10.03.2022 04:36, Jens Axboe wrote:
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.
No, ideally I would like to be able to send any type of SQE to a
different ring. For example, if I see that the current ring is
overloaded, I can create exactly the same SQEs as during usual
operation, but with a changed recipient ring.
Your approach with a new "sendable" NOP will allow to emulate it in
user-space, but it will involve unnecessary ring round-trip and will be
a bit less pleasant in user code, e.g. we would need to encode a
separate state "the task is being sent to a different ring" instead of
simply telling io-uring "read data and report CQE on this ring" without
any intermediate states.
