NAME

       io_uring_queue_init - setup io_uring submission and completion queues

SYNOPSIS

       #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);

       int io_uring_queue_init_mem(unsigned entries,
                                   struct io_uring *ring,
                                   struct io_uring_params *params,
                                   void *buf, size_t buf_size);

DESCRIPTION

       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
       for 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 if the kernel
       supports IORING_FEAT_NODROP the ring enters a CQ ring overflow state. Otherwise  it  drops
       the  CQEs  and increments cq.koverflow in struct io_uring with the number of CQEs dropped.
       The overflow state 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)).

       The  io_uring_queue_init_params(3)  and  io_uring_queue_init_mem(3) variants will pass the
       parameters indicated by params straight through to the io_uring_setup(2) system call.

       The io_uring_queue_init_mem(3) variant uses the provided buf with associated size buf_size
       as the memory for the ring, using the IORING_SETUP_NO_MMAP flag to io_uring_setup(2).  The
       buffer passed to io_uring_queue_init_mem(3) must already be zeroed.  Typically, the caller
       should  allocate  a  huge  page  and  pass  that  in to io_uring_queue_init_mem(3).  Pages
       allocated by mmap are already zeroed.  io_uring_queue_init_mem(3) returns  the  number  of
       bytes  used  from  the  provided  buffer,  so  that  the app can reuse the buffer with the
       returned offset to put more rings in the same huge page.

       On success, the resources held by ring should be released  via  a  corresponding  call  to
       io_uring_queue_exit(3).

RETURN VALUE

       io_uring_queue_init(3) and io_uring_queue_init_params(3) return 0 on success and -errno on
       failure.

       io_uring_queue_init_mem(3) returns the number of bytes used from the  provided  buffer  on
       success, and -errno on failure.

SEE ALSO

       io_uring_setup(2), io_uring_register_ring_fd(3), mmap(2), io_uring_queue_exit(3)