[LAD] hard realtime performance synth

[hollunder]

Excerpts from fons's message of Tue Jan 26 21:47:31 +0100 2010:
> On Tue, Jan 26, 2010 at 03:15:43PM -0500, David McClanahan wrote:
> 
> > 3. I'm a little worried about what some are calling realtime systems. The
> > realtime system that is part of Ubuntu Studio and others may be more
> > preemptible than the normal kernel(as in kernel calls themselves can be
> > preempted), but that's not a hard realtime system. A hard realtime
> > system(simplistic I know) might entail a task whose sole job is to pump out
> > a sinusoidal sound sample to the D-to-A on the sound card. A hard realtime
> > scheduler would run that task at 44Khz no matter what. This would entail
> > developing code that when the machine instructions were analyzed, would run
> > in the time constraints(aka the 44Khz). RTLinux appears to be suitable and
> > RTAI might be. Perhaps others.
> 
> That's not how Linux audio works. Processing is done in
> blocks of typically between 64 and 1024 samples. 
> 
> > The way things are now even with the "realtime kernel" on U. Studio. , xruns
> > can occur because there's no hard limit on accessing resources-only
> > priorities. This may work fine on newer/faster machines but not on the older
> > ones. Some may say, "Go buy a faster machine". My answer is that won't
> > necessarily solve the problem which is a proliferation of "systems" on top
> > of systems without any assurance they'll all work together on time.
> 
> With current kernels you don't even need any RT patches
> to get good performance, unless you need very low latency
> (i.e. very small block sizes). I've been working most of
> the day on a 64-channel system with several quite complex
> apps, and I haven't seen a single xrun. Standard unpatched
> 2.6.32 kernel.
> If you get xruns on a correectly set-up system (and that
> requires very little these days) it's probably because
> the apps you use have not been written in the way required
> to perform well in a realtime context. Unfortunately there
> are many like that.
> 
> Ciao,
> 
> -- 
> FA
> 
> O tu, che porte, correndo si ?
> E guerra e morte !

Well, I use the same distro and kernel and easily get xruns. It all
depends on a couple of factors, among them hardware, software and settings.

One reason why such hardware is not feasible for hard realtime is SMI.
Currently there's nothing you can do about those other than hoping that
they don't happen and that they don't block for too long. Well you can
go up with your latency settings which will make it less likely that
SMIs cause xruns. There are some slides from the last Linux Plumbers 
conference that explain what it basically is and talk about a possible 
future solution: 
linuxplumbersconf.org/2009/slides/Keith-Mannthey-SMI-plumers-2009.pdf

Other typical laptop hardware also is one reason for xruns, mainly
wireless, bluetooth or network in general, various acpi thingies, etc..
These fall in the settings category I guess, and I'm no expert on those
(yet) since I'm pretty new to laptops.

It certainly is possible to use laptops as reliable synths, Ken Restivo
uses a netbook as synth for live performances for example, but I think
SMIs alone will prevent hard realtime capabilities on any standard
hardware.

Regards,
Philipp