[linux-audio-user] low latency 2.4 help - needed

Jack O'Quin joq at io.com
Sun Mar 6 23:53:30 EST 2005

Atte André Jensen <atte.jensen at gmail.com> writes:

> To be honest I'm not sure exactly what is the cause of the problem
> (which is why I want to see what an optimized kernel will do), I can
> think of:
> 1) kernel.
> 2) unnecessary services
> 3) the fact that my soundcard is usb (Edirol UA-1A)
quite possible, but some have reported success with this device, IIRC
> 4) csound itself (doubt it)
> 5) computer. Hope not, it a P4 2.4Ghz laptop
very unlikely

>> For lowest latency, 2.6.10 with Ingo Molnar's realtime preemption
>> patches is currently the best.
> That's new for me... You're not talking about this, right?
> [atte at aarhus src]$ head linux-2.6.10-rt2.patch

No.  That patch is useful for granting RT privileges to non-root
users, but it has no effect on the inherent latency of the kernel.

> I tried applying the above patch, but I didn't see anything new under
> "security" in the kernel config, so I guess I did something wrong...

You should see a new CONFIG_SECURITY_REALTIME option after the patch
is applied.  If depends on CONFIG_SECURITY and requires that
CONFIG_SECURITY_CAPABILITES not be built in (=y).  I recommend
building it as a module (=m).

> Where to get the patches you're talking about, and what to do?

 [1] http://seclists.org/lists/linux-kernel/2005/Feb/1083.html

> Not that much, since I'm on a home brewn 2.6.9... This also means that I
> have a working alsa setup. If i wen't with 2.4 I would have to install
> alsa seperately, so...
>> A recent, stable 2.6.x kernel is also an easy option.  I'm getting as
>> good or better LL results with vanilla 2.6.10 than with 2.4.19 and the
>> LL patches.  I have not tried 2.6.11 yet, but expect it to be even
>> better.  IMO, latency is no longer a reason to avoid 2.6 kernels.
> As mentioned, I'm already on 2.6.9.

2.6.9 did not have good latency in my tests.  2.6.10 was much better.

Since you're already comfortable building and installing kernels, I
suggest you try 2.6.11 first.  Then, if you want to push the envelope
further, try Ingo's patchset[1].  

I suspect your USB device will become a latency bottleneck before
these kernels will, but only experimenting on your own system will
tell you for sure.

>> For the easiest solution, go with PlanetCCRMA (Fedora/RedHat) or
>> AGNULA/DeMuDi (Debian).  They've got this stuff all integrated and
>> readily available for binary download.
> I'm not interrested in "easy" but in "best". I'm on debian/unstable, so
> maybe agnula would be possible. I just want to make sure that my current
> system is not "infected" with all kinds of agnula stuff. Is it possible
> just to get the low-latency kernel and use on an unstable system?

I believe so.  And their LL kernels probably work very well.  But, I
can't prove that from personal experience.

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