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From: Jens Axboe <[email protected]>
To: Artyom Pavlov <[email protected]>, [email protected]
Subject: Re: Sending CQE to a different ring
Date: Wed, 9 Mar 2022 18:36:39 -0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 3/9/22 4:49 PM, Artyom Pavlov wrote:
> Greetings!
> 
> A common approach for multi-threaded servers is to have a number of
> threads equal to a number of cores and launch a separate ring in each
> one. AFAIK currently if we want to send an event to a different ring,
> we have to write-lock this ring, create SQE, and update the index
> ring. Alternatively, we could use some kind of user-space message
> passing.
> 
> Such approaches are somewhat inefficient and I think it can be solved
> elegantly by updating the io_uring_sqe type to allow accepting fd of a
> ring to which CQE must be sent by kernel. It can be done by
> introducing an IOSQE_ flag and using one of currently unused padding
> u64s.
> 
> Such feature could be useful for load balancing and message passing
> between threads which would ride on top of io-uring, i.e. you could
> send NOP with user_data pointing to a message payload.

So what you want is a NOP with 'fd' set to the fd of another ring, and
that nop posts a CQE on that other ring? I don't think we'd need IOSQE
flags for that, we just need a NOP that supports that. I see a few ways
of going about that:

1) Add a new 'NOP' that takes an fd, and validates that that fd is an
   io_uring instance. It can then grab the completion lock on that ring
   and post an empty CQE.

2) We add a FEAT flag saying NOP supports taking an 'fd' argument, where
   'fd' is another ring. Posting CQE same as above.

3) We add a specific opcode for this. Basically the same as #2, but
   maybe with a more descriptive name than NOP.

Might make sense to pair that with a CQE flag or something like that, as
there's no specific user_data that could be used as it doesn't match an
existing SQE that has been issued. IORING_CQE_F_WAKEUP for example.
Would be applicable to all the above cases.

I kind of like #3 the best. Add a IORING_OP_RING_WAKEUP command, require
that sqe->fd point to a ring (could even be the ring itself, doesn't
matter). And add IORING_CQE_F_WAKEUP as a specific flag for that.

-- 
Jens Axboe


  reply	other threads:[~2022-03-10  1:36 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-09 23:49 Sending CQE to a different ring Artyom Pavlov
2022-03-10  1:36 ` Jens Axboe [this message]
2022-03-10  1:55   ` Jens Axboe
2022-03-10  2:33     ` Jens Axboe
2022-03-10  9:15       ` Chris Panayis
2022-03-10 13:53       ` Pavel Begunkov
2022-03-10 15:38         ` Jens Axboe
2022-03-10  2:11   ` Artyom Pavlov
2022-03-10  3:00     ` Jens Axboe
2022-03-10  3:48       ` Artyom Pavlov
2022-03-10  4:03         ` Jens Axboe
2022-03-10  4:14           ` Jens Axboe
2022-03-10 14:00             ` Artyom Pavlov
2022-03-10 15:36             ` Artyom Pavlov
2022-03-10 15:43               ` Jens Axboe
2022-03-10 15:46                 ` Jens Axboe
2022-03-10 15:52                   ` Artyom Pavlov
2022-03-10 15:57                     ` Jens Axboe
2022-03-10 16:07                       ` Artyom Pavlov
2022-03-10 16:12                         ` Jens Axboe
2022-03-10 16:22                           ` Artyom Pavlov
2022-03-10 16:25                             ` Jens Axboe
2022-03-10 16:28                               ` Artyom Pavlov
2022-03-10 16:30                                 ` Jens Axboe
2022-03-10 13:34       ` Pavel Begunkov
2022-03-10 13:43         ` Jens Axboe
2022-03-10 13:51           ` Pavel Begunkov
2022-03-10  3:06     ` Jens Axboe

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