Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

[Olivier Langlois]

On Fri, 2021-06-25 at 08:15 +0000, David Laight wrote:
> From: Jens Axboe
> > Sent: 25 June 2021 01:45
> > 
> > On 6/22/21 6:17 AM, Olivier Langlois wrote:
> > > It is quite frequent that when an operation fails and returns
> > > EAGAIN,
> > > the data becomes available between that failure and the call to
> > > vfs_poll() done by io_arm_poll_handler().
> > > 
> > > Detecting the situation and reissuing the operation is much
> > > faster
> > > than going ahead and push the operation to the io-wq.
> > > 
> > > Performance improvement testing has been performed with:
> > > Single thread, 1 TCP connection receiving a 5 Mbps stream, no
> > > sqpoll.
> > > 
> > > 4 measurements have been taken:
> > > 1. The time it takes to process a read request when data is
> > > already available
> > > 2. The time it takes to process by calling twice io_issue_sqe()
> > > after vfs_poll() indicated that data
> > was available
> > > 3. The time it takes to execute io_queue_async_work()
> > > 4. The time it takes to complete a read request asynchronously
> > > 
> > > 2.25% of all the read operations did use the new path.
> 
> How much slower is it when the data to complete the read isn't
> available?
> 
> I suspect there are different workflows where that is almost
> always true.
> 
David,

in the case that the data to complete isn't available, the request will
be processed exactly as it was before the patch.

Ideally through io_uring fast polling feature. If not possible because
arming the poll has been aborted, the request will be punted to the io-
wq.

Greetings,