anb (Apr 18 2024 at 22:07): anb (Apr 18 2024 at 22:07):Hi, I'd like to implement a waker for async IO, but there's nothing similar to pipe(2) or eventfd(2) defined in preview2/wasi-io. I wonder if I missed anything.
Pat Hickey (Apr 18 2024 at 22:11):nope, no such thing exists in WASI. could you describe more what problem you are trying to solve? maybe there is another way to go about it
Pat Hickey (Apr 18 2024 at 22:19):just to expand a little: one way of thinking about this is, there is not (yet) a way to have more than one stack of control (thread, coroutine, whatever) per store with wasi. so there are no facilities for waking another stack/thread, since there can only be one
Pat Hickey (Apr 18 2024 at 22:21):when that restriction is lifted by landing threads and/or async (multiple stacks) in the component model in the 0.3 timeframe (roughly a year out, maybe longer), it will come with facilities for waking another thread/stack
anb (Apr 18 2024 at 22:26):Hi Pat, I want to make a async runtime on the guest side based on tokio/mio. The problem is that no waker is implemented for WASI at the moment, meaning tasks won't be wake up. It is often implemented via eventfd(2) on Linux, or pipe(2) in some other situations.[1] I tried to come up with something[2], but the interface is awful. I wonder if there was a discussion regarding the usefulness of the interface, as I vaguely remember the pipe interface was in wasi-io in the early days.
[1]: https://github.com/tokio-rs/mio/blob/master/src/waker.rs
[2]: https://github.com/tokio-rs/mio/pull/1662
anb (Apr 18 2024 at 22:30):I'm not sure if I fully understand the multi-stacks, but seems async tasks with single thread works for me with my mio waker implementation(with wasmtime-9).
Pat Hickey (Apr 18 2024 at 22:30):hmm. well, i havent tried to do that, but i suspect that the dependence on wakers is because tokio assumes a multi-threaded runtime, and wakers are the way to notify readiness between those
Pat Hickey (Apr 18 2024 at 22:30):oh ok, so if you're on wasmtime-9 you're using wasi preview 1 stuff
Pat Hickey (Apr 18 2024 at 22:30):have you looked at preview 2 yet?
Pat Hickey (Apr 18 2024 at 22:31):we changed a number of things to make it possible to wait on more kinds of things, but fundamentally there is still just the equivalent of poll_oneoff where the sole thread/stack of control is suspended until one of the arguments is ready
Pat Hickey (Apr 18 2024 at 22:33):again, im not really in touch with tokio on a deep enough level to know this for sure, but my suspicion is that mio is the wrong abstraction entirely for implementing a runtime on top of preview 2
Joel Dice (Apr 18 2024 at 22:33):In case it's of interest, I created experimental forks of Rust, mio, tokio, and tokio-postgres based on wasi:sockets@0.2.0, with a demo here: https://github.com/dicej/wasi-sockets-tests
Pat Hickey (Apr 18 2024 at 22:33):its interesting to me! i should look how you handled this stuff joel
Pat Hickey (Apr 18 2024 at 22:37):ah yeah so if you're using sockets on top of wasi-libc already then theyre back to being an fd so these mio changes make sense
Pat Hickey (Apr 18 2024 at 22:37):https://github.com/tokio-rs/mio/compare/master...dicej:mio:wasi-sockets
anb (Apr 18 2024 at 22:39):It's been a while, and I'm just picking up this project again, only walked through the wasmtime's changelog, haven't upgraded to preview2 yet. Not able to use stock tokio was a problem for us, so I wanted to try if I can improve the waker implementation if there's any chance.
Even with poll_oneoff, the a pair of fd could be used by tasks to "wakeup" each other because those fd are connected on the host side.
anb (Apr 18 2024 at 22:40):Thanks Joel, it looks interesting, I'll look into it in a while.
Joel Dice (Apr 18 2024 at 22:47):If you just need two async tasks within a guest to talk to each other, e.g. https://docs.rs/futures/latest/futures/channel/index.html plus either Tokio or a bare-bones executor should work fine. If you need multiple guests to talk to each other, a TCP connection to localhost could be an option.
anb (Apr 18 2024 at 22:51):Yeah, with customized hostcalls, it shouldn't be a problem. But I'd like to make something that fits mio's waker interface, e.g. Waker::new() does some magic internally. Socket will be cumbersome for this task I'm afraid.
Pat Hickey (Apr 18 2024 at 23:04):im not sure what you mean by "with customized hostcalls, it shouldnt be a problem" - the suggestion joel had, using futures, doesnt rely on the host at all. the futures implementation is in terms of atomics and (synchronous) locks, which are implemented in std and would boil down to regular memory and no-ops respectively because there arent multiple threads in wasi
Pat Hickey (Apr 18 2024 at 23:06):i think waker is a red herring here - the docs here https://docs.rs/mio/latest/mio/struct.Waker.html say waker is for cross-thread waking of Poll, and there is no such thing as cross-thread waking in this context
anb (Apr 18 2024 at 23:22):Sorry, I was still thinking about fitting into mio's interface, so it's easy to port "normal" program onto WASI, instead of changing how tasks communicate. Not sure if the single thread assumption will break by wasi-threads in the future.
Pat Hickey (Apr 18 2024 at 23:29):wasi-threads is a dead proposal, its not a design that will move forward. the folks who worked on that have changed their focus and are working on a new core wasm thread proposal that solves the problems that wasi-threads uncovered. the component model will get threads once that work is complete, and when it does this story will change. thats a year out, maybe more.
anb (Apr 18 2024 at 23:33):Oh that's good to know, I actually waited I could get wasi-threads working to reduce duplication with shared memory. I'll check the new component model later. For now I'm going to assume WASI is a single threaded env, and hope it won't shoot my foot in the future.
anb (Apr 18 2024 at 23:34):Thank you both!
Notification Bot (Apr 19 2024 at 00:11):anb has marked this topic as resolved.
Pat Hickey (Apr 19 2024 at 17:06):yep, when that assumption about wasi changes you can count on having a big effort from wasi contributors bringing the ecosystem along. so even if you do end up encountering it one day, it wont be alone!
Last updated: Dec 23 2024 at 12:05 UTC