[Jack-Devel] graph simplification using mathematics?

Fons Adriaensen fons at linuxaudio.org
Sun Jul 3 12:55:01 CEST 2016

On Sat, Jul 02, 2016 at 08:55:04PM -0400, Paul Davis wrote:

> There are all kinds of cool ideas for how to do things inside a single
> process - Faust is one of them. The inter-application stuff is really a
> different ball of wax. Fons sounds as if he has some interesting insights
> there, but it would be hard to integrate them into any host that wants to
> be able to run without JACK.

Not at all, and this was even an explicit design goal.

I'll try to explain how this works. 
There are three types of 'clients'. All three use essentially
the same API, only some options are different.

The first type is entirely equivalent to a Jack client as we
know it. It has a process callback and ports. Such clients
can run in parallel in same way as in Jack2, you can also
have more than one in a process, but that doesn't change
the way they are handled. Nothing new here.

The second and third types you'd use to create a complex
application consisting of an arbitrary graph of modules
in a single process, e.g. a DAW. 

You first create an 'outer client'. Such a client has ports,
but it doesn't have a process callback. Instead it will
encapsulate the internal multithreaded scheduler of the
application. To the end user (e.g. in qjackctl) it just
looks like the first type.

Next you create the modules, which are the third type. The
open() just takes one additional parameter: a reference to
the outer client. Otherwise the API is exactly the same,
you define a callback and create ports. The difference
is that these ports are not visible externally. They can
be used (by the application code only) in two ways: you
can connect them to other internal modules, or map them
to a port of the outer client. By doing the latter the
module becomes visible to the outer scheduler (but not
to the end user). The outer and internal scheduler work
together in such a way as if there were only one, seeing
the entire combined graph. At the same time, everything
that can be handled by the internal scheduler will be
taken care of locally, without any additional overhead
or context switches.

Now if the application doesn't want to be a 'Jack client'
but e.g. use ALSA directly, it still uses an internal
scheduler with the same API, and instead of mapping
internal (module) ports to the externally visible ones
of the outer client, it maps them to channels of the ALSA
interface. To the end user it just looks the same. To the
application, the only difference is that all external
triggers will always arrive at the same time, at the
start of an ALSA period. 



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