Ingo Molnar wrote:
* Ingo Molnar <mingo@xxxxxxx> wrote:
[...] "how do we give low latencies to audio applications (and other,
soft-RT alike applications), while not allowing them to lock up the
system."
ok, here is another approach, against 2.6.10/11-ish kernels:
http://redhat.com/~mingo/rt-limit-patches/
this patch adds the /proc/sys/kernel/rt_cpu_limit tunable: the maximum
amount of CPU time all RT tasks combined may use, in percent. Defaults
to 80%.
just apply the patch to 2.6.11-rc2 and you should be able to run e.g. "jackd -R" as an unprivileged user.
note that this allows the use of SCHED_FIFO/SCHED_RR policies, without
the need to add any new scheduling classes. The RT CPU-limit acts on the
existing RT-scheduling classes, by adding a pretty simple and
straightforward method of tracking their CPU usage, and limiting them if
they exceed the threshold. As long as the treshold is not violated the
scheduling/latency properties of those scheduling classes remains.
It would be very interesting to see how jackd/jack_test performs with
this patch applied, and rt_cpu_limit is set to different percentages,
compared against unpatched SCHED_FIFO performance.
Indeed it would be interesting because assuming there are no bugs in my SCHED_ISO implementation (which is unlikely) it should perform the same.
There are a number of features that it would be nice to have addressed if we take this route.
Superusers are unable to set anything higher priority than unprivileged users. Any restrictions placed on SCHED_RR/FIFO for unprivileged users affect superuser tasks as well. The default setting breaks the definition of these policies, yet changing the setting to 100 gives everyone full rt access.
ie it would be nice for there to be discrepancy between the default cpu limits and priority levels available to unprivileged vs superusers, and superusers' default settings to remain the same as current SCHED_RR/FIFO behaviour.
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