Re: io-uring and tcp sockets

[Jens Axboe <axboe@kernel.dk>]

On 11/4/20 4:21 AM, Stefan Metzmacher wrote:
> Hi David,
> 
>> New to io_uring but can't find this answer online, so reaching out.
>>
>> I was trying out io_uring with netperf - tcp stream sockets - and
>> noticed a submission is called complete even with a partial send
>> (io_send(), ret < sr->len). Saving the offset of what succeeded (plus
>> some other adjustments) and retrying the sqe again solves the problem.
>> But the issue seems fundamental so wondering if is intentional?
> 
> I guess this is just the way it is currently.
> 
> For Samba I'd also like to be sure to never get short write to a socket.
> 
> There I'd like to keep the pipeline full by submitting as much sqe's as possible
> (without waiting for completions on every single IORING_OP_SENDMSG/IORING_OP_SPLICE)
> using IOSQE_IO_DRAIN or IOSQE_IO_LINK and maybe IOSQE_ASYNC or IORING_SETUP_SQPOLL.
> 
> But for now I just used a single sqe with IOSQE_ASYNC at a time.
> 
> Jens, do you see a way to overcome that limitation?
> 
> As far as I understand the situation is completely fixed now and
> it's no possible to get short reads and writes for file io anymore, is that correct?

Right, the regular file IO will not return short reads or writes, unless a
blocking attempt returns 0 (or short). Which would be expected. The send/recvmsg
side just returns what the socket read/write would return, similarly to if you
did the normal system call variants of those calls.

It would not be impossible to make recvmsg/sendmsg handle this internally as
well, we just need a good way to indicate the intent of "please satisfy the
whole thing before return".

-- 
Jens Axboe