[CM] Question about threading and s7

[Iain Duncan]

Hi folks, I'm hoping someone can help me out with a question around S7.

In Max/MSP, when setup for live use there are (generally) two threads of
operation, with the high-priority/dsp thread able to interrupt the low/GUI.
If one doesn't do anything special, this means a max external (such as my
Scheme-for-Max) could be receiving messages in both: low for things
originating from a GUI action, high from metronomes or midi input.  I
assume that I should not expect all to be ok if I have one instance of an
s7 interpreter, which could be accessed from either thread, and it could
get interrupted part way through an eval operation to run another eval in
another thread that may access the same data. IE there's no magical thread
protection baked into S7 that I don't know about....

I'm wrestling with how to deal with this correctly. One option is to allow
users to designate an s4m instance as always-high or always-low, basically
saying if you need to mix low and high priority you should treat it like an
actor model and have two interpreters  that message each other and share
data through some non-scheme shared data structure (like max buffers or
tables). This might be ok because there is a way for me to insure incoming
max messages from any thread are either promoted or demoted.

I suppose another option is to get into critical sections, but I can't see
how that make sense if we don't want low priority actions to have the
chance of locking out high ones.

Strangely, I have not had any issues yet. But I presume that just means
I've been lucky. Cycling 74 does not (anymore) allow the javascript object
to work in both threads, and I'm thinking it must have been around thread
stability issues.

Any thoughts most welcome!
iain
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