                  int *err);

DESCRIPTION

The io_uring_setup_reg_wait(3) function allocates and registers a fixed wait region of nentries entries. Upon successful return, the function returns a non-NULL pointer. On error, it returns NULL and fills in err with the error value.

A registered wait region is a contiguous range of structio_uring_reg_wait, which look as follows:

    struct io_uring_reg_wait {
        struct __kernel_timespec ts;
        __u32                    min_wait_usec;
        __u32                    flags;
        __u64                    sigmask;
        __u32                    sigmask_sz;
        __u32                    pad[3];
        __u64                    pad2[2];
    };
where ts is the wait related information for this wait, min_wait_usec is the minimum wait period (for a two-stage wait), if set to non-zero, flags tells the kernel about the wait region, sigmask is a pointer to a signal mask, if used, and sigmask_sz is the size of the signal mask, if used.

Currently the only valid flag is IORING_REG_WAIT_TS , which, if set, indicates that the value in ts is valid and should be used for the wait operation.

A signal mask is used for the wait, if sigmask is set to a valid, non-zero, pointer value. If used, sigmask_sz must also be set.

Each of the wait regions are indicated by the offset of the structure. The first wait region is index 0 , the next is index 1 , and so forth, up to the registered number of regions set by nentries.

The wait regions may be modified by an application at any time before calling io_uring_submit_and_reg_wait(3). If modified while a wait region for that given offset is currently in use by the kernel, the results are undefined - the kernel may see the new value in time to use it, or it may not.

The main purpose of registered wait regions and the associated submit-and-wait helpers is to reduce the overhead of a wait operation. A normal wait for events with a timeout will pass in a structio_uring_getevents_arg which will need to be copied for each wait. For high frequency wait operations, this adds noticeable overhead for each wait. With registered wait regions, no such copying needs to take place for each wait.

Once a wait region has been setup, it persists for the life time of the ring. It's currently not possible to unregister or resize a wait region. Additionally, a wait region may currently only use a single page of memory. On a 4k page size system, this means an application is limited to 64 wait regions. That should be enough, as each wait index may be modified as needed. With at least 64 indices available, hopefully applications can just use the appropriately setup wait region for each specific type of wait, with different indices having different wait settings.

While a region cannot get unregistered from the kernel, once a ring has been closed, the application may free the associated memory by calling io_uring_free_reg_wait(3). An application may also do this before closing a ring, but then wait regions may no longer be modified by the application.

Available since kernel 6.13.

RETURN VALUE
On success io_uring_setup_reg_wait(3) returns a pointer to the start of the wait regions. On failure, it returns NULL and sets err to the appropriate -errno value.

SEE ALSO
io_uring_submit_and_wait_reg(3), io_uring_free_reg_wait(3)