Implementing Async MPMC Channel in Rust

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Implementing Async MPMC Channel in Rust Motoyuki Kimura 1

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MPMC Channel? ● Multi Producer Multi Consumer Channel ● i.e. have multiple senders and multiple receivers 2 Sender Sender Sender Receiver Receiver

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Channel Types ● Bounded / Unbounded ● Async / Sync ● SPSC, MPSC, MPMC 3

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Prior Art 4 kind bounded / unbounded sync / async std::mpmc mpmc, mpsc, etc both sync crossbeam mpmc, mpsc, etc both sync async_channel mpmc both async kanal mpmc both sync/async Go channel mpmc both async

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How do channels work? 5

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● Allocate a fixed length area Bounded 6 Sender Receiver

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Bounded 7 AAA BBB CCC Sender Receiver sender.send(CCC)

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Bounded 8 BBB CCC Sender AAA Receiver

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Bounded 9 HHH DDD EEE FFF GGG Sender CCC Receiver

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Bounded 10 Sender Receiver ● If there is no available data, reader would block Block!

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Bounded 11 HHH III CCC DDD EEE FFF GGG Sender Receiver ● If receiver is too slow, sender would block Block!

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Bounded 12 HHH DDD EEE FFF GGG Sender Sender Sender Receiver Receiver ● Actually, multiple senders and receivers work concurrently

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Implementation Details 13

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How to achieve block/unblock? 14

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Block / Unblock 15 ● Sync channel ○ thread api (park, unpark) ● Async channel ○ waker api

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Block / Unblock 16 Sender Receiver Block!

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Block / Unblock 17 Sender Receiver Waker Waker list ● When block happens, store waker somewhere

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Block / Unblock 18 CCC Sender Receiver Waker1 Waker list ● When block happens, store waker somewhere Pop from list

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Depth of Wakerlist … 🚀 19 ● For wakerlist, intrusive linked list would be much performant ○ What are intrusive linked lists? ○ new internal semaphore based on intrusive lists

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How to test concurrent code base? 20

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How to test concurrent code base? 21 ● Multi threaded ● Complex synchronization ● Atomic Operations

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How to test concurrent code base? 22

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How to test concurrent code base? 23 Detect Deadlocks! 🙌

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About Loom internals … 24 Video 🎥: https://youtu.be/-MRCgyiCxWU?si=a4NNwSxVgke6AFwZ Slide 📝: https://speakerdeck.com/kimuramotoyuki/bug-free-concurrency-in-rust