io_uring_submit_and_wait_min_timeout(3)

NAME
io_uring_submit_and_wait_min_timeout - submit requests to the submission queue and wait for the completion with both batch and normal timeout

SYNOPSIS
#include <liburing.h>

int io_uring_submit_and_wait_min_timeout(struct io_uring *ring,
                                         struct io_uring_cqe **cqe_ptr,
                                         unsigned wait_nr,
                                         struct __kernel_timespec *ts,
                                         unsigned int min_wait_usec,
                                         sigset_t *sigmask);

DESCRIPTION

The io_uring_submit_and_wait_min_timeout(3) function submits the next requests from the submission queue belonging to the ring and waits for wait_nr completion events, or until the timeout ts expires. The completion events are stored in the cqe_ptr array. If non-zero, min_wait_usec denotes a timeout for the wait_nr batch.

The sigmask specifies the set of signals to block. If set, it is equivalent to atomically executing the following calls:

    sigset_t origmask;

    pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
    ret = io_uring_submit_and_wait_min_timeout(ring, cqe, wait_nr, ts, min_wait, NULL);
    pthread_sigmask(SIG_SETMASK, &origmask, NULL);

This works like io_uring_submit_and_wait_timeout(3) with the twist that it applies a minimum timeout for the requested batch size of requests to wait for. While io_uring_submit_and_wait_timeout(3) waits for as long as ts specifies, or until wait_nr of request completions have been received, if min_wait_usec is set, then this is the timeout for the wait_nr number of requests. If the requested number of completions have been received within min_wait_usec number of microseconds, then the function returns successfully. If that isn't the case, once min_wait_usec time has passed, control is returned if any completions have been posted. If no completions have been posted, the kernel switches to a normal wait of up to ts specified amount of time, subtracting the time already waited. If any completions are posted after this happens, control is returned immediately to the application.

This differs from the normal timeout waiting in that waiting continues post the initial timeout, if and only if no completions have been posted. It's meant to be used to optimize batch waiting for requests, where the application allots a budget of min_wait_usec amount of time to receive wait_nr number of completions, but if none are received, then waiting can continue without incurring extra context switches or extra kernel/user transitions.

Can be used with any ring, as long as the kernel supports it. Support is indicated by checking the IORING_FEAT_MIN_TIMEOUT feature flag after the ring has been setup. Ideally used with a ring setup with IORING_SETUP_SINGLE_ISSUER|IORING_SETUP_DEFER_TASKRUN as that will greatly reduce the number of context switches that an application will see waiting on multiple requests.

Available since 6.12.

RETURN VALUE
On success io_uring_submit_and_wait_min_timeout(3) returns the number of submitted submission queue entries. On failure it returns -errno. If the kernel doesn't support this functionality, -EINVAL will be returned. See note on the feature flag. The most common failure case is not receiving a completion within the specified timeout, -ETIME is returned in this case.

SEE ALSO
io_uring_queue_init_params(3), io_uring_get_sqe(3), io_uring_submit(3), io_uring_submit_and_wait(3), io_uring_submit_and_wait_timeout(3), io_uring_wait_cqe(3)