Playing with @async is blocking

[gstavrinos]

Hey Daniel,

I have a MWE for my problem:

using WAV
y, fs = wavread("sound.wav")
@async wavplay(y, fs)
while true println("async!") end

I would expect to see multiple "async!" messages, but I see only one. Any thoughs?

Thanks

[dancasimiro]

Which wavplay implemention are you using? Is it the pulse audio implementation or the native macOS implementation?

[gstavrinos]

Oh, yeah, sorry. I am on Ubuntu 18.04, so pulse audio!

[gstavrinos]

Any updates on this?

[dancasimiro]

I’m sorry, but I don’t have time to track this issue down. I am accepting pull requests though.

[gstavrinos]

Sure, no problem. I'll see what I can do, but my schedule is tight too! Thank you!

[mgkuhn]

@async just creates a task (for co-routines) and does not run it in another thread, so your example works sequentially (single-threaded), as expected.

For what you wanted, try to spawn a task into an available thread:

$ cat test.jl
using WAV
using Base.Threads
y, fs = wavread("example.wav")
Threads.@spawn wavplay(y, fs)
while true println("async!") end
$ JULIA_NUM_THREADS=2 julia test.jl

Note that Threads.@spawn is still an experimental feature as of Julia 1.4.

What I think you probably really wanted: if a backend provides asynchronous playback (i.e., tell the C function call to return immediately), we could expose this to the user e.g. as wavplay(y, fs; async=true).

The Windows backend apparently can do this: we could replace the flag SND_SYNC with SND_ASYNC, although I'm not sure if this alone is memory safe (as Julia might garbage-connect the waveform buffer before it is played asynchronously). I don't know yet about the other backends.

[dancasimiro]

closing due to lack of follow up