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
next prev parent 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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