[LAT] [LAU] Are RT-patches needed anymore? (Was Re: >= 2.6.27 RT ETA?)

krgn k.gebbert at gmail.com
Sun Feb 1 22:13:41 EST 2009


On Sun, Feb 1, 2009 at 3:08 AM, <hollunder at gmx.at> wrote:

> On Sun, 01 Feb 2009 04:42:40 +0100
> Robin Gareus <robin at gareus.org> wrote:
>
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > Fernando Lopez-Lezcano wrote:
> > > On Fri, 2009-01-30 at 03:23 +0100, torbenh at gmx.de wrote:
> > >> On Thu, Jan 29, 2009 at 03:41:48PM -0800, Fernando Lopez-Lezcano
> > >> wrote:
> > >>> On Tue, 2009-01-27 at 14:06 +0100, Peder Hedlund wrote:
> > >>>> Quoting Ken Restivo <ken at restivo.org>:
> > >>>>
> > >>>>> And here is the next installment in the saga of trying to get
> > >>>>> Ingo RT going on my Asus EEE.
> > >>>>>
> > >>>>> I successfully built and ran the 2.6.26.8-rt12 with the
> > >>>>> alsa_seq patch. It ran.
> > >>>>>
> > >>>>> The problem is that neither the Ethernet (atl1e) or wireless
> > >>>>> (rt2860sta) work. So I pretty much had to reboot back out of
> > >>>>> it immediately.
> > >>>> I've been running the standard kernel from openSUSE 11.0 on my
> > >>>> Athlon 2000+ and can get down to at least 5.3ms latency on an
> > >>>> Audiophile 2496 using the limits.conf "trick".
> > >>>>
> > >>>> Do people really need lower latencies for music purposes or are
> > >>>> we just thinking "well, I needed the RT patch three years ago; I
> > >>>> ain't stopping now" ?
> > >>> It depends on your usage (this question seems to come up every
> > >>> couple of months lately). The current kernels are much better in
> > >>> low latency applications than three years ago. They are usable if
> > >>> you don't require "low" latencies (64 or 128 x 2). What you get
> > >>> also strongly depends on the hardware mix you have.
> > >>>
> > >>> If you want to use 64 or 128 frame periods (or less) you probably
> > >>> will need at rt patched kernel in most cases. Then again if an
> > >>> occasional xrun is not a problem then you would be fine with the
> > >>> stock kernel.
> > >> i am running with -p64 -n3 on an intel-hda with 2.6.28
> > >> of course internal cards have the greatest potential for
> > >> lowlatencies. so this might be unfair, compared to pci.
> > >
> > > Hmmm, I'm not sure, the load on pci itself by a soundcard should be
> > > nothing really hard. What would the internal card use? Would not
> > > that be pci or pci express anyway?
> > >
> > surely they are.
> >
> > $ lspci  | grep Audio
> > 00:1b.0 Audio device: Intel Corporation 82801G (ICH7 Family) High
> > Definition Audio Controller (rev 02)
> >
> > The RT patch does two things:
> > It allows to prioritize interrupts and it [almost] guarantees
> > real-time scheduling for a dedicated process or thread.
> >
> > While the soundcard is low bandwith on the PCI bus, IRQ prio may still
> > be required to override HDD and [sometimes] graphics I/O; at least
> > when playing or recording many tracks. NTL, you can get a perfect
> > x-run free system without the RT patch; you can just not rely on it
> > to be as x-run free as a RT patched kernel ;)
> >
> > >> and i havent really seen xruns which i could not relate to some
> > >> programm which wasnt RT-safe, and i am compiling stuff most of the
> > >> day... though perhaps i am not pushing the DSP load hard enough.
> > >>
> > >> i did not even turn preemptible RCU on.
> > >> the latency measurement instrumentation is also in 2.6.28 btw.
> > >
> > > Well, that's very good news then!
> > >
> > > I think the last time I tried to use a non-preempt was 2.6.27.x
> > > (maybe, I would have to double check). Playing 24 channels in
> > > ardour would result in xruns, not very often but they would happen,
> > > this is with 128x2 on an RME hdsp card runing on a quad core intel
> > > system. I should try again with the latest available.
> >
> > I just booted into a vanilla 2.6.28.2 #1 SMP PREEMPT
> >
> > right, there's no realtime patch, yet running jackd at 64 * 3 @48kSPS
> > on a HDA - ardour2 with 12 tracks, a couple of LADSPA effects and
> > jamin (lots of CPU!) - there's no xruns yet!
> >
> > I'll be back in the studio in two weeks from now to test it with USB
> > and 1394 devices. With <=2.6.24 kernels those were always working more
> > reliably that the HDA so I don't expect problems there.
> >
> > BTW. with 2.6.28 I needed to
> >
> >  `echo -1 > /proc/sys/kernel/sched_rt_runtime_us`
> >
> > or edit /etc/sysctl.conf and add
> >   sys.kernel.sched_rt_runtime_us = -1
> >
> > before JACK was able to acquire real-time privileges.
>
> May I ask what that does? This value is 950000 on my system.
> I think jack has rt privileges here on 2.6.28.2 but I'm not too certain.
> I don't even know how to check that reliably.
>

jackd would not start with -R if realtime permission can not be granted. I
think you would know if there was a problem ;)
I have to say I don't know either what this option is you are setting,
Robin. Is that the granularity of the timer (in u-sec's)? Setting to -1
seems to suggest that the highest possible value is used, although I'm only
wildly guessing. Would be great to know!

 cheers,

karsten
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