[PlanetCCRMA] The JACK Story (or "The State of (the Union) JACK" speech)

David Nielson naptastic at comcast.net
Mon Nov 23 13:09:48 PST 2009

I want to make sure I understand all this correctly.

1. When we are talking about "priority" in this context, are we talking 
about which processes have permission to preempt which other processes? 
A higher priority task can preempt a lower priority task but not vice-versa?

2. When we talk about "kernel threads" (rtc) are we talking about things 
like the scheduler, which should never ever be preempted, or are we 
talking about things like disk I/O?

3. Does the system in place distinguish between critical kernel 
processes, like the scheduler, and things that a user might want to put 
off until later, like disk I/O? (The reason I ask is that, with my 
setup, I can get 128x2 to be rock-solid *unless* there is an SD card in 
my computer's SD card reader, and if I am copying a file from the SD 
card to the hard disk, I get xruns of 40ms (forty milliseconds, REALLY!) 
so there is obviously something in the I/O that should get preempted but 

4. Would rt permissions for regular users be unacceptable to regular 
Fedora because of the possibility of a user crafting or unintentionally 
running a task with a high priority which then crashes, potentially 
taking down the entire system?

5. Does it really make that much difference? I know software I can write 
and run on a non-rt kernel as a regular user that basically takes down 
the entire system for everyone. So what's the big deal?

6. If I understand correctly, the ability of a user to start 
high-priority tasks depends on their membership in the jackuser group. 
Would it be acceptable to the Fedora people to have a "audio 
configuration wizard" applet (forgive me; I grew up with Windows) that 
requires the root user to grant membership to the jackuser group, and 
includes a Big Scary Warning about the potential for disaster?

(I think it's probably safe to assume that in most DAWs, the end user 
and superuser are the same person, but for the average Fedora 
installation, that's an unsafe assumption?)

David Nielson

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