Re: [patch, 2.6.11-rc2] sched: /proc/sys/kernel/rt_cpu_limit tunable

[Ingo Molnar]

* Jack O'Quin <joq@xxxxxx> wrote:

> At around 55 seconds into the run, JACK got in trouble and throttled
> itself back to approximately the 30% limit (actually a little above).
> Then, around second 240 it got in trouble again, this time collapsing
> completely.  I'm a bit confused by all the messages in that log, but
> it appears that approximately 9 of the 20 clients were evertually shut
> down by the JACK server.  It looks like the second collapse around 240
> also caused the scheduler to revoke RT privileges for the rest of the
> run (just a guess).

no, the scheduler doesnt revoke RT privileges, it just 'delays' RT tasks
that violate the threshold. In other words, SCHED_OTHER tasks will have
a higher effective priority than RT tasks, up until the CPU use average
drops below the limit again.

the effect is pretty similar to starting too many Jack clients - things
degrade quickly and _all_ clients start skipping, and the whole audio
experience escallates into a big xrun mess. Not much to be done about it
i suspect. Maybe if the current 'RT load' was available via /proc then
jackd could introduce some sort of threshold above which it would reject
new clients?

> JACK can probably do a better job of shutting down hyperactive
> realtime clients than the kernel, because it knows more about what the
> user is trying to do.  Multiplying incomprehesible rlimits values does
> not help much that I can see.

please debug this some more - the kernel certainly doesnt do anything
intrusive - the clients only get delayed for some time.

> Sometimes musicians want to "push the envelope" using CPU-hungry
> realtime effects like reverbs or Fourier Transforms.  It is often hard
> to predict how much of this sort of load a given system can handle.
> JACK reports its subsystem-wide "DSP load" as a moving average,
> allowing users to monitor it.  It might be helpful if the kernel's
> estimate of this number were also available somewhere (but maybe that
> value has no meaning to users).  Often, the easiest method is to push
> things to the breaking point, and then back off a bit.

yeah, i'll add this to /proc, so that utilities can access it. Jackd
could monitor it and refuse to start new clients if the RT load is
dangerously close to the limit (say within 10% of it)?

> The equivalent rlimits experiment probably requires:
> 
>   (1) editing /etc/security/limits.conf
>   (2) shutting everything down
>   (3) logout
>   (4) login
>   (5) restarting the test

well, there's setrlimit, so you could add a jackd client callback that
instructs all clients to change their RT_CPU_RATIO rlimit. In theory we
could try to add a new rlimit syscall that changes another task's rlimit
(right now the syscalls only allow the changing of the rlimit of the
current task) - that would enable utilities to change the rlimit of all
tasks in the system, achieving the equivalent of a global sysctl.

	Ingo
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