8 May
2006
8 May
'06
5:58 p.m.
On Monday 08 May 2006 20:14, Florian Paul Schmidt wrote:
On Mon, 8 May 2006 12:03:34 -0500
"Josh Lawrence" <hardbop200(a)gmail.com> wrote:
Apropos: on your page on -rt setup (excellent page, btw., many thanks! :-),
you suggest raising "softirq-timer/0" to prio 99, to make sleep() function
right (http://tapas.affenbande.org/?page_id=40 sleep() based/system timer).
I did so, and got strange latencies (> 40 ms) exactly once every 10 minutes,
caused by some routing-related action (rt_secret_rebuild) being run by the
softirq-timer/0 thread. Don't you get bit by that, too? Kernel is 2.6.16-rt16
My solution is to configure with CONFIG_HIGH_RES_TIMERS=y. Then, sleep() wakes
up correctly even with softirq-timer/0 being low-priority (SCHED_FIFO 1 or
even SCHED_OTHER).
In general I find adjusting priorities of the various softirq kernel threads a
bit of secret art. I can't find much documentation about "what kernel thread
runs which job" that would help making some proper decisions here. I found my
desktop "feels" most responsive when demoting all softirq thread to
SCHED_OTHER. I did so after seeing that with a non-rt kernel, bottom-half
handler don't run SCHED_FIFO/_RR at all. So -rt now gives me robust low
latencies for jackd, and still proper desktop feeling.
Well, maybe this is getting off-topic for this list. But it seems to me
trimming priorities between kernel and userland threads is a bit like no
man's land.
Wolfgang
fyi, I've followed Dana's instructions
more than once and found them
to be excellent. Just use a little common sense when following them
(there are newer versions of the kernel and the patch out) and you'll
be fine.
As Dana mentioned, there's a LOT more to getting realtime working than
just the kernel - be sure to read everything there.
I browsed over that page and didn't find any info on setting up the irq
handler priorities which is _the_ essential feature of -rt.