[Jack-Devel] Usage feasibility Q

[Filipe Coelho]

On 27.01.2018 21:55, Chris Caudle wrote:
> On Sat, January 27, 2018 12:42 pm, Filipe Coelho wrote:
>>> The jack design is not really made for having
>>> plugins in the server.
>> The JACK internal clients API disagrees with you ;)
> OK, I stated too strongly.  Perhaps a better way to state is that
> implementing the DSP as an external jack program may be more convenient,
> and would be the more typical way to implement.
>
> I'm not completely convinced that what I stated was actually too strong,
> though.
> To quote Paul Davis
> https://lists.linuxaudio.org/pipermail/linux-audio-user/2018-January/109472.html
> "In-process JACK clients are a very special case and almost nobody
> implements or uses them. They are like plugins for the JACK server, and
> that's not really the point of JACK (which was designed to connect distinct
> processes)."
>
> I consider Paul a reasonably good authority on jack programming.

Me too, and what he says is right.
JACK was made with external clients in mind, and it's what 99.9% of apps 
use.
But there are specific scenarios where internal clients are useful.


>> Internal clients are faster to run, since they dont require context
>> switches between different processes.
> Context switches are only a few microseconds.  Whether to implement as a
> plugin or external client should be based on development effort and ease
> of debugging, the context switch time should be less than 0.5% of any
> reasonably sized period so should not be a consideration.
>
>> They're harder to control externally though.
> Which functions are implemented as jack plugins?  There are several
> example clients available from the jack source repository, are there
> example internal plugins?

I responded to these 2 on the reply from Fons.
But in short, pretty much all of the MOD software runs as internal 
client (host, peak-meter, serial-midi-handler and file player).
This makes a real difference on their hardware, and was the way to go to 
maximize performance.

They're hard to control because you don't have direct access to the 
process running your client unless you start JACK yourself.
Typically you make your internal client listen to a known socket or 
other interface, and connect there.

On the other hand, running as internal client means if your process 
crashes, JACK crashes too (they're the same process).