5 ms·

One of the painful aspects of WASM is there's no blocking calls. You can't say "wait for the next event;" instead you must return to the outermost event loop, a

One of the painful aspects of WASM is there's no blocking calls. You can't say "wait for the next event;" instead you must return to the outermost event loop, and wait to be called back. How does Python-in-WASM work around that? For example, how does `for line in sys.stdin:` work if you can't actually block on stdin? Emscripten has some support for this via the "asyncify" transform, which layers additional control flow to enable return all the way up the call stack, and then "rewind" back down into it. But this bloats the code (and is also buggy) so maybe it's not being used.

bayesian_horse 5y ago
For Pyodide I made a proof of concept where you can run async/await with promises. If people actually develop stuff in Python for the web, they should do something like that.
trothamel 5y ago
In my experience, asyncify works pretty well everywhere but Safari on macOS/ARM or ios/ARM, where the stack sizes are too small to be useful. You do want to be a bit careful about where you block, which can minimize the number of functions that need to be transformed.
mimipp 5y ago
on point !
bkolobara 5y ago
It's on the embedder (Wasm VM) to provide this functionality. I'm working on a Wasm runtime [0] that is written in Rust and uses stack switching to allow you to call Rust async functions as if they were blocking. This keeps the Wasm bytecode simple (blocking), but at the same time provides high performance i/o. There is also a proposal to bring stack switching to the browser. [0]: https://github.com/lunatic-solutions/lunatic https://github.com/lunatic-solutions/lunatic
- dosshell 5y ago
  What is the difference between "stack switching" and a fiber based approach?
  - bkolobara 5y ago
    Fibers are usually implemented with stack switching. It should be the same approach.
ethanhsmith 5y ago
Yes, currently input goes into a propmpt() and it doesn't output anything unless you hit "Cancel" on the prompt, definitely a bad time. Python allows you to reach in and replace the core interpreter loop, so this may be an avenue to have our own asyncify-like function pop out to JS land and restore state correctly (which we can be smart about since we are the interpreter). It may also be possible to write something that runs Python in a webworker and communicate with it over a sharedarraybuffer, but that I'm a bit more hazy on. Pyodide has some discussion of this in https://github.com/pyodide/pyodide/issues/1219 https://github.com/pyodide/pyodide/issues/1219 and https://github.com/pyodide/pyodide/issues/1503 https://github.com/pyodide/pyodide/issues/1503. This is definitely the hardest part of getting Python to work. Well, hardest after the hardest part of building a compiler toolchain like Emscripten :)
- benno128 5y ago
  In WebAssembly.sh (https://github.com/wasmerio/webassembly.sh https://github.com/wasmerio/webassembly.sh) they run WASM binaries in a Web Worker and then use `SharedArrayBuffer` to block the WebWorker while the main thread does some work (e.g. collect input). You could use a similar solution. When building Runno (https://runno.dev https://runno.dev) I forked off that project and did a bunch of other things on top to get blocking to work in Safari and non-cross-origin-isolated contexts. Ultimately I think it's JavaScript's (or whichever host language) responsibility to block when the binary calls out (if that is the expected semantics).
  - deleted 5y ago
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  - est31 5y ago
    Does the SharedArrayBuffer approach offer any way to tell the OS scheduler to wait? Because the only waiting method I know of is busy waiting, aka wasting cycles at 100% of a CPU core. In normal processes you can call a sleep function, which saves CPU cycles, but there is no synchronous method for that available in javascript.
    - 5y ago
deleted 5y ago
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samwillis 5y ago
I know Absurd SQL[0] uses SharedArrayBuffer and Atomics to turn the async IndexDB into sync for use by Wasm. I wander if it’s possible to use that here too although it’s obviously a little different? 0: https://github.com/jlongster/absurd-sql https://github.com/jlongster/absurd-sql 1: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... 2: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
deleted 5y ago
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azakai 5y ago
> and is also buggy If you've found any bugs in Asyncify please file them! There are no open issues atm about any general bugs, aside from some corner cases with features like dynamic linking.
lights0123 5y ago
You can use Atomics.wait() to block a non-main thread.

bayesian_horse 5y ago

For Pyodide I made a proof of concept where you can run async/await with promises. If people actually develop stuff in Python for the web, they should do something like that.

trothamel 5y ago

In my experience, asyncify works pretty well everywhere but Safari on macOS/ARM or ios/ARM, where the stack sizes are too small to be useful. You do want to be a bit careful about where you block, which can minimize the number of functions that need to be transformed.

mimipp 5y ago

on point !

bkolobara 5y ago

It's on the embedder (Wasm VM) to provide this functionality. I'm working on a Wasm runtime [0] that is written in Rust and uses stack switching to allow you to call Rust async functions as if they were blocking. This keeps the Wasm bytecode simple (blocking), but at the same time provides high performance i/o. There is also a proposal to bring stack switching to the browser. [0]: https://github.com/lunatic-solutions/lunatic https://github.com/lunatic-solutions/lunatic

dosshell 5y ago
What is the difference between "stack switching" and a fiber based approach?
- bkolobara 5y ago
  Fibers are usually implemented with stack switching. It should be the same approach.

ethanhsmith 5y ago

Yes, currently input goes into a propmpt() and it doesn't output anything unless you hit "Cancel" on the prompt, definitely a bad time. Python allows you to reach in and replace the core interpreter loop, so this may be an avenue to have our own asyncify-like function pop out to JS land and restore state correctly (which we can be smart about since we are the interpreter). It may also be possible to write something that runs Python in a webworker and communicate with it over a sharedarraybuffer, but that I'm a bit more hazy on. Pyodide has some discussion of this in https://github.com/pyodide/pyodide/issues/1219 https://github.com/pyodide/pyodide/issues/1219 and https://github.com/pyodide/pyodide/issues/1503 https://github.com/pyodide/pyodide/issues/1503. This is definitely the hardest part of getting Python to work. Well, hardest after the hardest part of building a compiler toolchain like Emscripten :)

benno128 5y ago
In WebAssembly.sh (https://github.com/wasmerio/webassembly.sh https://github.com/wasmerio/webassembly.sh) they run WASM binaries in a Web Worker and then use `SharedArrayBuffer` to block the WebWorker while the main thread does some work (e.g. collect input). You could use a similar solution. When building Runno (https://runno.dev https://runno.dev) I forked off that project and did a bunch of other things on top to get blocking to work in Safari and non-cross-origin-isolated contexts. Ultimately I think it's JavaScript's (or whichever host language) responsibility to block when the binary calls out (if that is the expected semantics).
- deleted 5y ago
  [deleted]
- est31 5y ago
  Does the SharedArrayBuffer approach offer any way to tell the OS scheduler to wait? Because the only waiting method I know of is busy waiting, aka wasting cycles at 100% of a CPU core. In normal processes you can call a sleep function, which saves CPU cycles, but there is no synchronous method for that available in javascript.
  - 5y ago

deleted 5y ago

[deleted]

samwillis 5y ago

I know Absurd SQL[0] uses SharedArrayBuffer and Atomics to turn the async IndexDB into sync for use by Wasm. I wander if it’s possible to use that here too although it’s obviously a little different? 0: https://github.com/jlongster/absurd-sql https://github.com/jlongster/absurd-sql 1: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... 2: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

deleted 5y ago

[deleted]

azakai 5y ago

> and is also buggy If you've found any bugs in Asyncify please file them! There are no open issues atm about any general bugs, aside from some corner cases with features like dynamic linking.

lights0123 5y ago

You can use Atomics.wait() to block a non-main thread.