On 1/15/19 9:51 AM, Jonathan Corbet wrote: > On Mon, 14 Jan 2019 19:55:20 -0700 > Jens Axboe <axboe@xxxxxxxxx> wrote: > > So the [0/16] cover letter seems to have gone astray this time? It did go out, but I forgot to add a Subject line to it... https://marc.info/?l=linux-block&m=154752095709422&w=2 >> The submission queue (SQ) and completion queue (CQ) rings are shared >> between the application and the kernel. This eliminates the need to >> copy data back and forth to submit and complete IO. >> >> IO submissions use the io_uring_sqe data structure, and completions >> are generated in the form of io_uring_sqe data structures. The SQ >> ring is an index into the io_uring_sqe array, which makes it possible >> to submit a batch of IOs without them being contiguous in the ring. >> The CQ ring is always contiguous, as completion events are inherently >> unordered and can point to any io_uring_iocb. >> >> Two new system calls are added for this: >> >> io_uring_setup(entries, iovecs, params) >> Sets up a context for doing async IO. On success, returns a file >> descriptor that the application can mmap to gain access to the >> SQ ring, CQ ring, and io_uring_iocbs. > > Looking at the code, it would appear that the "iovecs" parameter doesn't > actually exist. Indeed, need to update that commit message. and io_uring_iocbs should now be io_uring_sqes. The iovec/file registration is done through io_uring_register(2). >> io_uring_enter(fd, to_submit, min_complete, flags) >> Initiates IO against the rings mapped to this fd, or waits for >> them to complete, or both The behavior is controlled by the >> parameters passed in. If 'min_complete' is non-zero, then we'll >> try and submit new IO. If IORING_ENTER_GETEVENTS is set, the >> kernel will wait for 'min_complete' events, if they aren't >> already available. > > I feel like I'm missing something here. Rather than have the > IORING_ENTER_GETEVENTS flag, why not just wait if min_complete > 0 ? For polled IO, it's useful to be able to check if we have events that can be readily reaped. If min_complete > 0, then you're asking the interface to wait/poll for these events. IORING_ENTER_GETEVENTS + min_complete == 0 is a valid combination to just reap events that are already completed. -- Jens Axboe
