Upgraded Kernel. Now my experimental io_uring branch of postgres is broken :( Different backends, inheriting io_urings from the postmaster, aren't allowed to submit events anymore. Only consume. Hm.
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Replying to @AndresFreundTec
Ah saw the email. Yes, I think we should revert this change for now. That’ll give us some time to discuss it more thoroughly.
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Replying to @axboe
Cool. Found another fork related one (fcb323cc53e2). I guess I better contribute a liburing test for sharing a uring across a fork... Shoul've upgrade the kernel earlier in the cycle :(
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Replying to @AndresFreundTec
Your situation is unique in that you're sharing across forks, so yeah would be great to have actual test cases for that in liburing. Always nice to test early, but test cases are better since it means I don't have to rely on others testing early.
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Replying to @axboe
Ok, will try to write one. Do you think sharing between processes is a bad path to go down, given the reasoning I outlined (https://lore.kernel.org/io-uring/20200126055457.5w4f5jyhkic7cixu@alap3.anarazel.de/ …)? I'd rather not work towards building on top of io_uring, that the author of uring finds too constraining for future development.
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Replying to @AndresFreundTec
Your sharing is fine, I have no issue with that. My only concern with sharing, and this isn't specific to across fork, is the implied extra overhead in terms of synchronization you need to have. I've got some ideas to improve that situation, though.
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Replying to @axboe
I'm curious about what those ideas to reduce the synchronization overhead are. Relatedly, one other reason to share urings more widely is that it's feasible to have one or two sqthreads, but not dozens.
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I don't think PG can currently benefit from polled submission, due to bottlenecks it has currently, but I hope to address them as part of the io_uring work. And after that using polling e.g. for the journal could be huge.
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Replying to @AndresFreundTec @axboe
I do wonder if it'd be feasible to, optionally, also share sqthreads between urings, not just the wq workers.
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Replying to @AndresFreundTec
Yeah I think that could be interesting proposition. I think we can just use the same "share wq backend" attachment to imply the same thing.
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