[Jack-Devel] Compiling jack2 in mixed mode

[Paul Davis]

On Sat, Jul 2, 2016 at 9:36 AM, marcin at saepia.net <marcin at saepia.net> wrote:

> Hello
>
> I have recently made stress tests of Jack under load and above +/- 100
> clients it starts to behave strange.
>
> Generally speaking it seem to not be designed for such use cases -  see my
> discussion on this list  2-3 weeks ago.
>

every external JACK client implies at least 1 context switch.

context switches are not free. their cost varies depending on the size of
the working set of the process (the amount of memory touched by the
process, notably during the JACK process() callback).

they might vary from <10 usecs to as much as 500usecs (the lowest possible
number is dependent on your CPU; the upper bound depends on the clients)

100 * 100 usecs = 10msecs

so, you've just used 10msecs of the time available for the process
callback. That's enormous (quite possibly larger than the actual time
available).

JACK is quite clever, but a process-level design (i.e. context switching
between clients) cannot scale to large numbers of clients. It would require
entirely different OS designs (e.g. 64 bit address space with hardware
protection rather than using an MMU) to reduce the context times to their
bare minimum. But even then, at say 10usec per context switch, 100 clients
is still 1 msec, which is a significant chunk of "very low latency
settings".

I don't know what you have in mind as "a replacement" but nobody who has
worked on JACK (or related ideas) in the last decade and a half has ever
suggested any way around this kind of limitation.

Internal clients (basically, plugins) do not have this issue, since there
is no context switch, just a thread-level switch, which is generally the
same as a fastest-possible context switch (no MMU invalidation).  Thus, you
can scale up better, but still not *that* high.
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