[LAU] difference between realtime-kernel and low-latency-kernel?

---------- Forwarded message ----------
From: porl sheean <porl42@email-addr-hidden>
Date: 16 Oct 2007 12:33
Subject: Re: [LAU] difference between realtime-kernel and
low-latency-kernel?
To: Fernando Lopez-Lezcano <nando@email-addr-hidden>

i think a few people have missed what i mean. after re-reading my post i can
see it wasn't all that clear. i am still using ubuntustudio, but i
downloaded the 64studio *kernel* (not the whole distribution, which i tried
but it wasn't quite what i wanted) to replace ubuntu's. after doing that,
everything runs perfectly. i assumed that the ubuntu kernel didn't have the
rt patches set right, but after downloading a vanilla kernel and patching it
myself i noticed that my custom kernel had the same problems as ubuntu's.
whilst i am happy to use the 64studio kernel, i am very curious as to what
they have done to it to make it work where both the ubuntu-rt and my own
custom kernel both have the same symptoms (*very* unstable audio at lower
than 40ms latency, and even then ardour often refuses to start plaback -
jack drops it off as soon as i press play sometimes on larger projects). i
did a diff of the config files from all kernels and couldn't find any
notable difference that would explain it (to my limited knowledge anyway),
so i assume it is a patch that 64studio has applied in addition to the
standard rt patch that is making the difference. are there any other audio
performance enhancing patches for the kernel?

thanks porl

On 16/10/2007, Fernando Lopez-Lezcano <nando@email-addr-hidden> wrote:
>
> On Sun, 2007-10-14 at 12:48 +1000, porl sheean wrote:
> > i have actually had no end of trouble with ubuntustudio's (and now
> > ubuntu's) rt kernel. on an amd 6000+ system with 1gig ram and a
> > rme9652 soundcard i can't get reliable performance under 40 or so ms.
> > i even tried a vanilla kernel with the rt patches and had the same
> > trouble. the 64studio kernel worked fine, however. i'm currently
> > running at 5ms with it and have had no problems. this is even with
> > compiz fusion running and spinning the cube whilst playing back audio
> > from an 18 channel ardour project. what patches would cause such a
> > difference in performance? it isn't any options selected in 'make
> > menuconfig' - i loaded the 64studio's ones in and used them. still no
> > luck. i can only assume they have added more patches to do with
> > realtime performance than just the -rt patchset.
>
> >From what I gather ubuntustudio does not have an rt patched kernel (ie:
> patched with Ingo Molnar's realtime preemption patch). See observations
> below:
>
> > On 05/10/2007, thomas fisher <studio1@email-addr-hidden> wrote:
> > I can supply no quantifications for the 32 bit 2.6.20-16-realtime
> > kernel in ubuntustudio other than no xruns have been observed.
>
> So, Ubuntu Studio has a kernel named realtime and they have observed no
> xruns (which is not your case, and not the case of other posters).
> Obviously it depends on how they test (hardware used, size of buffers,
> load on the machine under test, etc, etc) and there's no info on that
> post about that.
>
> > With the low latency kernel, xruns were observed.
>
> This implies they don't have the low latency patches applied and that,
> in their experience, the low latency patch was worse than the mainline
> kernel.
>
> If you have access to that kernel and you can check its build
> configuration you could grep for "PREEMPT" there and post the results.
> That will definitely tell us which options were used for building it (I
> suspect you will find just "PREEMPT_VOLUNTARY" there).
>
> In my experience, a mainline kernel will lead to xruns at low latencies
> (there's always an exception, of course).
>
> > Jack is the only app that has a -20 priority assigned.
>
> This implies they are not using SCHED_FIFO for running Jack. Apparently
> they are boosting the normal scheduler ring priority of Jack to -20. I
> have not experimented with this so I can't comment, except to say that
> everyone else (that I know of) is using SCHED_FIFO for running Jack -
> SCHED_FIFO is a higher priority scheduling method that can't be
> preempted by regular linux tasks, and while it is more risky as a badly
> designed application can hang the machine, the tradeoff is of course
> much better realtime performance.
>
> > The general workstation has been running without fault. The general
> > Debian / Ubuntu philosophy tends towards system stability.
>
> The realtime preemption patch is certainly less stable than the mainline
> kernel. But if you hardware runs it fine then it is more effective than
> mainline for achieving low latencies.
>
> -- Fernando
>
>
> > Tom
> > On Wednesday 03 October 2007 14:54:32 Fernando Lopez-Lezcano
> > wrote:
> > > On Wed, 2007-10-03 at 18:39 +0200, Frank Barknecht wrote:
> > > > Hallo,
> > > >
> > > > Matthias Schönborn hat gesagt: // Matthias Schönborn
> > wrote:
> > > > > I've just read that there's a difference between a
> > realtime-kernel and
> > > > > the low-latency-kernel provided by ubuntustudio. The
> > text in the german
> > > > > wiki on ubuntuusers.de said, that a realtime-kernel is
> > slightly better
> > > > > than the lowlatencykernel
> > ( http://wiki.ubuntuusers.de/Echtzeitkernel ) -
> > > > > then why isn't it used in ubuntustudio? Or do I just mix
> > something up?
> > > >
> > > > I think, this wiki and maybe Ubuntustudio as well are
> > using a very
> > > > confusing terminology.
> > > >
> > > > Generally we have two kinds of kernels: The "vanilla"
> > kernel as
> > > > downloadable on kernel.org and the same kernel, but
> > patched with Ingo
> > > > Molnars RT-patches. The vanilla kernel, if configured
> > properly with
> > > > CONFIG_PREEMPT etc., already gives very good performance
> > in the low
> > > > latency department, enough for many users, even audio
> > users. I run one
> > > > of these.
> > > >
> > > > If you want more, then you can install a RT-patched
> > kernel, as is
> > > > provided in the linux-rt or linux-realtime packages. I
> > would call the
> > > > Ingo-Molnar-patched kernels Realtime-Kernels or
> > Low-Latency-Kernels.
> > >
> > > To further clarify (or confuse?) the issue, how "low
> > latency" the kernel
> > > is also depends on how you configure the kernel build
> > options before or
> > > after patching the kernel with Ingo's patch. For Ingo's
> > patch
> > > CONFIG_PREEMPT_RT is the best option in terms of latency but
> > there are
> > > others (CONFIG_PREEMPT_DESKTOP) that have a more
> > conservative approach
> > > but have (relatively speaking) higher latencies. So from
> > worst to best
> > > it would be something like:
> > >
> > > vanilla linuz + CONFIG_PREEMPT_NONE
> > > vanilla + CONFIG_PREEMPT_VOLUNTARY (used by the stock
> > Fedora kernel)
> > > vanilla + Ingo + CONFIG_PREEMPT_DESKTOP
> > > vanilla + Ingo + CONFIG_PREEMPT_RT (the one I use for
> > Planet CCRMA)
> > >
> > > (there's more granularity and options in the CONFIG_PREEMPT*
> > world but
> > > those are the ones that have the biggest impact as far as I
> > can
> > > remember)
> > >
> > > -- Fernando
> > >
> > >
> > > _______________________________________________
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> > >
> >
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> >
> >
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Received on Tue Oct 16 08:15:02 2007