[Ardour-Users] smp questions etc

Steven Chamberlain steven at pyro.eu.org
Tue Aug 21 05:23:11 PDT 2007

Matthew Polashek wrote:
> Ok, I'm throwing together a new box in which I will install UbuntuStudio
> with the intention of using it as a live processing rig for live
> performance using PD.

Some time ago I started using a Linux machine to do realtime processing
as part of a PA system.  I've been through a variety of hardware trying
to get there.  I had some unavoidable latency issues with an old VIA
motherboard, so I decided to avoid those.

I may be wrong, but I came to the conclusion that floating-point ops.
are a good measure of overall performance for live audio processing.
After a little bit of experimenting I decided that having more CPU cores
was generally better than having fewer, faster cores.  Therefore in your
situation I would choose the quad-core.

When I started using an SMP kernel on a dual-core processor, I managed
to get lower latencies even with single-threaded audio processing.  I
imagine this is because as Sampo suggested, the other core can handle
other tasks that might otherwise have slowed down the audio thread.

If you can make sure your audio processing is multi-threaded, the
quad-core should give you very good performance.  I used 'jackdmp' to do
this, because I was using several instances of 'jack-rack' to process
different input signals so jackdmp could run those at the same time in
separate threads.

Something to look out for is that if you run one LADSPA host, running
many plugins, it might run each plugin one-by-one from a single thread.
 Even running jackdmp would not help in this situation.  I have not
tried using 'pd' so I don't know if it would run the plugins
multi-threaded or not.

Related to this, I think it would be interesting if some host could run
and interconnect LADSPA plugins *and* JACK clients, running clients
simultaneously where possible like jackdmp does.

So, I would recommend the quad-core.  Once you can get the audio
processing software to make use of all four cores, you should get much
better results.

Good luck,
Steven Chamberlain
steven at pyro.eu.org

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