Re: [PATCH 05/16] Add io_uring IO interface

On Mon, 14 Jan 2019 19:55:20 -0700
Jens Axboe [off-list ref] wrote:

So the [0/16] cover letter seems to have gone astray this time?

Anyway, a couple of minor notes/questions:

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.

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 ?

Thanks,

jon