3 Mar
2011
3 Mar
'11
4:18 p.m.
hi all,
I just came across this:
http://www.cs.unc.edu/~anderson/litmus-rt/index.html. From the site:
The LITMUSRT project is a soft real-time extension of the Linux kernel with
focus on multiprocessor real-time scheduling and
synchronization. The Linux
kernel is modified to support the sporadic task model and modular scheduler
plugins. Both partitioned and global scheduling is supported.
It seems their latest patch is against 2.6.36. I realize this is not a plug
and play alternative to Ingo's work by any means, but I was wondering if
anyone more knowledgeable has some insight into what exactly the Litmus
kernel may offer...
-michael
5 Mar
5 Mar
2:25 p.m.
On 03/03/2011 04:17 PM, michael noble wrote:
hi all,
I just came across this:
http://www.cs.unc.edu/~anderson/litmus-rt/index.html. From the site:
thanks for the link.
The LITMUSRT project is a soft real-time extension of
the Linux kernel with
...for linux-audio?
quick glance: I got the impression that it only addresses scheduling
policy (task partitioning) and not realtime task preemption. Also
software needs to be written dedicated for Litmus RT.
focus on multiprocessor real-time scheduling and
synchronization. The Linux
kernel is modified to support the sporadic task model and modular scheduler
plugins. Both partitioned and global scheduling is supported.
It seems their latest patch is against 2.6.36. I realize this is not a plug
and play alternative to Ingo's work by any means, but I was wondering if
anyone more knowledgeable has some insight into what exactly the Litmus
kernel may offer... From the site: "LITMUS RT is not a
production-quality system, and we
have currently no plans to turn it into
one."
The concept may become interesting when more audio software makes use of
multiple processors and rt-preemt is already built into the kernel.
I suppose that many of the lessons they're learning will be useful for
developing a new scheduler for Linux at some point, but it's very far
from being useful for linux-audio in the current stage.
2c,
robin
-michael
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4:26 p.m.
On Sat, Mar 5, 2011 at 10:25 PM, Robin Gareus <robin(a)gareus.org> wrote:
> quick glance: I got the impression that it only addresses scheduling
> policy (task partitioning) and not realtime task preemption. Also
> software needs to be written dedicated for Litmus RT.
Judging from the installation instructions, it seems preemption is there:
> cd litmus-rt
> patch -p1 < ../litmus-rt-2011.1.patch
> # create a working kernel configuration
> # - select HZ=1000
> # - enable in-kernel preemptions
> # - disable NO_HZ
> # - don't use power management options like frequency scaling
> # - disable support for group scheduling
> # - disable "Write protect kernel read-only data structures" (in kernel
debug)
> I got the feeling from reading that the
main thing missing is some kind of
sane interface for assigning RT priviliges to non-root users. Then again,
that maybe covered to some degree in the provided liblitmus which in turn
provides:
rt_launch, which can be used to start arbitrary programs as real-time tasks.
If I get adventurous I may try compiling this thing to see what I can see.
Most of it is over my head and I'll take your word for it that this is far
from userland friendly or ready, but it looks like a fun learning exercise.
-m
5:08 p.m.
On 03/05/2011 04:26 PM, michael noble wrote:
yes, but you'll want IRQ handlers to be processes and tweak their RT
priority; and you will need to patch libjack to start audio-threads as
/RT task/, and not run the whole application process (GUI) with elevated
priority.
For non-jack apps: each application will need to set up its audio/midi
i/o thread in RT. running a whole GUI with RT privileges will make
things worse because GUI interaction will also be able to interrupt
audio i/o.
On Sat, Mar 5, 2011 at 10:25 PM, Robin Gareus
<robin(a)gareus.org> wrote:
> quick glance: I got the impression that it only addresses scheduling
> policy (task partitioning) and not realtime task preemption. Also
> software needs to be written dedicated for Litmus RT.
Judging from the installation instructions, it seems preemption is there:
yes but not AFAICT not real-time preemtion.
Vanialla linux does offer some process preemption models (adopted from
earlier RT linux patches), but what makes Thomas Gleixner's Preempt-RT
outstanding for audio is that it allows to preemt all process including
IRQ handlers. Lots of work has gone into that and this part is not
merged into Linus' tree [yet].
cd litmus-rt
patch -p1 < ../litmus-rt-2011.1.patch
# create a working kernel configuration
# - select HZ=1000
# - enable in-kernel preemptions
# - disable NO_HZ
# - don't use power management options like frequency scaling
# - disable support for group scheduling
# - disable "Write protect kernel read-only data structures" (in kernel
debug)
I got the feeling from reading that the main thing missing is some kind of
sane
interface for assigning RT priviliges to non-root users. Then again,
that maybe covered to some degree in the provided liblitmus which in turn
provides:
rt_launch, which can be used to start arbitrary programs as real-time tasks. If I get adventurous I may try compiling this thing to
see what I can see.
Most of it is over my head and I'll take your word for it that this is far
from userland friendly or ready, but it looks like a fun learning exercise.
-m
have fun.
You'll need to come up with a test-case where multiple processors are
involved in audio processing.. - don't let me discourage you but be
prepared that it will longer than a quick afternoon adventure.
I don't know if the same tools used for testing the preempt-rt-kernel
can be used for litmus, but you may want to look into
http://git.kernel.org/?p=linux/kernel/git/clrkwllms/rt-tests.git
in particular https://rt.wiki.kernel.org/index.php/Cyclictest
ciao,
robin
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michael noble -
Robin Gareus