io_uring_queue_init - setup io_uring submission and completion queues
#include <liburing.h>
int io_uring_queue_init(unsigned entries,
struct io_uring *ring,
unsigned flags);
int io_uring_queue_init_params(unsigned entries,
struct io_uring *ring,
struct io_uring_params *params);
The
io_uring_queue_init(3) function executes the
io_uring_setup(2)
system call to initialize the submission and completion queues in the kernel
with at least
entries entries in the submission queue and then maps the
resulting file descriptor to memory shared between the application and the
kernel.
By default, the CQ ring will have twice the number of entries as specified by
entries for the SQ ring. This is adequate for regular file or storage
workloads, but may be too small networked workloads. The SQ ring entries do
not impose a limit on the number of in-flight requests that the ring can
support, it merely limits the number that can be submitted to the kernel in
one go (batch). if the CQ ring overflows, e.g. more entries are generated than
fits in the ring before the application can reap them, then the ring enters a
CQ ring overflow state. This is indicated by
IORING_SQ_CQ_OVERFLOW
being set in the SQ ring flags. Unless the kernel runs out of available
memory, entries are not dropped, but it is a much slower completion path and
will slow down request processing. For that reason it should be avoided and
the CQ ring sized appropriately for the workload. Setting
cq_entries in
struct io_uring_params will tell the kernel to allocate this many
entries for the CQ ring, independent of the SQ ring size in given in
entries. If the value isn't a power of 2, it will be rounded up to the
nearest power of 2.
On success,
io_uring_queue_init(3) returns 0 and
ring will point
to the shared memory containing the io_uring queues. On failure
-errno
is returned.
flags will be passed through to the io_uring_setup syscall (see
io_uring_setup(2)).
If the variant is used, then the parameters
indicated by
params will be passed straight through to the
io_uring_setup(2) system call.
On success, the resources held by
ring should be released via a
corresponding call to
io_uring_queue_exit(3).
io_uring_queue_init(3) returns 0 on success and
-errno on failure.
io_uring_setup(2),
io_uring_register_ring_fd(3),
mmap(2),
io_uring_queue_exit(3)