Re: [Jack-Devel] Profiling xruns

this involves kernel "debugging". there are facilities in the kernel
to trace delays in RT-scheduled tasks. googling with reveal more. you
will end up with very detailed information. the process is not
particularly smooth or easy - its targetted at kernel developers and
highly technical user space developers - but it is quite powerful.

On 3/17/12, Dénes Almási <[hidden]> wrote:
> Hi!
>
> I am developing a jack client that is handling different input devices
> (like joysticks, touch screens, etc.) and capable of using them as
> musical input devices.
>
> The architecture is relatively simple: A directed acyclic graph can be
> made up of different boxes, each of them performing different tasks,
> like
> reading input, generating oscilating values, passing data to a jack
> midi
> or audio port, etc. When the graph is modified, a non-rt thread
> generates
> a topologic ordering of the nodes into a linked list and a pointer to
> the head
> is passed in a ringbuffer to the process function which at its
> beginning examines
> the content of the buffer and can store the new list. In each process
> call, the list is iterated and the process function is called for each
> box.
> The edges have buffers which are read and written by the boxes and the
> topologic
> ordering ensures that only those edges are read in a cycle whose data
> are
> already computed.
>
> The problem is: Even without having ANY nodes, thus the process
> function doing
> practically nothing, sometimes the application does xruns. I am sure
> that the
> source of these xruns is the application as they show up in the
> messages
> window of qjackctl with the app's name. Also, I am running in soft
> mode, which
> I assume, means that xruns indicating that the backend is not fast
> enough, are
> ignored.
>
> How could I locate the cause of these xruns? Is there a good practice
> for profiling
> these problems? I think it is hard because using tools like gprof will
> make the
> app slow and cause xruns which would never occur without the profiling
> itself.
>
> I am using mlockall and a big memory pool, a thread pool (though
> currently no threads
> are created when process is called), process shielding for the sound
> card soft irq,
> jackd and the app.
>
> Jack settings: Rt priority 83, Frames/Period: 128, Sample rate: 48000,
> Periods/Buffer: 2.
> The latency is 5.33 msec and apps like ardour xrun only very rarely,
> under heavy usage.
> The machine is a samsung notebook with a core i3 cpu, 3Gb ram and
> unfortunately an onboard
> soundcard.
>
> Is it better for xrun debugging to use the dummy interface in order to
> be totally
> independent of the sound card? Also, could it be the problem that I am
> using
> clutter for gui? I am quite a beginner in developing audio
> applications...
>
> Thank you in advance,
> Dennis
> 
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>

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