The different realtime kernels

Thu Sep 30 16:45:30 BST 2010

Hi Ricardo :)

sorry for my broken English, especially at the moment, because I do have
an influenza.

On Thu, 2010-09-30 at 16:18 +0100, Ricardo Lameiro wrote:
> Hi Ralf, 
> 
> I didn't understood what did you meant with:
> > For what do multimedia users (producers, but consumers) need more,
> but
> > vanilla + rt-patch? Does somebody run a multi-user data server on
> the
> > same machine, as he is using in his audio or audio-video studio?
> This
> > would be nonsense.
> 
> What would be nonsense? audio producers using hard RT preemption on
> the kernel?
> Do you think that a webserver needs more Realtime preemption than
> audio work?

No, I guess for audio and audio-video productions we only need a vanilla
+ rt-patch kernel and nothing more.

Nobody should run a web-server or anything else on a DAW, so there are
no other kernel patches needed.

I'm pro PREEMPT RT and against PREEMPT only ;) or any kernel patches
that don't make sense for audio, audio-video productions.

I was asking for reasons to patch a kernel with something like a
'generic'-patch. A DAW, resp. audio-video-MIDI workstation don't need a
special server-kernel, or desktop-kernel etc., just a vanilla kernel +
rt-patch.

Why does Ubuntu Studio comes without PREEMPT RT, but just PREEMPT?!
This is my intension.

FWIW, I'm a professional audio and video engineer and did program oldish
computers and I'm missing hard real-time for modern PCs. Even the
kernel-rt isn't able to do hard real-time, so I don't understand why
Ubuntu Studio does prefer a kernel without rt-patch. Today the rt-patch
isn't good enough and any kernel without this patch is useless for
multimedia production.

So a misunderstanding ;)!

> 
> As I see, If a webserver used a RT kernel, it would have a lot of
> problems, because it will probably lock in some tasks until they are
> finished.
> 
> Audio needs a very low latency, high resolution timer etc, because the
> Interrupts given by sound cards and by audio software need to be
> addressed as fast as possible,

This is what I'm thinking off, I sometimes use the hr timer, that on
Linux still is a PITA on some machines and for some apps.

Anyway, if possible a multimedia distro should use hr timer (HPET), but
always a kernel-rt only.

>  if they arent, what happens is that the audio buffers, either for the
> souncard playback, or capture will run out of data, and then the
> continuos steam of audio data will be over, and wait until receive
> more info. In a Nutshell, you LOSE audio data, and you will never get
> it back, for professional audio that is unacceptable. Also if You give
> software RT priorities, it less possible that, for instance, Ardour is
> left behind of a twitter client.... unaceptable to...
> 
> I am going to make some simple math with a not so professional cenario
> to ilstrate just the data stream, not audo software CPU time.
> 
> 
> Recording and monitoring out 8 channels (8 in 8 out) at 48KS/s at 24
> bits
> 
> 48000 * 24 = 115200 bps = 14.0625KB/s
> 
> 14.0625 * 16 = 225 KB/s = 1.76MB/s
> 
> Well, 1.76 MB/s is not to much really, well this calc is simple
> cenario, provided that the sound card uses real 24 bit audio data
> stream, if it used 32 bit, welll do the math.
> 
> Now to a PRO setup.. 192 KS/s @ 24bits
> 
> 192000 * 24 = 4608000 = 0.55 MB/s
> 
> 0.55 * 16  = 8,78 MB/s
> 
> 8.78 MBytes per second, not mbits, FIrewire is rated at 400 MBit per
> second... USB in practice is a lot less + Communication overhead.
> 
> This is only on the Audio tranfer side, then you need to send this
> streams from each different software, make dsp calculations for
> Amplitude (volume) or mixing. This takes time.... so YES a Real time
> kernels is better for audio users than for normal users. Specially if
> you use Externals Firewire/USB card with high outputs
> 
> note: this are simple calculations made fast, just to demonstrate the
> kind of stream we talk about. I assumed 24 bits, this is very rare,
> usually it goes with 32 bit, that is a lot more data to transfer.
> 
> If some more explanation on why a RT kernel is prefered for audio, i
> can try to answer some more questions, i am not a pro in this tough.
> 
> Ricardo Lameiro
> 
> 2010/9/30 Ralf Mardorf <ralf.mardorf at alice-dsl.net>
>         
>         On Thu, 2010-09-30 at 16:38 +0200, Ralf Mardorf wrote:
>         > On Thu, 2010-09-30 at 07:35 -0400, Ronan Jouchet wrote:
>         > > Hello everybody,
>         > >
>         > > Many are confused about the various realtime kernels, so
>         here is a
>         > > reminder of the situation as of Sept. 2010 (but _please_
>         see
>         > >
>         https://help.ubuntu.com/community/UbuntuStudio/RealTimeKernel , which is
>         > > more detailed and continuously updated).
>         > >
>         > > ***Summary***
>         > > vanilla = unpatched kernel straight from kernel.org
>         > > generic = vanilla + ubuntu sauce (it's the default ubuntu
>         kernel)
>         > >
>         > > The *soft realtime kernels, prepared by changing
>         build-time parameters*
>         > >      preempt = generic + mild configuration to reduce
>         latency
>         > >      lowlatency = generic + aggressive configuration to
>         reduce latency
>         > >
>         > > The *hard realtime kernels, prepared by applying a big
>         patch* from Ingo
>         > > Molnar to the kernel source before building:
>         > >      realtime = vanilla + patch (hard to maintain and
>         stabilize because
>         > > merging 2 pieces of code is never easy)
>         > >      rt = generic + patch (even harder to maintain and
>         stabilize because
>         > > merging 3 pieces of code is harder than 2)
>         > >
>         > > ***Availability***
>         > > - for Maverick, generic will be the only kernel in the
>         archives, thus
>         > > the default kernel for ubuntu and ubuntustudio, but
>         Alessio has been
>         > > maintaining a PPA providing lowlatency and realtime
>         > > - for Natty or later: work is being done to include
>         lowlatency in the
>         > > official archives and make it the default ubuntustudio
>         kernel
>         > >
>         > > I hope this clears some doubts. By the way, this confusion
>         is only going
>         > > to get more intense at release time (less informed /
>         technical users).
>         > > Could we include some kind of note informing users about
>         this? Why not a
>         > > "RealTime kernel help" item in the Audio Production menu,
>         redirecting to
>         > > the wiki page?
>         > >
>         > > Good day,
>         > >
>         > > -- Ronan Jouchet
>         >
>         
>         >
>         > 2 cents,
>         > Ralf
>         
>         
>         PS: Ok, on 32-bit architecture some might need support for
>         large RAM in
>         addition, this might be an additional patch, hat's not needed
>         on 64-bit
>         architecture.
>         
>         
>         
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> 
> 
> 
> -- 
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