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From: Constantine Gavrilov <[email protected]>
To: Jens Axboe <[email protected]>
Cc: [email protected]
Subject: Re: Short sends returned in IORING
Date: Wed, 4 May 2022 18:21:04 +0300	[thread overview]
Message-ID: <CAAL3td3Em=MBPa9iJitYTAkndymzuj2DbSnbQRf=0Emsr5qHVw@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>

On Wed, May 4, 2022 at 4:54 PM Jens Axboe <[email protected]> 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
----------------------------------------

  reply	other threads:[~2022-05-04 15:22 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-03 23:05 Short sends returned in IORING Constantine Gavrilov
2022-05-04 13:54 ` Jens Axboe
2022-05-04 15:21   ` Constantine Gavrilov [this message]
2022-05-04 15:28     ` Jens Axboe
2022-05-04 15:55       ` Jens Axboe
2022-05-11 14:56         ` Constantine Gavrilov
2022-05-11 15:11           ` Constantine Gavrilov
2022-05-11 15:33             ` Constantine Gavrilov
2022-05-11 19:30               ` Constantine Gavrilov
2022-05-12 16:28           ` Jens Axboe
2022-05-15 13:36             ` Constantine Gavrilov
2022-05-16 12:50               ` Constantine Gavrilov
2022-05-04 16:18   ` Constantine Gavrilov

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