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From: Jens Axboe <axboe@kernel.dk>
To: Mark Papadakis <markuspapadakis@icloud.com>, io-uring@vger.kernel.org
Subject: Re: io_uring and spurious wake-ups from eventfd
Date: Tue, 7 Jan 2020 13:26:56 -0700
Message-ID: <55243723-480f-0220-2b93-74cc033c6e1d@kernel.dk> (raw)
In-Reply-To: <2005CB9A-0883-4C35-B975-1931C3640AA1@icloud.com>

On 1/7/20 8:55 AM, Mark Papadakis wrote:
> This is perhaps an odd request, but if it’s trivial to implement
> support for this described feature, it could help others like it ‘d
> help me (I ‘ve been experimenting with io_uring for some time now).
> 
> Being able to register an eventfd with an io_uring context is very
> handy, if you e.g have some sort of reactor thread multiplexing I/O
> using epoll etc, where you want to be notified when there are pending
> CQEs to drain. The problem, such as it is, is that this can result in
> un-necessary/spurious wake-ups.
> 
> If, for example, you are monitoring some sockets for EPOLLIN, and when
> poll says you have pending bytes to read from their sockets, and said
> sockets are non-blocking, and for each some reported event you reserve
> an SQE for preadv() to read that data and then you io_uring_enter to
> submit the SQEs, because the data is readily available, as soon as
> io_uring_enter returns, you will have your completions available -
> which you can process.  The “problem” is that poll will wake up
> immediately thereafter in the next reactor loop iteration because
> eventfd was tripped (which is reasonable but un-necessary).
> 
> What if there was a flag for io_uring_setup() so that the eventfd
> would only be tripped for CQEs that were processed asynchronously, or,
> if that’s non-trivial, only for CQEs that reference file FDs?
> 
> That’d help with that spurious wake-up.

One easy way to do that would be for the application to signal that it
doesn't want eventfd notifications for certain requests. Like using an
IOSQE_ flag for that. Then you could set that on the requests you submit
in response to triggering an eventfd event.

-- 
Jens Axboe


  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-07 15:55 Mark Papadakis
2020-01-07 20:26 ` Jens Axboe [this message]
2020-01-07 20:34   ` Jens Axboe
2020-01-08  7:36     ` Mark Papadakis
2020-01-08 16:24       ` Jens Axboe
2020-01-08 16:46         ` Mark Papadakis
2020-01-08 16:50           ` Jens Axboe
2020-01-08 17:20             ` Jens Axboe
2020-01-08 18:08               ` Jens Axboe
2020-01-09  6:09         ` Daurnimator
2020-01-09 15:14           ` Jens Axboe

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