Re: difference between realtime-kernel and low-latency-kernel?

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Tue, 2007-10-16 at 12:34 +1000, porl sheean wrote:
> 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. 

Arghh, sorry about that, I thought you were running Ubuntu's kernel. So,
everybody else, ignore my comments! :-) (maybe that was explained
somewhere else in the thread and I missed that, sorry again). 

> 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). 

That is strange (and interesting). 

> 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), 

Would you mind sending me (or posting) the configuration options for
both kernels? (or rather, three, the one for Ubuntu Studio, 64studio and
your hand rolled kernel?)

> 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? 

Not that I know of, the only one I'm applying that is related to low
lantency is Ingo's. And I imagine that 64studio uses something like the
rtirq script to tune irq priorities, and that it would treat both
kernels the same. 

Puzzling. 

-- Fernando




> On 16/10/2007, Fernando Lopez-Lezcano <nando@xxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx> 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
>         >         >
>         >         >
>         >         > _______________________________________________
>         >         > Linux-audio-user mailing list 
>         >         > Linux-audio-user@xxxxxxxxxxxxxxxxxxxx
>         >         >
>         >
>         http://lists.linuxaudio.org/mailman/listinfo.cgi/linux-audio-user
>         >
>         >
>         >         _______________________________________________
>         >         Linux-audio-user mailing list
>         >         Linux-audio-user@xxxxxxxxxxxxxxxxxxxx
>         >
>         http://lists.linuxaudio.org/mailman/listinfo.cgi/linux-audio-user
>         >
>         > _______________________________________________ 
>         > Linux-audio-user mailing list
>         > Linux-audio-user@xxxxxxxxxxxxxxxxxxxx
>         >
>         http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
>         
> 
> 
> _______________________________________________
> Linux-audio-user mailing list
> Linux-audio-user@xxxxxxxxxxxxxxxxxxxx
> http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user

_______________________________________________
Linux-audio-user mailing list
Linux-audio-user@xxxxxxxxxxxxxxxxxxxx
http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user


[Index of Archives]     [Linux Sound]     [ALSA Users]     [Pulse Audio]     [ALSA Devel]     [Sox Users]     [Linux Media]     [Kernel]     [Photo Sharing]     [Gimp]     [Yosemite News]     [Linux Media]

  Powered by Linux