16 Oct
2007
16 Oct
'07
12:35 a.m.
---------- Forwarded message ----------
From: porl sheean <porl42(a)gmail.com>
Date: 16 Oct 2007 12:33
Subject: Re: [LAU] difference between realtime-kernel and
low-latency-kernel?
To: Fernando Lopez-Lezcano <nando(a)ccrma.stanford.edu>
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(a)ccrma.stanford.edu> 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(a)commspeed.net> 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:
http://lists.linuxaudio.org/mailman/listinfo.cgi/linux-audio-user
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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