Re: Short sends returned in IORING

[Constantine Gavrilov <constantine.gavrilov@gmail.com>]

On Wed, May 4, 2022 at 4:54 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 5/3/22 5:05 PM, Constantine Gavrilov wrote:
> > Jens:
> >
> > This is related to the previous thread "Fix MSG_WAITALL for
> > IORING_OP_RECV/RECVMSG".
> >
> > We have a similar issue with TCP socket sends. I see short sends
> > regarding of the method (I tried write, writev, send, and sendmsg
> > opcodes, while using MSG_WAITALL for send and sendmsg). It does not
> > make a difference.
> >
> > Most of the time, sends are not short, and I never saw short sends
> > with loopback and my app. But on real network media, I see short
> > sends.
> >
> > This is a real problem, since because of this it is not possible to
> > implement queue size of > 1 on a TCP socket, which limits the benefit
> > of IORING. When we have a short send, the next send in queue will
> > "corrupt" the stream.
> >
> > Can we have complete send before it completes, unless the socket is
> > disconnected?
>
> I'm guessing that this happens because we get a task_work item queued
> after we've processed some of the send, but not all. What kernel are you
> using?
>
> This:
>
> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.19/io_uring&id=4c3c09439c08b03d9503df0ca4c7619c5842892e
>
> is queued up for 5.19, would be worth trying.
>
> --
> Jens Axboe
>

Jens:

Thank you for your reply.

The kernel is 5.17.4-200.fc35.x86_64. I have looked at the patch. With
the solution in place, I am wondering whether it will be possible to
use multiple uring send IOs on the same socket. I expect that Linux
TCP will serialize multiple send operations on the same socket. I am
not sure it happens with uring (meaning that socket is blocked for
processing a new IO until the pending IO completes). Do I need
IOSQE_IO_DRAIN / IOSQE_IO_LINK for this to work? Would not be optimal
because of multiple different sockets in the same uring. While I
already have a workaround in the form of a "software" queue for
streaming data on TCP sockets, I would rather have kernel to do
"native" queueing in sockets layer, and have exrtra CPU cycles
available to the  application.

-- 
----------------------------------------
Constantine Gavrilov
Storage Architect
Master Inventor
Tel-Aviv IBM Storage Lab
1 Azrieli Center, Tel-Aviv
----------------------------------------