On 10/08/2011 04:02 AM, Clemens Ladisch wrote:
Michael Ost wrote:
The higher priority threads in the system are:
...
* IRQ8 (rtc) - FIFO/99
Why does this interrupt get such a high priority?
(Not that it matters as I don't expect it to be used at all ...)
No, it isn't used. This is just a by-product of the rtirq script that we
run at startup.
Have you ever seen "migration" or "watchdog" hold the CPU for any length
of time? I was curious about "migration" since
/proc/sys/kernel/sched_migration_cost = 500000
so I set cpu affinity on the threads, but it didn't change things.
Are there issues with memory mapping, that can block other unrelated
threads?
Then you would have seen page faults.
Yes, that was what I figured too. Thanks for the confirmation.
There do appear to be "involuntary context switches" (as reported by
getrusage) when the spikes happen. This makes it seem like the
scheduler is interrupting our threads. But how do you figure out why
that is happening? [...] All of the 5 processing threads are
SCHED_RR/76. [...] Are there just too danged many SCHED_RR threads
fighting for two cores?
RR means Round Robin, i.e., all threads with the same priority get
an equal amount of CPU without much delay, so the scheduler has to
switch between them quite often. RR is intended for threads that must
make some progress continuously. (See "man sched_setscheduler" and
especially "man sched_rr_get_interval".)
If you want to run a thread until it has finished for now, use
SCHED_FIFO.
I assume it is your intention that all those threads have equal priority
(which means to the kernel "don't care which one of them gets executed
first").
SCHED_RR is what we want. Our realtime audio processing (and midi input)
threads all need to be able to run. They just must not be interrupted
ever by GUI or user level threads.
I'm not totally clear on how SCHED_RR and SCHED_FIFO relate though.
Would (a) a SCHED_RR/50 thread be run ahead of a SCHED_FIFO/49 thread?
And would (b) a SCHED_RR/50 thread interrupt a running SCHED_FIFO/49 thread?
Thanks for your reply,
Michael Ost
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