8 Mar
2019
8 Mar
'19
12:22 p.m.
On 3/8/19 7:12 PM, Ethan Funk wrote:
Hello all.
1) At first, JACK seems to be designed for a single jackd server running
on a system, with the server control API lacking, as far as I can tell,
a method to control multiple servers by name. But then, with the name
command line option, I can actually run multiple servers tied to
different audio devices. Does the server control API simple not support
names at this point? Maybe this is just how JACK evolved, originally one
jackd per machine, then named server support added, but not fully
implemented across all the API?
I think the root of the problem is that the people who came up with the
idea of multiple named servers seem to really regret it. It's not
supported consistently by clients, it will break things for any but
really advanced users, and it creates a support problem in the
community. I wouldn't necessarily rely on it. Most scenarios can be
solved with a single server. And running separate graphs on one machine,
possibly as different users, to create some sort of separation of
concerns, is bound to fail - this being realtime jobs, each server can
totally hose all the others if it misbehaves. I know this is the age of
VMs, but here's a reason to really, really push a separate iron into
your rack...
Data exchange between JACK servers on separate hosts is easy and
reliable, if needed.
2) Again, with the named server: I can start a named
jackd server.
It's probably a bug. It's definitely God's way of telling you not to.
3) Regarding the jack_client_open() behavior when no
server is yet
running: the function call does seem to execute the first line of
$HOME/.jackdrc and start the jackd server running. However, it appears
to inherit all the file descriptors from my application. This is a
problem because my application is designed to self-restart on a crash.
With jackd holding my application's TCP control socket open, my
application can't restart (bind again to the desired TCP port) until
after I kill the jackd process. I assume the auto-jackd startup code is
forking and execing, and the code simply isn't closing the parent's file
descriptors. Is this a bug or intentional? Is there a way I can detect
if a jackd server is running ahead of time, so I can start the server
myself using my own fork/exec which would closing my descriptors on the
child, then, once I know jackd is running, call jack_client_open() in my
app?
First of all, as a pro user, I find apps auto-starting JACK extremely
annoying.
The behaviour you describe sounds like a bug, but (just a guess) it
might be because of the semaphore communication between JACK clients...
In a professional setting, don't auto-start JACK. Bring it up as a
system service (so you get all of systemd's watchdog features), or if
you must, tie it to a user's login session, although that's usually not
the best way.
Then test for JACK when you start, and if it isn't there, fail. A user
that is going to have a use for a radio automation system won't need
this 2% usability improvement, but will dislike the 150% burden on
debugging and tuning the system.
4) because my audio-engine is a faceless application,
and can be run
without a desktop session, I need it to be able to connect to a jackd
server run by other users, or to start a jackd server that can be used
by other users.
AFAIK, that's not going to happen by default. Look at /dev/shm/jack-* -
this is how libjack passes the baton from one client to the next.
There may be a way to make these world-readable (I recall people have
tried this), but it's pretty much the single most un-unixy thing I can
imagine.
I can verify that with Ubuntu 18.10, I can not start
jackd from a user account, then connect to it from my application
running as a different user, even if I run my app as root, not that I
intend to do that as a real world workaround. Is there some approach,
group permissions possibly, to allowing other users to access a jackd
server?
As stated above, since there is no real compartmentalisation from one
user to the next if they share a realtime graph, maybe it's not the best
approach. Why not run all JACK jobs as one user, and provide user
interfaces to several users if you must?
Then again, I will admit that I have no idea about a complex radio
workflow, so my argument might be valid from a JACK perspective only.
But if it's as simple as having N people hosting N radio shows at the
same time that will be handled by a single playout server: give each of
them a playout machine with all relevant functionality and a sound card
for local monitoring and external equipment only. The program out goes
to a network feed. I recommend zita-njbridge. It will totally isolate
your clients from the central server, and even provide adaptive
resampling if you have no way of synchronizing clocks. If you are in a
studio facility with the necessary infrastructure, you can even hook
everything up to the house wordclock and tell zita-njbridge to not
bother resampling, in which case it will be bit-transparent.
I'm sure I haven't even scratched the surface of your workflow needs,
but I hope it might give you ideas.
5) Wishful thinking: Give jackd the ability to read
QjackCtl config
files, so I could configure things from the GUI, then stop jackd and be
able to restart it from the server-control API or command line with a
command line option pointing to a config file. Better yet, make a
persistent JACK-aware place to store such file in the file hierarchy.
Will that be worth the trouble? All the state of JACK that I need to
know in my daily work is in .jackdrc.
If you find yourself needing to assemble a complex graph from several
JACK clients that is nonetheless static, i.e. needs to be recreated in
exactly the same way every boot, maybe consider hardcoding it into a set
of systemd service files, with calls to "jack_connect" in ExecPost. Has
the nice side effect that if a component dies, systemd will log it for
you and restart it if you want.
Quoting from memory, it's better to solve the right problem badly than
the wrong problem well :-D
Best,
Jörn
--
Jörn Nettingsmeier
Tuinbouwstraat 180, 1097 ZB Amsterdam, Nederland
Tel. +49 177 7937487
Meister für Veranstaltungstechnik (Bühne/Studio), Tonmeister VDT
http://stackingdwarves.net